Combining Plasticizers/Retarders And Accelerators

Transcription

Katholieke Universiteit Leuven
Faculteit Ingenieurswetenschappen
Departement Burgerlijke Bouwkunde
Norwegian University of Science and Technology
Faculty of Natural Sciences and Technology
Department of Materials Science and Engineering
Combining Plasticizers/Retarders
And Accelerators
E2006
Promotor: prof. dr. H. Justnes
prof. dr. ir. D. Van Gemert
Klaartje De Weerdt
Dirk Reynders
Academiejaar: 2005-2006
Departement: Burgerlijke Bouwkunde
Adres en telefoon: Kasteelpark Arenberg 40 – 3001 Heverlee – 016/32 16 54
Naam en voornaam studenten: De Weerdt Klaartje
Reynders Dirk
Titel eindwerk: Combineren van plastificeerders/vertragers en versnellers
Korte inhoud eindwerk:
De combinatie van plastificeerders/vertragers en versnellers werd bestudeerd met drie
mogelijke toepassingen in het achterhoofd: 1) het tegengaan van het vertragend effect van
plastificeerders zonder de reologie sterk te wijzigen, 2) de activatie van vertraagd beton op de
werf na veilig transport in warme streken of steden met onvoorspelbaar verkeer en 3) het
oververtragen van overschotten aan vers beton gevolgd door activatie na één of meerdere
dagen.
De experimenten werden grotendeels uitgevoerd op cementpasta. Een Paar-Physica MCR 300
rheometer werd gebruikt ter bepaling van de reologie en een TAM Air isotherme calorimeter
ter bepaling van de hydratiecurves.
Er werd vastgesteld voor toepassing 1) dat calciumnitraat het vertragend effect van natrium en
calcium lignosulfonaat sterk terugschroeft en in het geval van polyacrylaat zelfs volledig
wegneemt terwijl de combinaties werken als plastificeerders, voor toepassing 2) dat de
combinatie natriumgluconaat/calciumnitraat een mogelijk werkend systeem is en voor
toepassing 3) dat de combinatie citroenzuur/calciumnitraat het hergebruik van overschotten
aan vers beton op een later tijdstip mogelijk maakt.
Promotor: prof. dr. ir. D. Van Gemert – prof. dr. H. Justnes
Assessoren: prof. dr. ir. L. Vandewalle – ir. G. Heirman
Year: 2005-2006
Department: Burgerlijke Bouwkunde
Address en tel.: Kasteelpark Arenberg 40 – 3001 Heverlee – 016/32 16 54
Name and surname students: De Weerdt Klaartje
Reynders Dirk
Title of thesis: Combining plasticizers/retarders and accelerators
Summary of thesis:
The combination of plasticizers/retarders with accelerators has been studied in view of three
potential concrete applications: 1) counteracting retardation of plasticizers without negatively
affecting rheology too much, 2) activating retarded concrete at site after safe transport in hot
climate or cities with unpredictable traffic and 3) over-retarding residual fresh concrete one
day and activating it next day or after several days.
The experimental work is largely carried out on cement paste using a Paar-Physica MCR 300
rheometer to determine flow curves and gel strength and a TAM Air isothermal calorimeter
for determination of heat of hydration curves.
It has been found for application 1) that calcium nitrate strongly reduces retardation of sodium
and calcium lignosulphonates and even cancels retardation of polyacrylates, whereas the
blend also has plasticizing effects, for 2) that sodium gluconate/calcium nitrate is a potentially
effective system and for 3) that citric acid/calcium nitrate may facilitate later use of residual
fresh concrete.
Promotor: prof. dr. ir. D. Van Gemert – prof. dr. H. Justnes
Assessors: prof. dr. ir. L. Vandewalle – ir. G. Heirman
Table of Contents
1
Introduction
1
2
Background on cement, cement hydration, rheology and admixtures
4
2.1
2.2
2.3
2.4
2.5
4
5
9
13
22
3
Cement ..................................................................................................................
Cement hydration ..................................................................................................
Rheology ...............................................................................................................
Plasticizers/retarders .............................................................................................
Calcium nitrate ......................................................................................................
Materials and apparatus
24
3.1 Materials................................................................................................................ 24
3.2 Apparatus .............................................................................................................. 27
4
5
6
7
Counteracting plasticizer retardation
34
4.1
4.2
4.3
4.4
34
35
75
80
Introduction ...........................................................................................................
Calorimetric and rheological measurements.........................................................
Mortar measurements............................................................................................
General conclusion................................................................................................
Long transport of fresh concrete
81
5.1
5.2
5.3
5.4
5.5
5.6
81
81
95
98
101
110
Introduction ...........................................................................................................
Sodium lignosulphonate........................................................................................
Citric acid ..............................................................................................................
Lead nitrate............................................................................................................
Sodium gluconate..................................................................................................
Reutilizing residual fresh concrete
111
6.1
6.2
6.3
6.4
6.5
6.6
111
111
114
116
120
126
Introduction ...........................................................................................................
Phase I – Screening of retarders............................................................................
Phase II – Determination of required retarder dosage ..........................................
Phase III – Activation using calcium nitrate .........................................................
Phase IV – Strength measurements.......................................................................
Conclusions
127
Chapter 1
Introduction
This thesis continues a long tradition of Erasmus exchanges between the “Katholieke
Universiteit Leuven” (Belgium) and the “Norges Teknisk-Naturvitenskapelige Universitet i
Trondheim” (Norway). For many years students have been studying advanced aspects of
cementitious materials. Thys, A. and Vanparijs, F. ([1]) studied the longterm performance of
concrete with calcium nitrate, Ardoullie, B. and Hendrix, E. ([2]) focused on the chemical
shrinkage of cementitious pastes and mortars, Clemmens, F. and Depuydt, P. ([3])
investigated early hydration of Portland cements, the thesis of Van Dooren, M. ([4])
concerned the factors influencing the workability of fresh concrete, and Brouwers, K. ([5])
studied a number of cold weather accelerators.
In this thesis the combination of plasticizers/retarders and accelerators has been investigated
in view of three different potential concrete applications.
The first application, which made up the major part of this study, focused on the fact that
plasticizers that are used to increase flow for cementitious materials at equal water-to-cement
ratio also to a variable extent retard setting as a side effect. The objective was to find an
accelerator that at least partially would counteract this retardation without negatively affecting
the rheology too much. Whereas earlier studies on this topic focused on plastic viscosity at
high shear rate (i.e. relevant for mixing) and relatively low dosages of plasticizer, the study
reported here focused on the lower shear rate range (i.e. relevant for pouring concrete) and
higher dosages of plasticizer. The results of this study are presented in Chapter 4. These
results are valuable elements in evaluating the combined use of plasticizers and accelerators,
as it was e.g. applied during construction of Statoil’s Troll platform (Figure 1.1), a huge gas
platform located 80 km north-west of Bergen (Norway) that reaches 303 m below the surface
of the sea. During the construction of its 350 m tall base an accelerator has been used to speed
1
Chapter 1: Introduction
2
up the slip forming process of the plasticized concrete as construction works were behind
schedule.
The second application concerns long transport of fresh concrete. The preliminary study was
largely carried out on paste. It was investigated if a concrete mix from a ready mix plant after
being deliberately over-retarded for long transport in for instance hot climate or cities with
unpredictable traffic (e.g. traffic jam) could be activated by adding an accelerator in the
revolving drum close to the construction site before pumping the concrete in place. Results
are discussed in Chapter 5.
The third potential application, presented in Chapter 6, concerns the search for a system to
preserve residual fresh concrete for a few days (e.g. over a weekend) followed by activation
before use. However, it might also be used as an overnight concept. Whereas recently a
freezing preservation technique has been proposed as method for reutilizing left-over
concrete, this study concentrated on a technique consisting of over-retardation of residual
fresh concrete followed by later activation using an accelerator.
Figure 1.1 Troll gas platform (1996)
Chapter 1: Introduction
3
The necessary background on cement, cement hydration, rheology and admixtures is given in
Chapter 2. Chapter 3 introduces and describes the materials and the apparatus that have been
used throughout this work.
Chapter 2
Background on cement, cement
hydration, rheology and admixtures
2.1 Cement
Cement chemists use in general a short hand notation, C = CaO, S = SiO2, A =Al2O3,
F = Fe2O3 and S = SO3, for the main elements in the chemical analyses of cement, in
addition to H = H2O to describe hydration processes. The elements are determined by
X-ray fluorescence or analytical chemistry and given as the corresponding oxides.
Assuming that the only minerals in the cement are alite (C3S), belite (C2S), aluminate
phase (C3A), ferrite phase (C4AF) and anhydrite ( C S ) the content of these minerals
may be calculated through mass balances. The first four minerals are formed during
equilibrium conditions in the burning of the cement clinker, while the latter mineral
(or gypsum, C S H 2 ) is added to the mill when clinker is ground to cement. In
specification sheets, the content of other oxides is also given: N (Na2O), K (K2O) and
M (MgO). “Free lime” is the content of free CaO due to insufficient burning or due to
the decomposition of C3S into C2S and “free lime” if the cooling rate is too low.
The specific surface area (m2/kg) of cement is commonly determined directly by an
air permeability method called the Blaine method. In addition to the specific area, the
particle size is of importance for the hydration rate of cement, since the hydration
takes place at the interface between the cement grain and the water phase. However, it
is important to realise that the surface of a cement grain is inhomogeneous. The
distribution of C3S/C2S- and C3A/C4AF-domains are determined by the milling
process and the difference in resistance against fracture. Since cement grains are
composite grains with possibly all 4 major phases in one grain, efforts to simulate
4
Chapter 2: Background
5
cement by adding corresponding amounts of individual minerals will therefore fail.
(Justnes, H., [6], p.10)
2.2. Cement hydration
In the discussion of rheology of cement paste and the interaction with plasticizing
admixtures and retarders, it is of importance to know something about the hydration
until setting. It is sometimes believed that no hydration takes place in the so-called
“dormant” period between water addition and initial setting, while actually a
substantial growth of hydration products takes place on the surface of the cement
grains. (Justnes, H., [6], p.10)
2.2.1 The interstitial phases C3A/C4AF
In the absence of calcium sulphates the first hydration product of C3A which appears
to grow at the C3A surface is gel-like. Later this material transforms into hexagonal
crystals corresponding to the phases C2AH8 and C4AH19. The formation of the
hexagonal phases slows down further hydration of C3A as they function as a hydration
barrier. Finally the hexagonal phases convert to the thermodynamically stable cubic
phase C3AH6 disrupting the diffusion barrier, after which the hydration proceeds with
a fairly high speed. The overall hydration process may thus be written as
2 C 3 A + 27 H → C 2 AH 8 + C 4 AH19 → 2 C 3 AH 6 + 15 H
(hexagonal phases)
(cubic phase)
In the presence of calcium sulphate (as in a Portland cement) the amount of hydration
of C3A in the initial state of hydration is distinctly reduced when compared to that
consumed in the absence of C S . Needle-shaped crystals of ettringite are formed as the
main hydration product:
C3 A + 3 CSH 2 + 26 H → C6 AS3 H32
Minor amounts of the monosulphate C 4 A S H 12 or even C 4 AH19 may also be formed
if an imbalance exists between the reactivity of C3A and the dissolution rate of
calcium sulphate, resulting in an insufficient supply of SO 42- - ions.
Then ettringite formation is accompanied by a significant liberation of heat. After a
rapid initial reaction, the hydration rate is slowed down significantly. The length of
this dormant period may vary and increases with increasing amounts of calcium
sulphate in the original paste.
6
A faster hydration, associated with a second heat release maximum, gets under way
after all the available amount of calcium sulphate has been consumed. Under these
conditions the ettringite, formed initially, reacts with additional amounts of tricalcium
aluminate, resulting in the formation of calcium aluminate monosulphate hydrate
(monosulphate):
C 6 A S3 H 32 + 2 C 3 A + 4 H → 3 C 4 A S H 12
As ettringite is gradually consumed, hexagonal calcium aluminate hydrate ( C 4 AH19 )
also starts to form. It may be present in the form of a solid solution with C 4 A S H 12 or
as separate crystals.
The origin of the dormant period, characterised by a distinctly reduced hydration rate,
is not obvious and several theories have been forwarded to explain it. The theory most
widely accepted assumes the build-up of a layer of ettringite at the surface of C3A that
acts as a barrier responsible for slowing down the hydration. Ettringite is formed in a
through-solution reaction and precipitates at the surface of C3A due to its limited
solubility in the presence of sulphates. The validity of this theory has been questioned
arguing that the deposited ettringite crystals are not dense enough to account for the
retardation of hydration. The four proceeding alternative theories have been proposed:
i)
The impervious layer consists of water-deficient hexagonal hydrate
stabilised by incorporation of SO 42- . It is formed on the surface of C3A and
ii)
becomes covered by ettringite.
C3A dissolves incongruently in the liquid phase, leaving an aluminate rich
layer on the surface. Ca2+ - ions are adsorbed on it, thus reducing the
number of active dissolution sites and thereby the rate of C3A dissolution.
A subsequent adsorption of sulphate ions results in a further reduction of
the dissolution rate.
iii)
SO 42- - ions are adsorbed on the surface of C3A forming a barrier. Contrary
to this theory it has been found that C3A is not slowed down if the calcium
iv)
sulphate is replaced by sodium sulphate.
Formation of an amorphous layer at the C3A surface that acts as an
osmotic membrane and slows down the hydration of C3A.
The termination of the dormant period appears to be due to a breakdown of the
protective layer, as the added calcium sulphate becomes consumed and ettringite is
converted to monosulphate. In this through-solution reaction both C3A and ettringite
dissolve and monosulphate is precipitated from the liquid phase in the matrix.
7
The composition of the calcium aluminoferrite phase (ferrite phase), usually written
as C4AF, may vary between about C4A1.4F0.6 and C4A0.6F1.4. Under comparable
conditions the hydration products formed in the hydration of the ferrite phase are in
many aspects similar to those formed by the hydration of C3A although the rates differ
and the aluminium in the products is partially substituted by ferric ions. The reactivity
of the ferrite may vary over a wide range, but seems to increase with increasing A/F –
ratio.
2.2.2 The main mineral alite C3S
The hydration of alite can be divided into 4 periods:
a) Pre-induction period: Immediately after contact with water, an intense, but
short-lived hydration of C3S gets under way. An intense liberation of heat may
be observed in this stage of hydration. The duration of this period is typically
no more than a few minutes.
b) Induction (dormant) period: The pre-induction period is followed by a period
in which the rate of reaction slows down significantly. At the same time the
liberation of heat is significantly reduced. This period lasts typically a few
hours.
c) Acceleration (post-induction) period: After several hours the rate of hydration
accelerates suddenly and reaches a maximum within about 5 to 10 hours. The
beginning of the acceleration period coincides roughly with the beginning of
the second main heat evolution peak. The Ca(OH)2 concentration in the liquid
phase attains a maximum at this time and begins to decline. Crystalline
calcium hydroxide (portlandite) starts to precipitate. The initial set as
determined by Vicat-needle is often just after the start of this period and the
final setting time just before the ending of it.
d) Deceleration period: After reaching a maximum the rate of hydration starts to
slow down gradually, however, a measurable reaction may still persist even
after months of curing. The reason for this is that the hydration reaction
becomes diffusion controlled due to hydration products growing around the
unhydrated cement core in increasingly thickness.
8
The overall alite hydration reaction may ideally be written as
2 C 3S + 7 H → C 3S 2 H 4 + 3 CH
The calcium hydroxide, CH, is crystalline, while the calcium silicate hydrate is
amorphous with a variable composition and therefore often simply denoted CSH-gel.
2.2.3 Hydration and setting of ordinary Portland cement
The overall hydration of ordinary Portland cement is basically a combination of the
description of the interstitial phase with gypsum and alite as discussed in the
preceding sections. Which of the two dominates the setting is still a matter of
discussion and probably depends on the cement composition
The hydration of Portland cement can be associated with the liberation of hydration
Rate of Heat Evolution
heat. Figure 2.1 shows the heat evolution curve for a typical Portland cement.
Dissolution Ettringite
and CSH gel Formation
Formation of
Monosulfate
Rapid Formation
of CSH and CH
Induction Period
Increase in Ca2+ and
OH- Concentration
DiffusionControlled
Reactions
Final Set
Initial Set
Min
Hours
Days
Time of Hydration
Figure 2.1 Hydration heat evolution of an ordinary Portland cement. (Justnes, H., [6],
p. 10)
In cements containing at least a fraction of the K+ in the form of potassium sulphate,
the hydration process may be marked by a distinct initial endothermic peak
immediately after mixing which is due to the dissolution of this cement constituent in
the mixing water. A rather intense liberation of heat with a maximum within a few
9
minutes is due to the initial rapid hydration of C3S and C3A. Hydration of calcium
sulphate hemihydrate to dehydrate may also contribute to this exothermic peak. After
a distinct minimum, due to the existence of a dormant period in which the overall rate
of hydration is slowed down, a second, mean exothermic peak, with a maximum after
a few hours, becomes apparent. It is mainly due to the hydration of C3S and the
formation of the CSH phase and portlandite. After that, the rate of heat release slows
down gradually and reaches very low values within a few days. In most but not all
cements, a shoulder or small peak may be observed at the descending branch of the
main peak, which is probably due to renewed ettringite formation, there may even be
a second shoulder which is attributed to ettringite-monosulphate conversion. (Hewlett,
P., [7], p. 270-271)
2.3 Rheology
2.3.1 General viscosity
In his “Principa” published in 1687, Isaac Newton formulated the following
hypothesis about steady simple shearing flow: “The resistance which arises from the
lack of slipperiness of the parts of the liquid, other things being equal, is proportional
to the velocity with which the parts of the liquid are separated from each other”. This
is shown in Figure 2.2.
Figure 2.2 Steady simple shearing flow. (Justnes, H., [6], p. 3)
This lack of slipperiness is what we now call “viscosity”. It is synonymous with
“internal friction” and is a measure of “resistance to flow”. The force per unit area
required to produce the motion F/A is denoted shear stress ( τ ) and is proportional to
the “velocity gradient” U/d (or “shear rate”, γɺ ). The constant of proportionality, η ,
is called the shear viscosity (also called “apparent” viscosity):
η=
τ
γɺ
10
The simplest rheological behaviour for liquids is the Newtonian viscous flow and
Hooke’s law for solid materials. Ideal viscous (or Newtonian) flow behaviour is
described using Newton’s law
τ = η ⋅ γɺ
Examples of ideal viscous materials are low molecular liquids such as water, solvents,
mineral oils, etc. and they are often called Newtonian liquids.
Hooke’s law states that the shear force acting on a solid is proportional to the resulting
deformation
τ = G ⋅γ
where G is the “rigidity modulus”.
Many materials – especially those of colloidal nature – show a mechanic behaviour in
between these to border lines (Hooke’s an Newton’s laws), i.e. they have both plastic
and elastic properties and are called viscoelastic.
Samples with a yield point only begin to flow when the external forces acting on the
material are larger than the internal structural forces. Below the yield point, the
material shows elastic behaviour, i.e. it behaves like a rigid solid that under load
displays only a very small degree of deformation that does not remain after removing
the load. To describe the rheology of samples showing a yield point the Bingham
model is often used. The Bingham model was extended by Herschel/Bulkley to
include samples with apparent yield point due to shear thinning or thickening:
τ = τ 0 + µ p ⋅ γɺ p
p = 1 for samples with Bingham behaviour (true yield point)
p < 1 for samples exhibiting shear thinning (apparent yield point)
p > 1 for samples with shear thickening behaviour
Shear thinning is a reduction of viscosity with increasing shear rate in steady flow.
Samples with shear thinning behaviour can be macromolecule solutions or melts
where the individual molecules are entangled. Under high shear load the
macromolecules will stretch out and may be disentangled, causing a reduction of the
viscosity. Furthermore, in dispersions or suspensions shearing can cause particles to
orient in the flow direction, agglomerates to disintegrate or particles to change their
11
form. During this process the interaction forces between the particles usually decrease
and this also lowers the flow resistance.
Shear thickening is an increase of viscosity with increasing shear rate. Shear
thickening flow behaviour occurs in concentrated chemically unlinked polymers due
to mechanical entanglements between the mostly branched molecule chains. The
higher the shear load the more the molecule chains prevent each other from moving.
If, during the shear process with highly concentrated suspensions, the particles touch
each other more and more the consequences are similar: the resistance to flow
increases.
Cement paste has shear thinning properties due to both agglomerates of cement grains
and growth of needle-shaped ettringite in the fresh state. An extreme case of
“particles” that will change shape under shear load easily are entrained air bubbles.
There is often more air in concrete than in cement paste, and this may make it difficult
to correlate the concrete rheological properties with those of the “same” paste using
the particle-matrix model. Note that concrete with 5 volume percentage air
corresponds to 15 – 20 volume percentage air in the matrix, something that clearly
will affect the matrix rheology.
2.3.2 Flow resistance
Numerous rheological models have been proposed to describe cementitious materials.
The Bingham model has become very popular due to its simplicity and ability to
describe cementitious flow. The model describes the shear stress ( τ ) as a function of
yield stress ( τ̂0 ), plastic viscosity ( µ p ) and shear rate ( γɺ ) as
τ = τˆ 0 + µp ⋅ γɺ
The concept of yield stress is sometimes a very good approximation for practical
purposes. It is however clear that the Bingham model often only applies for limited
parts of the flow curve if the tested material has shear thinning or shear thickening
flow behaviour. The Bingham model is dependent on the shear rate range for shear
thickening materials. The shear thickening behaviour results furthermore in negative
yield stress values at the high shear rate, which has no physical meaning (see Figure
2.3). There is a similar strong effect of the shear rate range on the flow parameters of
a shear thinning paste.
12
τ
µp
γɺ
τ̂0
Figure 2.3 Shear thickening behaviour resulting in negative yield stress values when
using the Bingham model.
The Hershel/Buckley equation τ = τˆ 0 + µp ⋅ γɺ p can be used to fit flow curves of pastes
showing shear thinning or shear thickening behaviour. However, it may be difficult to
compare
( µp )
viscosities
for
different
mixes
with
different
p-factors. Negative yield stress values ( τˆ0 ) with no physical meaning can sometimes
also be obtained using the Hershel/Buckley equation. Therefore the area under the
flow curve (Vikan, H. and Justnes, H., [8]) was chosen as a measure of “flow
resistance” (Figure 2.4). This parameter, from here on referred to as “flow resistance”,
shall be used throughout to work to describe the flow curve. The flow resistance will
always be a positive value and not depend on curve shape.
τ
flow resistance
γɺ
Figure 2.4 Flow resistance.
13
Furthermore, the choice between two parameters for correlation, as for the Bingham
model, can be omitted. It can be shown (Vikan, H. and Justnes, H., [8]) that the area
under the flow curve represents something more “physical” than an “apparent” yield
stress from Bingham modeling. In a parallel plate set-up with shear area, A [m2], and
gap h [m] between the plates:
τ=
F
A
∆γɺ =
[N/m2 or Pa]
∆v
h
[m/s.m or s-1]
where F [N] is the force used to rotate the upper plate and v [m/s] the velocity.
 F   ∆v  F ⋅ ∆v F ⋅ ∆v
Area under the curve = τ ⋅ ∆γɺ =   ⋅   =
=
A⋅ h
V
 A  h 
where V [m3] is the volume of the sample. The unit of the area under the curve is then
[N.m/m3.s or J/m3.s or W/m3]. It is in other words the power required to make a unit
volume of the paste flow with the prescribed rate in the selected range. The power,
P [W], required to mix concrete for a certain time interval is actually sometimes
measured by simply monitoring voltage (U [V]) and current (I [A]) driving the
electrical motor of the mixer, since P = U.I.
2.4 Plasticizers/retarders
2.4.1. Introduction
Water-reducing admixtures or plasticizers are all hydrophilic surfactants which, when
dissolved in water, deflocculate and disperse particles of cement. By preventing the
formation of conglomerates of cement particles in suspension, less water is required to
produce a paste of a given consistency or concrete of particular workability.
Maintaining low water contents whilst achieving an acceptable level of workability
results in higher strengths for given cement content as well as lower permeability and
reduced shrinkage. An important consequence of the reduction in the permeability is a
major enhancement of its durability. The permeability of concrete to gases (oxygen,
CO2), and water (carrying chlorides, sulfates, acids and carbonates) is of major
importance with respect to its durability.
Retarding admixtures, which extend the hydration induction period and thereby
lengthening the setting times, are often treated together with plasticizing admixtures
as the main components used for retarding mixtures are also present in water-reducing
14
admixtures. As a result, many retarders tend to reduce mixing water and many water
reducers tend to retard the setting of concrete.
A much greater reduction in the volume of mixing water can be achieved using socalled superplasticizers or high-range water-reducing admixtures in case of concretes
of normal workability. Normal water reducers are capable of reducing water
requirement by about 10-15%. Further reductions can be obtained at higher dosages
but this may result in undesirable effect on setting, air content, bleeding, segregation
and hardening characteristics of concrete. Superplasticizers are capable of reducing
water contents by about 30%. (Ramachandran, V.S., [9], p. 211)
Much of the following is based on ‘Rheology of Cement based Binders – State-of-theArt’ by H. Justnes ([6]).
2.4.2. Common plasticizer types
There are four generations of plasticizers/water reducers in terms of time of
discovery/use:
1. Salts of hydrocarboxylic acids with strong retarding effects
2. Calcium or sodium lignosulphonate (denoted CLS or NLS) as by-products
from pulping industry with medium retarding properties.
3. Synthetic compounds like naphtalene-sulphonate-formaldehyde condensates
(SNF) and sulphonated melamine-formaldehyde condensates (SMF) with
small retarding properties.
4. Synthetic polyacrylates with grafted polyether side chains (PA) with small
retarding properties.
The first generation plasticizers, the salts of organic hydroxycarboxylic acids, are
mostly used for their dominating retarding behavior. As the name implies, the
hydrocarboxylic acids have several hydroxyl (OH) groups and either one or two
terminal carboxylic acids (COOH) groups attached to a relatively short carbon chain.
Figure 2.5 illustrates some typical hydroxycarboxylic acids which can be used as
water reducing or retarding admixtures. Gluconic acid is perhaps the most widely
used admixture. Citric, tartaric, mucic, malic, salicylic, heptonic, saccharic and tannic
acid can also be used for the same purpose. Usually they are synthetized chemically
15
Figure 2.5 Typical hydrocarboxylic acids used in water reducing admixtures.
(Ramachandran, V.S., [9], p.126)
and have a very high degree of purity as they are used as raw materials by
pharmaceutical and food industries. Some aliphatic hydrocarboxylic acids, however,
can also be produced from fermentation or oxidation of carbohydrates and for this
reason are also called sugar acids. Hydrocarboxylic acids can be used alone as
retarders or water-reducing and retarding admixtures. For use as normal and
accelerating water reducers they must be mixed with an accelerator. (Ramachandran,
V.S., [9], p. 125)
The second generation plasticizers, the lignosulphonates, are still the most widely
used raw material in the production of water reducing admixtures. Lignosulphonates
are sulphonated macromolecules from partial decomposition of lignin by calcium
hydrogen sulphite. Under sulphite pulping, lignin is sulphonated and rendered water
soluble. The spent sulphite liquor contains sulphonated lignin fragments of different
molecular sizes and sugar monomers after removing the pulp. It can be further
purified by fermentation to remove hexoses and by ultrafiltration to enrich larger
molecular fractions. In addition to chemical modification of functional groups for
special applications, simple treatment by sodium sulphate will ion exchange calcium
16
through formation of gypsum that is removed. A fragment of a lignosulphonate is
illustrated in Figure 2.6. Fractionation to enrich larger molecular fractions increases
the effectiveness of lignosulphonate as a dispersant for cement in water and reduces
the retarding effect. Sodium lignosulphonates retard in general less than calcium
lignosulphonates.
Figure 2.6 Fragment of lignosulphonate. (Justnes, H., [6], p. 30)
Due to the size of the molecule, it cannot be ruled out that lignosulphonates disperse
cement both through electrostatic repulsion and steric hindrance. The average
molecular weight of common lignosulphonates used as plasticizers for cement may be
about 5,000-10,000. It is assumed that the structure of lignosulphonates in solution
consists of a mainly hydrophobic hydrocarbon core with sulphonic groups positioned
at the surface. The bulk of the model is assumed to be made up of cross linked, polyaromatic chains which are randomly coiled. The negatively charged groups are
positioned mainly on the surface or near the surface of the particle, and a double layer
17
of counter ions is present in the solvent. The lignosulphonate molecules behave as
expanding polyelectrolytes as they expand at low and contract at high salt
concentrations.
The third generation plasticizers, the synthesized polymers with sulphonated groups,
are not covered here as they were not used in this work.
The fourth generation of plasticizers is based on a polyacrylate (PA) backbone that is
obtained by free radical polymerization of different vinyl monomers. This backbone
may vary widely in composition depending on the choice of monomers as shown in
Figure 2.7. The next step is to graft on side chains of polyether (polyethylene oxide).
Variations in the nature and relative proportions of the different monomers in the
copolymer yield a group of products having broad ranges of physico-chemical and
functional properties. Since some of the polyacrylates seem to enhance the
segregation tendencies, they are often combined with viscosifiers to counteract this
effect.
Figure 2.7 Illustration of a generic group of polyacrylate copolymers where R1 equals
H or CH3, R2 is a poly-ether side chain (e.g., polyethylene oxide) and X is a polar
(e.g., CN) or ionic (e.g., SO3) group. (Ramachandran, V.S. et al., [10], p.52)
2.4.3. Mechanisms of dispersion
There are generally two main mechanisms which explain how plasticizers disperse
particles in a suspension: electrostatic repulsion and steric hindrance. These two
mechanisms are sketched Figure 2.8 and Figure 2.9 respectively. Since its ionic lattice
is cut, any fractured mineral particle will have domains of positive and negative
charged sites. Negatively charged polymers (common feature of most plasticizers)
will absorb to the positive charged sites and render the total particle surface negatively
charged. As negatively charged particles approach each other there will be an
electrostatic repulsion preventing them from getting close and attach to form
18
Figure 2.8 Sketch of how negative charged polymers may adsorb to both positively
and negatively charged domains of particles. The resulting overall negative charge of
the particles will prevent them to form agglomerates by electrostatic repulsion and
they will stay dispersed. The electrostatic repulsion effect increases with increasing
charge density of the adsorbed molecule. (Ramachandran, V.S. et al, [10], p.200)
Figure 2.9 Sketch of branched macromolecules adsorbing on the surface of grains
that will create steric hindrance for them to get close enough to form agglomerates.
The size effect of steric hindrance increases with increasing molecular weight (or
actual size) of the adsorbed molecule. (Ramachandran, V.S. et al, [10], p.201)
agglomerates. The latest generation of grafted polymers may also have some negative
charges on their backbone that can co-ordinate on the positive sites but it should be
noted that the ester group of acrylates may co-ordinate strongly to calcium anyway
without any charge. The grafted polyether chains perpendicular to the backbone may
stretch out and hinder the particles to get close enough to form agglomerates. This
so-called steric hindrance is based on the size of the adsorbed molecules
perpendicular to the particle surface. This is shown in Figure 2.10.
19
Figure 2.10 Idealized model on how a grafted polymer will lead to steric hindrance
by adsorbing the polymer backbone to the surface and stretching the grafted side
chains into the water phase. (Justnes, H., [6], p. 26)
The model of the grafted polymer dispersing according to steric hindrance in Figure
2.10 may be a simplification. It would then be necessary for all the intermolecular
bonds (van der Waals type hydrogen bonds) to break and unwind the polyether chains
to let them stretch out into the water phase (even though the hydrophilic nature of
polyethers may aid in stabilizing such configuration). Alternatively, the molecules
may stay unwound as polymeric balls or “micelles” that equally well will lead to
steric hindrance (see Figure 2.11).
While the first three generations of plasticizers are said to rely on electrostatic
repulsion as mechanism for their dispersion of cement agglomerates, the fourth
generation is the first to be designed to function through steric hindrance.
Macromolecular micelles
Cement surface
Figure 2.11 Model of how macromolecules with strong intramolecular forces still
may disperse through steric hindrance as polymer “balls” or “micelles” (after Justnes,
H., [6], p. 26)
Another effect that will prevent agglomerates formation is called depletion as
sketched in Figure 2.12. The mechanism of this is that surplus polymer will not be
adsorbed and will stay in the water phase between the particles and for this reason
prevents them from getting close enough to form agglomerates.
cement
particle
s
20
cement
particle
s
polymer
Figure 2.12 Surplus polymer in the water phase (not adsorbed) may prevent the
cement particles to get close enough to form agglomerates. This depletion effect will
not disperse by itself, but rather help stabilize dispersions by preventing flocculation.
(after Justnes, H., [6], p. 27)
Rheology may also be improved by a tribology effect as sketched in Figure 2.13.
Tribology is the science of friction, abrasion and lubrication. Low molecular weight
compounds may reduce the friction between particles and also reduce the surface
tension of the water face.
cement
particle
s
cement
particle
s
Low molecular weight
compound
Figure 2.13 Low molecular compounds in the water phase may improve rheology of
particle suspensions by lubrication and by lowering the surface tension of the water
phase, which may be denoted as a tribology effect. (after Justnes, H., [6], p. 27)
Initial rheology of cement paste is also governed by early hydration, unlike inert
particles suspensions (e.g. limestone). Thus, there are other mechanisms of how
plasticizers may improve rheology of cement pastes. One is adsorption to active sites
21
and retardation of the formation of hydration products (see Figure 2.14), another is
changing the morphology of the hydration products formed by reducing growth (see
Figure 2.15) or by intercalation in the hydration products (see Figure 2.16).
Figure 2.14 Rheology in cement pastes may improve due to less hydration caused by
adsorbed polymers co-ordinating to active sites (■). The effect increases with
decreasing size of the molecules. LMW = low molecular weight and HMW = high
molecular weight. (Ramachandran, V.S. et al, [10], p.201)
Figure 2.15 Schematic illustration of hydration nucleation and growth inhibition by
adsorbed molecules. Selective adsorption on crystal planes can give morphology
changes. (Ramachandran, V.S. et al, [10], p.208)
22
Figure 2.16 Intercalation of plasticizer in hydration product with structural alteration
(e.g. lignosulphonates with hydration products of C3A). (Ramachandran, V.S. et al,
[10], p.209)
2.5 Calcium nitrate
This section is based on the paper Setting Accelerator Calcium Nitrate,
Fundamentals, Performance and Applications by Justnes, H. and Nygaard, E. ([11]).
In the past a growing concern about the chloride-induced corrosion of reinforcing bars
embedded in Portland cement concrete has led to the development of a number of
chloride-free set accelerating admixtures to replace the widely used calcium chloride
accelerator. In 1981, calcium nitrate, Ca(NO3)2, was proposed as a basic component
of a set accelerating admixture. Calcium nitrate, denoted as CN, works as a pure set
accelerator (see Figure 2.17), and not as a strength development accelerator. The pure
set accelerating effect is beneficial in preventing any increase in maximum
temperature in massive constructions due to the heat of hydration. In spite of this, an
increase in long term compressive strength is often observed, probably due to binder
morphology changes.
Hardening
Setting
Reference
Figure 2.17 Difference between set and hardening accelerators.
23
The effectiveness of CN as a setting accelerator for cement is dependent on the
cement type. The set accelerating efficiency appeared to be correlated with the belite,
C2S, content, while no correlation between set accelerating efficiency and C3A has
been found. In order to find the reason for the linear correlation between accelerator
efficiency and belite content, and possibly the mechanism of CN as set accelerator for
cement, Justnes and Nygaard undertook a thorough analysis of the water in cement
pastes from mixing to paste setting for two different cement types (HS65 and P30).
For
both
cement
pastes
the
most
noticeable
change
when
1.55 % CN by weight of the cement was added, was that the calcium concentration
increased and the sulphate concentration decreased. Thus, the mechanism for
accelerated setting is twofold:
i)
an increased calcium concentration leads to a faster super-saturation of the
fluid with respect to calcium hydroxide, Ca(OH)2, while
ii)
a lower sulphate concentration will lead to slower/less formation of ettringite
which will shorten the onset of aluminate, C3A, hydration.
The difference between the two cements was that P30 contained much more of the
mineral aphthitalite, K3Na(SO4)2, which leads to a high initial sulphate concentration
in the fluid. When CN was added, much of the calcium precipitated as sparingly
soluble gypsum. Even when 1.55 % CN was added to the P30 paste, the sulphate
concentration in the fluid was higher than in the water of HS65 paste without CN. At
the same time, the calcium concentration in the fluid of P30 with CN was only
slightly higher than for HS65 without CN. The Ca2+ concentration in the water of
HS65 paste, on the other hand, was increased with about 4 times when 1.55 % CN
was added. Thus, the reason why CN did not accelerate the setting of P30 was that it
contained a very soluble alkali sulphate originating from the clinker process.
The correlation between belite content and set accelerating efficiency is
understandable since belite can incorporate a portion of the total alkalies in its
structure and consequently prevent them from taking part in the early fluid chemistry
since belite is a slow reacting mineral. Hence, for a series of cements, with about
equal total alkali content and increasing belite content, it is expected that the set
accelerating efficiency of CN will increase. On the other hand, in an investigation of
calcium acetate, chloride and nitrate on belite hydration, it has been found that after 1
day, the chemically bound water was 6 times larger when 2 % CN was mixed in the
water, while 2 % calcium acetate and 2 % calcium chloride only increased the 1 day
chemically bound water by 30 % compared with the reference. Therefore, a special
influence of CN on β-C2S can not be excluded.
Chapter 3
Materials and apparatus
The purpose of this chapter is to introduce and describe the materials and the apparatus that
have been used frequently throughout this work.
3.1 Materials
3.1.1. Cements
Two Portland cements have been used in this thesis. Their physical characteristics are given
in Table 3.1, chemical analysis according to producer and minerals by Bogue estimation is
given in Table 3.2 and the mineralogy of the cements determined by multicomponent Rietveld
analyses of XRD profiles, specific surface determined by the Blaine method and content of
easily soluble alkalis determined by plasmaemissionspectrometry are given in Table 3.3.
Table 3.1 Physical characteristics of Portland cements according to EN 196
Cement type
Fineness:
Grains + 90 µm
Grains + 64 µm
Grains – 24 µm
Grains – 30 µm
Blaine (m2/kg)
Water demand
Le Chatelier
Initial set time
σc (MPa) at
1 day
2 days
7 days
28 days
CEM I
52.5 R - LA
CEM I
42.5 RR*
1.7%
4.1%
66.3%
75.6%
359
26.7%
0.5 mm
145 min.
0.1%
0.5%
89.2%
94.8%
546
32.0%
0 mm
115 min.
17.1
27.5
42.5
58.6
32.7
39.9
49.3
58.9
24
Chapter 3: Materials and apparatus
25
Table 3.2 Chemical analysis (%) of the Portland cements according to producer and minerals
(%) by Bogue estimation.
Cement
type
Chemical
analyses
CaO
SiO2
Al2O3
Fe2O3
SO3
MgO
Free CaO
K2O
Na2O
Equiv. Na2O
Cr6+ (ppm)
Carbon
Chloride
LOI
Fly Ash
Minerals
by Bogue
C3 S
C2 S
C3 A
C4AF
CS
CEM I
52.5 R - LA
CEM I
42.5 RR*
63.71
20.92
4.21
3.49
2.67
1.87
0.84
0.46
0.19
0.49
0.30
0.17
0.02
1.72
-
61.98
20.15
4.99
3.36
3.55
2.36
1.23
1.08
0.42
1.13
0.00
0.04
0.03
1.34
-
50.4
22.0
5.3
10.6
5.8
50.7
19.5
7.5
10.2
7.7
(* The RR term refers to the Norwegian standard NS 3086 (2003) where RR means extra
demands to 1 and 2 day strength compared to R. 42.5 RR should then have characteristic 1
day strength ≥ 20.0 MPa and 2 day strength ≥ 30.0 MPa.)
It can be seen that the CEM I 42.5 RR cement had a higher alkali and C3A content and a
higher specific surface than the CEM I 52.5 R LA cement and, as a consequence of the latter
two, had a higher water demand. CEM I 42.5 RR cement pastes were therefore prepared with
a w/c ratio of 0.50, whereas CEM I 52.5 R LA cement pastes were prepared with a w/c ratio
of 0.40 throughout this work.
26
Table 3.3 Mineral composition (%) and alkali content of Portland cements obtained by
QXRD and plasmaemissionspectrometry
Cement
type
Alite
Belite
Ferrite
Cubic
aluminate
Orthorombic
aluminate
Lime
Periclase
Gypsum
Hemihydrate
Anhydrite
Calcite
Portlandite
Quartz
Arcanite
Mullite
Amporhous
Blaine
K (%)
Na (%)
Naeqv (%)
CEM I
52.5 R - LA
65.0
12.9
9.6
0.5
CEM I
42.5 RR
64.7
14.8
7.5
5.9
3.0
1.1
0.6
0.3
1.4
1.5
0.4
4.0
0.3
0.4
0.0
364
0.32
0.74
0.26
1.0
1.6
0.0
1.8
0.6
0.5
0.3
0.0
0.3
546
0.92
0.22
0.76
3.1.2. Plasticizers/retarders
Borregaard Lignotech, Sarpsborg, Norway delivered two lignosulphonate powders denoted as
Ultrazine Na and Ultrazine Ca. Ultrazine Ca (CLS) was sugar reduced and large molecular
size enriched by ultra filtration of the basic calcium lignosulphonate obtained in the sulfite
process on spruce. In Ultrazine Na (NLS) the calcium in Ultrazine Ca has been ion exchanged
with sodium. Solutions with 30% dry matter were prepared before use.
A polyether grafted polyacrylate water solution containing 18% solids and a viscosifying
agent has also been used as a plasticizer. The molecular weight of the polyacrylate was
220,000.
A number of substances were used as retarders. They were all of analytical laboratory grade:
- citric acid (C6H8O7 ⋅ H2O )
- sodium salt of gluconic acid (C6H11NaO7)
- sodium salt of tartaric acid (Na2C4H4O6 ⋅ 2H2O, right-turning form)
- lead nitrate (Pb(NO3)2)
- zinc acetate (Zn(CH3OO)2 ⋅ 2H2O)
- sucrose (C12H22O11)
27
The trisodiumphosphate (Na3PO4 ⋅ 12H2O) used in this work was from technical quality.
Household sugar was also used as a retarder.
3.1.3. Accelerator
Technical calcium nitrate (CN) was used as an accelerator. Its formula may be written as
xNH4NO3 ⋅ yCa(NO3)2 ⋅ zH2O, and named xyz CN according to short hand practice. The CN
used in the present work had x = 0.092, y = 0.500 and z = 0.826, or in other words 19.00%
Ca2+, 1.57% NH +4 , 64.68% NO3- and 14.10% H2O. The CN was delivered in the form of
granules by Yara, Porsgrunn, Norway.
Calcium nitrate was also used in the form of a 50% aqueous solution of pure calcium nitrate
Ca(NO3)2, also obtained from Yara. The fluid is colourless, viscous and can easily be blended
into the mixing water.
3.2. Apparatus
3.2.1. Mixer
The cement pastes were blended in a high shear mixer by Braun (MR5550CA) and by Tefal
(Rondo 500) as illustrated in Figure 3.1. The mixers had a rotational speed of approximately
800 rpm. It will be notified which of the blenders has been used in each chapter. The blending
was performed by adding cement to the water and mixing for ½ minute, resting for 5 minutes
and blending again for 1 minute.
Figure 3.1 High shear blenders from Braun (left) and Tefal (right)
28
3.2.2. Rheometer
Rheological measurements have been performed with a MCR 300 rheometer produced by
Paar Physica (Figure 3.2). A parallel-plate measuring system was used as illustrated in Figure
3.3. This measuring system consisted of two plates. The surfaces of both the bob and the
motionless plate were flat, but the upper plate had a serrated surface of 150 µm depth to avoid
slippage.
Figure 3.2 MCR 300 rheometer by Paar Physica
Figure 3.3 The parallel plate measuring system (Mezger T., [12], p. 177)
The geometry of the upper plate is determined by the plate radius R being 2.5 cm. The
distance H between the two parallel plates must be much smaller than the radius R and has
been recommended to be at least 10 times larger than the largest of the particles of the sample
(Mezger T., [12], p. 177-179). The average particle size of unhydrated cement being
29
approximately 10 µm (Taylor, [13]), the gap between the plates was set to 1 mm for all
measurements. The temperature controlled bottom plate was set to 20° C.
The parallel plate measuring system makes it possible to measure dispersions containing
relatively large particles as well as samples with three-dimensional structures. The measuring
system has however also a number of disadvantages. There is no constant shear gradient in
the measurement gap because the shear rate (or shear deformation) increases in value from
zero at the center of the plate to the maximum at the edge. Furthermore, several unwanted
phenomena can occur at the edge of the plate: inhomogeneities, emptying of the gap, flowingoff and spreading of the sample, evaporation of water, or skin formation (Mezger T., [12], p.
180-181). To reduce evaporation both upper and lower plates were covered with a plastic ring
and a metallic lid while a water trap attached to the upper plate was filled with water to ensure
saturated water pressure.
The following measuring sequence was used to determine the flow resistance (area under the
(down) flow curve in the range from 2 to 50 1/s), the gel strength after 10 seconds of resting
and the gel strength after 10 minutes of resting:
1. 1 minute with constant shear rate ( γɺ ) of 100 1/s to stir up the paste
2. 1 minute resting
3. Stress ( τ ) – shear rate ( γɺ ) curve with linear sweep of γɺ from 2 up to 200 1/s in 30
points lasting 6 s each (up curve)
4. Stress ( τ ) – shear rate ( γɺ ) curve with linear sweep of γɺ from 200 down to 2 1/s in 30
points lasting 6 s each (down curve)
5. 10 s resting
6. Shear rate ( γɺ ) – stress ( τ ) curve with logarithmic sweep of τ from 1 to 100 Pa in 30
points lasting 6 s each to measure the gel strength after 10 s rest
7. 10 minutes resting
8. Shear rate ( γɺ ) – stress ( τ ) curve with logarithmic sweep of τ from 1 to 400 Pa in 70
points lasting 6 s each to measure the gel strength after 10 minutes rest
The recording of the shear rate ( γɺ ) – stress ( τ ) curves was stopped whenever the shear rate
( γɺ ) exceeded 300 1/s to prevent the sample from being lost from the measurement gap.
A flow chart of the mixing and measurement sequence is shown in Figure 3.4.
30
Shear rate
mixing
½ minute
mixing
1 minute
gel
strength
up
curve
gel
strength
down
curve
1 minute
at 100 1/s
transfer to
rheometer
5 minutes
rest
8 ½ minutes
1 minute
rest
10 seconds
rest
10 minutes
rest
Time
Figure 3.4 Flow chart of the mixing and measurement sequence
The reproducibility of the rheological measurements was investigated for two different
cement pastes. The cement pastes were made with distilled water. The plasticizer was added
to the water. Cement paste 1 was prepared with CEM I 52.5 R LA cement and 0.30% sodium
lignosulphonate by weight and a w/c ratio of 0.40. Paste 2 was prepared with CEM I 42.5 RR
cement and 0.50% sodium lignosulphonate by weight and a w/c ratio of 0.50. Total paste
volume was approximately 250 ml.
Each of the two cement pastes was prepared 5 times. The rheological data has been
transformed into flow resistance (area under the flow curve in the range from 2 to 50 1/s), gel
strength after 10 seconds of rest and gel strength after 10 minutes of rest. The results are
shown in Table 3.3 for cement paste 1 and Table 3.5 for paste 2.
The data show that the reproducibility of the flow resistance is reasonable. Measurements of
the gel strength show higher deviations, especially for the 10 minute gel strength of the CEM
I 52.5 R LA cement pastes which had a standard deviation of 27%.
31
Table 3.4 Reproducibility of rheological measurements for cement paste 1
(w/c=0.40 – CEM I 52.5 R LA – 0.30% Ultrazine Na)
PASTE 1
Average
Standard deviation
% standard dev.
Flow resistance
[Pa/s]
391
383
394
419
384
394
15
4%
Gel strength [Pa]
10 sec.
10 min.
2.4
14.2
2.4
13.0
2.8
9.2
2.8
10.0
2.8
7.1
2.7
10.7
0.2
3
9%
27%
Table 3.5 Reproducibility of rheological measurements for cement paste 2
(w/c=0.50 – CEM I 42.5 RR – 0.50% Ultrazine Na)
PASTE 2
Average
Standard deviation
% standard dev.
Flow resistance
[Pa/s]
2119
2375
2455
2343
2392
2337
128
5%
Gel strength [Pa]
10 sec.
10 min.
22.2
36.8
22.2
36.8
26.1
40.1
22.2
40.1
22.2
36.8
2.7
38.1
1.7
2
7%
5%
3.2.3. Calorimeter
An eight-channel TAM Air Isothermal Calorimeter from Thermometric AB, Sweden was
used for the heat of hydration measurements (Figure 3.5). The calorimeter was calibrated at
20° C. The hydration heat was measured by weighing 6 to 7 grams of cement paste into a
glass ampoule after which the ampoule was sealed and loaded into the calorimeter. The
ampoules were wiped with a paper tissue to make sure that they were perfectly clean and dry
when they were inserted into the calorimeter.
When studying the heat of hydration measurements it should be kept in mind that when an
ampoule is loaded into the calorimeter the temperature of the calorimeter will be disturbed. If
the temperature of the ampoule is 2 degrees higher than the thermostat temperature, an
exothermic heat flow, showing an exponential decay, of roughly 400 mW is observed. This
phenomenon explains the exponential decay in specific heat which is observed in the first
hour after mixing.
32
Figure 3.5 TAM Air Isothermal Calorimeter
3.2.4. Adsorption of plasticizers
To measure the consumed amount of lignosulfonate on the cement a UV Spectrophotometer
from Thermo Spectronic was used as illustrated in Figure 3.6. The adsorption measurements
in this work utilized a wavelength of 285 nm. Pore solutions were extracted from the cement
pastes by filtering the pastes through 0.45 µm filter paper on a Büchner funnel using low
vacuum 15 minutes after water addition. They were then diluted 25, 50 or 100 times with a
solution of ‘artificial pore water’ (NaOH and KOH with a K/Na molar ratio equal to 2 and pH
= 13.2). The amount of plasticizer in the water phase was read from calibration curves which
had been made with a dilution series of each of the two lignosulfonates being used in this
work. The difference between the added and the measured content of plasticizer gave the
bound portion.
Figure 3.6 UV Spectrophotometer from Thermo Spectronic
33
The consumption of polyacrylate on cement was determined by measuring Total Organic
Carbon (TOC) left in the pore water with a Shimadzu TOC Analyzer 5000A. The Shimadzu
TOC 5000A works by converting organic matter to carbon dioxide by combustion with a
catalyst that promotes the redox reaction with oxygen. The reaction takes place at a
temperature of 680° C. The amount of carbon dioxide formed is measured to determine the
carbon content. The amount of plasticizer bound to the cement is given by the difference
between the added and the measured content of organic carbon.
Chapter 4
Counteracting plasticizer retardation
4.1 Introduction
Plasticizers are used to increase flow for cementitious materials at equal water-to-cement
ratio, but will also to a variable extent retard cement setting as a side effect. The objective was
to find an accelerator that at least partially would counteract this retardation without
negatively affecting the rheology too much. Earlier papers (Justnes, H., Petersen, B.G., [14]
and [15]) focusing on this topic studied rheological properties at high shear rate (i.e. relevant
for mixing) for relatively low dosages of plasticizer, whereas the study reported in this
chapter focused on the lower shear rate range (i.e. relevant for pouring concrete) and higher
dosages of plasticizer. Three different plasticizers were tested in the present study, but the
accelerator was chosen to be calcium nitrate.
The experimental work is largely carried out on cement paste using a Physica MCR 300
rheometer to determine flow curves and gel strength and an isothermal calorimeter for
determination of heat of hydration curves.
Two promising admixture blends were also tried out in mortar.
34
Chapter 4: Counteracting plasticizer retardation
35
4.2 Calorimetric and rheological measurements
4.2.1. Experimental
The investigated cement pastes were made with distilled water. Plasticizer and accelerator
were added to the water before mixing, except for one series of pastes marked with DA
(delayed addition), where the plasticizer was added 5 minutes after the start of initial blending
in a 30% aqueous solution. Both a CEM I 52.5 R LA and a CEM I 42.5 RR Portland cement
were used. Three different plasticizers were studied: a sodium lignosulphonate (NLS), a
calcium lignosulphonate (CLS) and a polyether grafted polyacrylate (PA). The setting
accelerator calcium nitrate (CN), available in a 50% aqueous solution, was used to counteract
the retardation. A more detailed description of both plasticizers and accelerator can be found
in Chapter 3. Table 4.1 provides an overview of the experimental program.
Table 4.1 Experimental program
Cement type
CEM I 52.5 R LA
(w/c = 0.40)
Plasticizer
Accelerator
Reference (0%)
0.15% NLS*
0.15% NLS DA*
0.30% NLS
0.50% NLS
0.00% CN
0.30% CLS
0.25% CN
0.50% CLS
0.50% CN
0.75% CN
0.10% PA
1.00% CN
CEM I 42.5 RR
Reference (0%)
(w/c = 0.50)
0.50% NLS
1.00% NLS
0.50% CLS
1.00% CLS
0.10% PA
(* The 1.00% CN dosage was not studied for these series.)
In Chapter 3 it was pointed out that the CEM I 42.5 RR cement had a higher alkali and C3A
content and a higher specific surface than the CEM I 52.5 R LA cement and, as a consequence
of the latter two, had a higher water demand. CEM I 42.5 RR cement pastes were therefore
prepared with a w/c ratio of 0.50, whereas CEM I 52.5 R LA cement pastes were prepared
with a w/c ratio of 0.40 throughout this work. Total paste volume was approximately 250 ml.
The blending was performed in a high shear mixer of Braun (see 3.2.1) by adding the cement
to the water containing plasticizer and/or accelerator and mixing for ½ minute, resting for 5
36
minutes and blending again for 1 minute. The cement pastes containing 0.15% sodium
lignosulphonate were mixed with a high shear mixer by Tefal using the same blending
sequence.
The heat of hydration versus time curves were measured by accurately weighing 6 to 7 grams
of cement paste into a glass ampoule after which the ampoule was sealed and loaded into the
calorimeter.
The rheological properties were studied by performing the measurement sequence discussed
in section 3.2.2 on the cement pastes 15 minutes after the start of the blending:
To measure the consumed (adsorbed and intercalated) amount of plasticizer by cement, pore
solutions were extracted from the cement pastes by filtering the pastes through 0.45 µm filter
paper on a Büchner funnel using low vacuum 15 minutes after water addition.
The consumed amount of lignosulphonate was determined using a UV Spectrophotometer
from Thermo Spectronic. The adsorption measurements in this work utilized a wavelength of
285 nm. The pore solutions were diluted 25, 50 or 100 times with a solution of ‘artificial pore
water’ (NaOH and KOH with a K/Na molar ratio equal to 2 and pH = 13.5). The amount of
plasticizer in the water phase was read from calibration curves which had been made with a
dilution series of each of the two lignosulphonates being used in this work. The calibration
curves for NLS and CLS are given in Figure 4.1 and Figure 4.2 respectively. The difference
between the added and the measured content of plasticizer gave the consumed amount.
The consumption of polyacrylate by cement was determined by measuring Total Organic
Carbon (TOC) left in the pore water with a Shimadzu TOC Analyzer 5000A.
37
Calibration curve, NLS
0.9
0.8
y = 137.5844x
R2 = 0.9992
Absorbance
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
% Added
Figure 4.1 Calibration curve for adsorbance of sodium lignosulphonate (NLS).
Absorbance
Calibration curve, CLS
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
y = 137.2718x
R2 = 0.9997
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
% Added
Figure 4.2 Calibration curve for adsorbance of calcium lignosulphonate (CLS).
Prior to discussing the results, we shall provide an overview of the way in which the read outs
from the rheometer were converted into flow resistance (area under the flow curve in the
range from 2 to 50 1/s, see also Chapter 2), gel strength after 10 seconds of rest and gel
strength after 10 minutes of rest. The measurements on the cement paste made with
CEM I 52.5 R LA cement without any admixtures shall be used to illustrate this:
38
1. The flow resistance is defined as the area under the down flow curve in the range from
2 to 50 1/s. The down curve for the paste made with CEM I 52.5 R LA cement is
shown in Figure 4.3. Table 4.2 shows the read outs from the rheometer. The area
under the curve was determined by calculating the average of the shear stresses for
every two consecutive measuring points in the range from 2 to 50 1/s and multiplying
this by the difference in shear rate for these points. In this case a value of 2283 Pa/s
was found for the flow resistance.
Table 4.2 Rheometer read outs for the down curve.
Meas. Pt.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Shear Rate [1/s]
200
193
186
180
173
166
159
152
145
139
132
125
118
111
104
97.6
90.8
83.9
77.1
70.3
63.4
56.6
49.8
43.0
36.1
29.3
22.5
15.7
8.83
2.01
Shear Stress [Pa]
98.2
96.9
95.7
94.6
93.4
92.9
91.1
89.9
88.8
87.5
86.4
85.1
83.8
82.5
81.1
79.5
77.7
75.7
73.6
71.4
69.4
67.3
64.7
61.4
58.0
53.3
47.3
40.1
30.8
22.2
39
Down Curve
120
Shear Stress [Pa]
100
80
60
40
20
0
0
50
100
150
200
Shear Rate [1/s]
Figure 4.3 Down curve.
2. The 10 sec. gel strength can be derived from the shear rate ( γɺ ) – stress ( τ ) curve with
logarithmic sweep of τ from 1 to 100 Pa in 30 points lasting 6 s each. The curve is
plotted in Figure 4.4. The rheometer read outs are given in Table 4.3. The 10 sec. gel
strength was calculated by taking the average of the shear stresses of measuring points
19 and 20 (Table 4.3) as the breakthrough happened somewhere in between. That way
a value of 19 Pa was found for the 10 sec. gel strength.
10 sec. gel strength
180
160
Shear Rate [1/s]
140
120
100
gel strength
80
60
40
20
0
0
20
40
60
80
100
Shear Stress [Pa]
Figure 4.4 Shear rate – stress curve to determine the 10 sec. gel strength.
40
Table 4.3 Rheometer read outs to determine the 10 sec gel strength.
Meas. Pt.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Shear Rate [1/s]
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
3.44
9.32
12.9
16.0
21.3
26.1
35.0
48.7
73.6
110
155
Shear Stress [Pa]
1.00
1.17
1.37
1.61
1.89
2.21
2.59
3.04
3.56
4.18
4.89
5.74
6.72
7.88
9.24
10.8
12.7
14.9
17.4
20.4
24.0
28.1
32.9
38.6
45.2
53.0
62.1
72.8
85.3
100
3. The calculation of the 10 min. gel strength is completely similar to that of the 10 sec.
gel strength and shall therefore not be treated.
41
4.2.2. Results and discussion for reference pastes
Figure 4.5 shows the flow resistances for both CEM I 52.5 R LA and CEM I 42.5 RR
reference cement pastes. The flow resistance of the CEM I 42.5 RR cement paste (w/c = 0.50)
is higher than the CEM I 52.5 R LA paste (w/c = 0.40) in spite of the higher water-to-cement
ratio. This is due to the higher specific surface and the content of cubic C3A. Addition of
calcium nitrate appeared to have no effect on the flow resistance of these pastes.
reference
Flow resistance (Pa/s)
3500
CEM I 52.5 R LA
3000
CEM I 42.5 RR
2500
2000
1500
1000
500
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.5 Flow resistance for CEM I 52.5 R LA (w/c = 0.40) and CEM I 42.5 RR reference
cement pastes (w/c = 0.50) for different dosages of calcium nitrate.
The gel strengths after 10 seconds of rest are depicted in Figure 4.6. In case of CEM I 52.5 R
LA cement paste, an increasing 10 seconds gel strength was observed for increasing calcium
nitrate dosages up to 0.50%. Figure 4.7 shows the gel strengths after 10 minutes of rest. For
both cement types an increasing (albeit less pronounced in case of CEM I 42.5 RR cement)
gel strength can be seen for increasing calcium nitrate dosages.
42
reference
35
10 sec. gel strength (Pa)
CEM I 52.5 R LA
30
CEM I 42.5 RR
25
20
15
10
5
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.6 Gel strength after 10 seconds of rest for CEM I 52.5 R LA (w/c = 0.40) and
CEM I 42.5 RR reference cement pastes (w/c = 0.50) for different dosages of calcium nitrate.
reference
10 min. gel strength (Pa)
300
CEM I 52.5 R LA
CEM I 42.5 RR
250
200
150
100
50
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.7 Gel strength after 10 minutes of rest for CEM I 52.5 R LA (w/c = 0.40) and
CEM I 42.5 RR reference cement pastes (w/c = 0.50) for different dosages of calcium nitrate.
43
The heat of hydration curves are shown in Figure 4.8 and Figure 4.9. It can be seen that
calcium nitrate speeded up hydration with approximately two hours for both cement types.
The peak in the hydration curve for the pastes without calcium nitrate was seen at about 9
hours after water addition.
CEM I 52.5 R LA - w/c = 0.40 - reference
2.5
1.00 % CN
2
Rate of hydration heat (mW/g)
0.75 % CN
0.50 % CN
1.5
0.25 % CN
1
0.00 % CN
0.5
0
1
3
5
7
9
11
13
15
17
19
21
23
25
Time (hours)
Figure 4.8 Heat of hydration curves for CEM I 52.5 R LA cement pastes (w/c = 0.40) for
different dosages of calcium nitrate.
CEM I 42.5 RR - w/c = 0.50 - reference
4
3.5
1.00 % CN
3
0.75 % CN
2.5
2
1.5
0.00 % CN
0.25 % CN
1
0.50 % CN
0.5
0
1
3
5
7
9
11
13
15
17
19
Time (hours)
Figure 4.9 Heat of hydration curves for CEM I 42.5 RR cement pastes (w/c = 0.50) for
different dosages of calcium nitrate.
44
4.2.3. Results and discussion for sodium lignosulphonate
Flow resistances, gel strengths after 10 seconds and 10 minutes of rest measured on
CEM I 52.5 R LA cement pastes (w/c = 0.40) are listed in Table 4.4, Table 4.5 and Table 4.6,
respectively. Those measured on CEM I 42.5 RR cement pastes (w/c = 0.50) are listed in
Table 4.7, Table 4.8 and Table 4.9.
Table 4.4 Flow resistance (Pa/s) for CEM I 52.5 R LA cement paste (w/c=0.40).
Flow resistance
[Pa/s]
Reference
0.15% NLS
0.15% NLS DA
0.30% NLS
0.50% NLS
0.00
2283
1552
683
353
147
Calcium nitrate [%]
0.25
0.50
0.75
2253
2515
2418
1973
1815
2060
618
727
839
651
819
1030
287
528
671
1.00
2372
1201
881
Table 4.5 Gel strength after 10 seconds of rest (Pa) for CEM I 52.5 R LA cement paste
(w/c=0.40).
10 sec. gel
strength [Pa]
Reference
0.15% NLS
0.15% NLS DA
0.30% NLS
0.50% NLS
0.00
18.9
22.2
5.3
2.4
<1
Calcium nitrate [%]
0.25
0.50
0.75
22.2
30.5
30.5
35.8
30.5
35.8
3.9
4.5
6.2
4.5
6.2
8.6
3.3
6.2
7.3
1.00
30.5
13.1
10.0
Table 4.6 Gel strength after 10 minutes of rest (Pa) for CEM I 52.5 R LA cement paste
(w/c=0.40).
10 min. gel
strength [Pa]
Reference
0.15% NLS
0.15% NLS DA
0.30% NLS
0.50% NLS
0.00
73.6
52.1
20.1
7.1
3.9
Calcium nitrate [%]
0.25
0.50
0.75
104
114
161
87.6
95.6
35.6
20.1
26.0
30.9
15.5
30.9
52.1
11.9
36.8
47.7
1.00
271
67.5
73.6
45
Table 4.7 Flow resistance (Pa/s) for CEM I 42.5 RR cement paste (w/c=0.50).
Flow resistance
[Pa/s]
Reference
0.50% NLS
1.00% NLS
0.00
2788
2138
231
Calcium nitrate [%]
0.25
0.50
0.75
3161
2644
3099
2884
2614
2542
425
416
492
1.00
3160
2364
581
Table 4.8 Gel strength after 10 seconds of rest (Pa) for CEM I 42.5 RR cement paste
(w/c=0.50).
10 sec. gel
strength [Pa]
Reference
0.50% NLS
1.00% NLS
0.00
22.2
22.2
<1
Calcium nitrate [%]
0.25
0.50
0.75
30.5
22.2
26.1
30.5
26.1
30.5
8.6
10.0
10.0
1.00
26.1
26.1
13.1
Table 4.9 Gel strength after 10 minutes of rest (Pa) for CEM I 42.5 RR cement paste
(w/c=0.50).
10 min. gel
strength [Pa]
Reference
0.50% NLS
1.00% NLS
0.00
67.5
33.7
7.7
Calcium nitrate [%]
0.25
0.50
0.75
104
80.3
104
52.1
52.1
56.8
14.2
16.9
5.9
1.00
95.6
61.9
33.7
The flow resistances for CEM I 52.5 R LA cement pastes are also shown in Figure 4.10. In
case of the reference no significant influence of the addition of calcium nitrate on the flow
resistance could be measured. When sodium lignosulphonate (NLS) was added, however,
calcium nitrate had a clear increasing effect on the flow resistance as can be seen in Figure
4.10. The values found for the flow resistance are nevertheless still far below those of the
reference. From Figure 4.11, which shows the increase in flow resistance relative to the flow
resistance of the respective reference without calcium nitrate, it can be seen that the increasing
effect of calcium nitrate on the flow resistance became more pronounced when higher dosages
of sodium lignosulphonate were used. An interesting observation for the flow resistance was
that simply delayed addition of 0.15% sodium lignosulphonate makes it in excess of 50%
more effective as plasticizer than when it is added with the mixing water. This effect is
attributed to less intercalation of lignosulphonate in the early hydration products of cement,
leaving more lignosulphonate available to function as plasticizer through physical absorption
on the grain surface.
46
CEM I 52.5 R LA - w/c = 0.40
3000
Reference
0.15% NLS
0.15% NLS DA
0.30% NLS
0.50% NLS
2500
2000
1500
1000
500
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.10 Flow resistance for CEM I 52.5 R LA cement pastes (w/c = 0.40) for different
dosages of calcium nitrate.
CEM I 52.5 R LA - w/c = 0.40
700
Reference
0.15% NLS
0.15% NLS DA
0.30% NLS
0.50% NLS
Flow resistance (%)
600
500
400
300
200
100
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.11 Increase in flow resistance relative to the flow resistance of a reference without
calcium nitrate for CEM I 52.5 R LA cement pastes (w/c = 0.40) for different dosages of
calcium nitrate.
47
Figure 4.12 shows the flow resistances for CEM I 42.5 RR cement pastes. Neither clear
increasing nor decreasing effect of calcium nitrate on the flow resistance could be denoted in
case of the reference or in case of the pastes prepared with 0.50% sodium lignosulphonate.
The pastes prepared with 1.00% sodium lignosulphonate, however, again show the increasing
trend also observed for the CEM I 52.5 R LA pastes.
When comparing CEM I 42.5 RR cement paste (w/c = 0.50) and CEM I 52.5 R LA paste
(w/c = 0.40), one can see that higher dosages of plasticizer were required to achieve
comparable reductions in flow resistance in spite of the higher water-to-cement ratio. The
tendency of increasing flow resistance with increasing calcium nitrate dosage is less
pronounced for CEM I 42.5 RR than for CEM I 52.5 R LA paste. This may be associated with
calcium nitrate being a less effective accelerator for this cement compared to the other
according to the mineralogy: CEM I 42.5 RR cement has a lower belite and a higher alkali
content than CEM I 52.5 LA cement (see section 2.5 and Table 3.2).
As only a limited number of plasticizer concentrations were studied in case of CEM I 42.5 RR
and as the effect of 0.50% sodium lignosulphonate on the flow resistance was rather small, no
noteworthy conclusions can be drawn from Figure 4.13, which shows the increase in flow
resistance relative to the flow resistance of the respective reference without calcium nitrate.
CEM I 42.5 RR - w/c = 0.50
3500
Reference
0.50% NLS
3000
1.00% NLS
2500
2000
1500
1000
500
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.12 Flow resistance for CEM I 42.5 RR cement pastes (w/c = 0.50) for different
48
CEM I 42.5 RR - w/c = 0.50
300
Reference
0.50% NLS
Flow resistance (%)
250
1.00% NLS
200
150
100
50
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
calcium nitrate for CEM I 42.5 RR cement pastes (w/c = 0.50) for different dosages of
calcium nitrate.
Figure 4.14 shows the gel strengths after 10 seconds of rest for CEM I 52.5 R LA cement
pastes. An increasing effect of calcium nitrate on the gel strength can be seen. This increasing
effect on the gelling tendency may be beneficial in some cases since tendencies to segregation
will be reduced.
49
CEM I 52.5 R LA - w/c = 0.40
40
Reference
0.15% NLS
35
0.15% NLS DA
30
0.30% NLS
0.50% NLS
25
20
15
10
5
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.14 Gel strength after 10 seconds of rest for CEM I 52.5 R LA cement pastes (w/c =
0.40) for different dosages of calcium nitrate.
The gel strengths after 10 seconds of rest for CEM I 42.5 RR cement pastes are depicted in
Figure 4.15. Only in case of the pastes prepared with 1.00% sodium lignosulphonate a clear,
increasing, trend can be seen.
Figure 4.16 and Figure 4.17 show the gel strengths after 10 minutes of rest for
CEM I 52.5 R LA and CEM I 42.5 RR cement pastes, respectively. For all mixtures an
increasing effect of calcium nitrate on the 10 minutes gel strength was measured.
50
CEM I 42.5 RR - w/c = 0.50
35
Reference
0.50% NLS
30
1.00% NLS
25
20
15
10
5
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.15 Gel strength after 10 seconds of rest for CEM I 42.5 RR cement pastes (w/c =
CEM I 52.5 R LA - w/c = 0.40
300
Reference
0.15% NLS
0.15% NLS DA
0.30% NLS
0.50% NLS
250
200
150
100
50
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.16 Gel strength after 10 minutes of rest for CEM I 52.5 R LA cement pastes (w/c =
51
CEM I 42.5 RR - w/c = 0.50
120
Reference
0.50% NLS
100
1.00% NLS
80
60
40
20
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.17 Gel strength after 10 minutes of rest for CEM I 42.5 RR cement pastes (w/c =
The heat of hydration curves for CEM I 52.5 R LA cement pastes depicted in Figure 4.18,
Figure 4.19, Figure 4.20 and Figure 4.21 show that calcium nitrate is able to counteract the
retardation for the investigated dosages of sodium lignosulphonate. For lower dosages of
calcium nitrate the peak in the hydration curves not only shifted to earlier times but also got
higher. At higher dosages the peak reached a maximum and declined when the dosage of
calcium nitrate was further increased.
52
CEM I 52.5 R LA - w/c = 0.40 - 0.15% NLS
2.5
0.50 % CN
0.25 % CN
0.00 % CN
2
1.5
0.75 % CN
1
0.5
0
1
5
9
13
17
21
25
29
33
Time (hours)
Figure 4.18 Heat of hydration curves for CEM I 52.5 R LA cement pastes (w/c = 0.40) with
0.15% sodium lignosulphonate for different dosages of calcium nitrate.
CEM I 52.5 R LA - w/c = 0.40 - 0,15% NLS DA
2.5
0.50 % CN
0.25 % CN
0.00 % CN
2
1.5
0.75 % CN
1
0.5
0
1
5
9
13
17
21
25
29
33
Time (hours)
0.15% sodium lignosulphonate (delayed addition) for different dosages of calcium nitrate.
53
CEM I 52.5 R LA - w/c = 0.40 - 0.30% NLS
2.5
0.50 % CN
0.25 % CN
0.00 % CN
2
1.5
0.75 % CN
1.00 % CN
1
0.5
0
1
5
9
13
17
21
25
29
33
37
41
45
Time (hours)
CEM I 52.5 R LA - w/c = 0.40 - 0.50% NLS
3
0.25 % CN
0.50 % CN
0.75 % CN
2.5
0.00 % CN
1.00 % CN
2
1.5
1
0.5
0
1
5
9
13
17
21
25
29
33
37
41
45
Time (hours)
54
The heat of hydration curves for CEM I 42.5 RR cement pastes are shown in Figure 4.22 and
Figure 4.23. They also show that calcium nitrate is able to counteract retardation.
Furthermore, it can be observed that as more calcium nitrate was added the peak in the heat of
hydration curve became lower.
CEM I 42.5 RR - w/c = 0.50 - 0.50% NLS
4
0.00 % CN
3.5
0.25 % CN
0.50 % CN
3
2.5
2
0.75 % CN
1.5
1.00 % CN
1
0.5
0
1
5
9
13
17
21
25
29
33
Time (hours)
Figure 4.22 Heat of hydration curves for CEM I 42.5 RR cement pastes (w/c = 0.50) with
CEM I 42.5 RR - w/c = 0.50 - 1.00% NLS
4
0.00 % CN
0.25 % CN
3.5
0.50 % CN
0.75 % CN
3
1.00 % CN
2.5
2
1.5
1
0.5
0
1
5
9
13
17
21
25
29
33
Time (hours)
55
Figure 4.24 shows the consumed amounts (as a percentage of the added amount due to both
adsorption and intercalation) of sodium lignosulphonate for CEM I 52.5 R LA cement paste
for different dosages of calcium nitrate. One can observe that when the dosage of calcium
nitrate was increased more plasticizer was consumed and that higher dosages of plasticizer
resulted in lower plasticizer consumption. The increasing effect of calcium nitrate on
plasticizer consumption might be explained by the fact that addition of calcium nitrate results
CEM I 52.5 R LA - w/c = 0.40
Consumed NLS (% of added)
95
90
85
80
0.15% NLS
75
0.15% NLS DA
0.30% NLS
70
0.50% NLS
65
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.24 Consumed amounts (% of added) of NLS for CEM I 52.5 R LA cement paste
(w/c=0.40) for different dosages of calcium nitrate.
in increased hydration rate and increased adsorption of plasticizer due to the increased number
of adsorption sites. When we compare the plasticizer consumption for the pastes prepared
with 0.15% sodium lignosulphonate through immediate addition with those of the pastes
prepared with 0.15% sodium lignosulphonate through delayed addition, we can conclude that
delayed addition resulted in lower plasticizer consumption. Delayed addition, however,
appeared to result in much lower flow resistances. This observation brings us to the
conclusion that, in case of immediate addition, great amounts of sodium lignosulphonate
intercalated in the hydration products rather then being adsorbed on the hydration products
where they can fulfil their role as plasticizing agent. Comparison of the heat of hydration
curves depicted in Figure 4.18 and Figure 4.19, leads to the conclusion that delayed addition
has a more pronounced retarding effect on hydration than immediate addition. This
56
conclusion, in turn, might suggest that the retarding effect of lignosulphonates on cement
hydration is due to absorption on hydration products rather than to intercalation in the
hydration products.
The consumed amounts (as a percentage of the added amount) of sodium lignosulphonate for
CEM I 42.5 RR cement pastes are shown in Figure 4.25. Similar conclusions as for
CEM I 52.5 R LA pastes can be drawn: addition of calcium nitrate led to higher plasticizer
consumption and relatively less plasticizer was consumed as the plasticizer dosage was
increased.
CEM I 42.5 RR - w/c = 0.50
Consumed NLS (% of added)
85
80
75
70
0.50% NLS
65
1.00% NLS
60
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.25 Consumed amounts (% of added) of NLS for CEM I 42.5 RR cement paste
57
4.2.4. Results and discussion for calcium lignosulphonate
CEM I 52.5 R LA cement pastes (w/c = 0.40) are listed in Table 4.10, Table 4.11 and
Table 4.12, respectively. Those measured on CEM I 42.5 RR cement pastes (w/c = 0.50) are
listed in Table 4.13, Table 4.14 and Table 4.15.
Flow resistance
Calcium nitrate [%]
[Pa/s]
0.00
0.25
0.50
0.75
1.00
Reference
2283
2253
2515
2418
2372
0.30% CLS
543
687
914
1194
1391
0.50% CLS
194
438
618
908
1040
(w/c=0.40).
10 sec. gel
strength [Pa]
Reference
0.30% CLS
0.50% CLS
0.00
18.9
4.5
<1
Calcium nitrate [%]
0.25
0.50
0.75
22.2
30.5
30.5
6.2
8.6
11.8
5.3
6.2
10.0
1.00
30.5
16.2
11.8
(w/c=0.40).
10 min. gel
strength [Pa]
Reference
0.30% CLS
0.50% CLS
0.00
73.6
15.5
5.9
Calcium nitrate [%]
0.25
0.50
0.75
104
114
161
21.9
40.1
61.9
16.9
26.0
52.1
1.00
271
61.9
56.8
Flow resistance
[Pa/s]
Reference
0.50% CLS
1.00% CLS
0.00
2788
2530
206
Calcium nitrate [%]
0.25
0.50
0.75
3161
2644
3099
2742
2675
2754
481
435
450
1.00
3160
2884
602
58
(w/c=0.50).
10 sec. gel
strength [Pa]
Reference
0.50% CLS
1.00% CLS
0.00
22.2
26.1
<1
Calcium nitrate [%]
0.25
0.50
0.75
30.5
22.2
26.1
26.1
30.5
35.8
10.0
10.0
13.8
1.00
26.1
30.5
18.9
(w/c=0.50).
10 min. gel
strength [Pa]
Reference
0.50% CLS
1.00% CLS
0.00
67.5
43.7
5.9
Calcium nitrate [%]
0.25
0.50
0.75
104
80.3
104
47.7
61.9
73.6
20.1
15.5
26.0
1.00
95.6
80.3
40.1
Figure 4.26 shows the flow resistances for CEM I 52.5 R LA cement pastes. Figure 4.27
shows the increase in flow resistance relative to the flow resistance of the respective reference
without calcium nitrate. From these graphs it can be seen that the effect of calcium nitrate on
CEM I 52.5 R LA - w/c = 0.40
3000
Reference
0.30% CLS
2500
0.50% CLS
2000
1500
1000
500
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
59
CEM I 52.5 R LA - w/c = 0.40
600
Reference
0.30% CLS
0.50% CLS
Flow resistance (%)
500
400
300
200
100
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
calcium nitrate for CEM I 52.5 R LA cement pastes (w/c = 0.40) for different dosages of
calcium nitrate.
the flow resistance of CEM I 52.5 R LA cement pastes was similar to the one found for
sodium lignosulphonate: addition of calcium nitrate increased the flow resistance and this
increasing effect on the flow resistance became more pronounced when the dosage of calcium
lignosulphonate was increased. Figure 4.28 and Figure 4.29 show the flow resistance and the
relative increase in flow resistance for CEM I 42.5 RR cement paste. A 0.50% calcium
lignosulphonate dosage had only a relatively small plasticizing effect. In case of the 1.00%
dosage one can again observe the increasing effect of calcium nitrate on the flow resistance.
When comparing these results with those found for sodium lignosulphonate, one can see that
sodium lignosulphonate was a marginally better plasticizer than calcium lignosulphonate.
60
CEM I 42.5 RR - w/c = 0.50
3500
Reference
0.50% CLS
1.00% CLS
3000
2500
2000
1500
1000
500
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
CEM I 42.5 RR - w/c = 0.50
350
Reference
0.50% NLS
1.00% NLS
Flow resistance (%)
300
250
200
150
100
50
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
calcium nitrate for CEM I 42.5 RR cement pastes (w/c = 0.50) for different dosages of
calcium nitrate.
61
Figure 4.30, Figure 4.31, Figure 4.32 and Figure 4.33 show the gel strength after 10 seconds
and 10 minutes of rest for CEM I 52.5 R LA cement pastes and CEM I 42.5 RR cement
pastes. It can be seen that addition of calcium nitrate had an increasing effect on both 10
seconds and 10 minutes gel strength for pastes prepared with calcium lignosulphonate.
CEM I 52.5 R LA - w/c = 0.40
35
Reference
0.30% CLS
0.50% CLS
30
25
20
15
10
5
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.30 Gel strength after 10 seconds of rest for CEM I 52.5 R LA cement pastes
(w/c = 0.40) for different dosages of calcium nitrate.
CEM I 42.5 RR - w/c = 0.50
40
Reference
35
0.50% CLS
1.00% CLS
30
25
20
15
10
5
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.31 Gel strength after 10 seconds of rest for CEM I 42.5 RR cement pastes
62
CEM I 52.5 R LA - w/c = 0.40
300
Reference
0.30% CLS
0.50% CLS
250
200
150
100
50
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.32 Gel strength after 10 minutes of rest for CEM I 52.5 R LA cement pastes
CEM I 42.5 RR - w/c = 0.50
120
Reference
0.50% CLS
1.00% CLS
100
80
60
40
20
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.33 Gel strength after 10 minutes of rest for CEM I 42.5 RR cement pastes
The heat of hydration curves for CEM I 52.5 R LA cement pastes are depicted in Figure 4.34
and Figure 4.35. They clearly show that calcium nitrate is able to counteract the retardation
caused by calcium lignosulphonate. The hydration curves found for calcium lignosulphonate
differ only slightly from those found for sodium lignosulphonate (Figure 4.20 and
Figure 4.21).
63
CEM I 52.5 R LA - w/c = 0.40 - 0.30% CLS
2.5
0.00 % CN
2
0.25 % CN
0.50 % CN
1.5
1
0.75 % CN
1.00 % CN
0.5
0
1
5
9
13
17
21
25
29
33
37
41
45
Time (hours)
0.30% calcium lignosulphonate for different dosages of calcium nitrate.
CEM I 52.5 R LA - w/c = 0.40 - 0.50% CLS
3
0.50 % CN
0.75 % CN
0.25 % CN
2.5
1.00 % CN
0.00 % CN
2
1.5
1
0.5
0
1
5
9
13
17
21
25
29
33
37
41
45
Time (hours)
64
The heat of hydration curves for CEM I 42.5 RR cement pastes are given in Figure 4.36 and
Figure 4.37. They also show that calcium nitrate is able to counteract retardation. One can,
however, see that increasing the calcium nitrate dose from 0.75% tot 1.00% did not accelerate
the hydration reaction and even lowered the peak in the hydration curve. Except for the paste
prepared with 0.50% lignosulphonate and no calcium nitrate (calcium lignosulphonate
appeared to retard hydration much more than sodium lignosulphonate), the hydration curves
did not differ much from those found for sodium lignosulphonate (Figure 4.22 and
Figure 4.23).
CEM I 42.5 RR - w/c = 0.50 - 0.50% CLS
4
0.75 % CN
1.00 % CN
0.50 % CN
3.5
0.25 % CN
0.00 % CN
3
2.5
2
1.5
1
0.5
0
1
5
9
13
17
21
25
29
Time (hours)
65
CEM I 42.5 RR - w/c = 0.50 - 1.00% CLS
4
0.25 % CN 0.00 % CN
0.50 % CN
3.5
0.75 % CN
3
1.00 % CN
2.5
2
1.5
1
0.5
0
1
5
9
13
17
21
25
29
Time (hours)
Figure 4.38 and Figure 4.39 show the consumed amounts (as a percentage of the added
amount) of calcium lignosulphonate for CEM I 52.5 R LA and CEM I 42.5 RR cement pastes,
respectively. The conclusions that can be drawn are the same as those for sodium
lignosulphonate: addition of calcium nitrate led to higher plasticizer consumption and
relatively less plasticizer was consumed as the plasticizer dosage was increased.
66
CEM I 52.5 R LA - w/c = 0.40
Consumed CLS (% of added)
95
90
85
80
75
0.30% CLS
70
0.50% CLS
65
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.38 Consumed amounts (% of added) of CLS for CEM I 52.5 R LA cement paste
CEM I 42.5 RR - w/c = 0.50
Consumed CLS (% of added)
90
85
80
75
70
0.50% CLS
65
1.00% CLS
60
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.39 Consumed amounts (% of added) of CLS for CEM I 42.5 RR cement paste
67
4.2.5. Results and discussion for polyacrylate
CEM I 52.5 R LA cement pastes (w/c = 0.40) are listed in Table 4.16, Table 4.17 and Table
4.18, respectively. Those measured on CEM I 42.5 RR cement paste (w/c = 0.50) are listed in
Table 4.19, Table 4.20 and Table 4.21.
Flow resistance
Calcium nitrate [%]
[Pa/s]
0.00
0.25
0.50
0.75
1.00
Reference
2283
2253
2515
2418
2372
0.10% PA
98
121
206
273
313
(w/c=0.40).
10 sec. gel
strength [Pa]
Reference
0.10% PA
0.00
18.9
<1
Calcium nitrate [%]
0.25
0.50
0.75
22.2
30.5
30.5
<1
1.1
2.8
1.00
30.5
3.3
(w/c=0.40).
10 min. gel
strength [Pa]
Reference
0.10% PA
0.00
73.6
2.7
Calcium nitrate [%]
0.25
0.50
0.75
104
114
161
1.4
3.2
4.6
1.00
271
5.5
Flow resistance
[Pa/s]
Reference
0.10% PA
0.00
2788
1139
Calcium nitrate [%]
0.25
0.50
0.75
3161
2644
3099
1020
951
904
1.00
3160
886
(w/c=0.50).
10 sec. gel
strength [Pa]
Reference
0.10% PA
0.00
22.2
16.2
Calcium nitrate [%]
0.25
0.50
0.75
30.5
22.2
26.1
11.8
10.0
8.6
1.00
26.1
8.6
68
(w/c=0.50).
10 min. gel
strength [Pa]
Reference
0.10% PA
0.00
67.5
20.1
Calcium nitrate [%]
0.25
0.50
0.75
104
80.3
104
16.9
21.9
18.4
1.00
95.6
23.8
Figure 4.40 and Figure 4.41 show the flow resistances for CEM I 52.5 R LA and
CEM I 42.5 RR cement pastes, respectively. It can be seen that polyacrylate is a much more
effective plasticizer than NLS and CLS (in particular considering the dosage). The effect of
calcium nitrate on the plasticizing effect differed for the two cement types. In case of CEM I
52.5 R LA cement paste an increasing effect of calcium nitrate on the flow resistance can be
observed. In case of CEM I 42.5 RR cement paste, however, addition of calcium nitrate
appeared to have a decreasing effect on the flow resistance thereby improving workability.
CEM I 52.5 R LA - w/c = 0.40
3000
Reference
0.10% PA
2500
2000
1500
1000
500
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
69
CEM I 42.5 RR - w/c = 0.50
3500
Reference
0.10% PA
3000
2500
2000
1500
1000
500
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.42 and 4.43 show the gel strength after 10 seconds of rest for CEM I 52.5 R LA and
CEM I 42.5 RR cement pastes, respectively. Calcium nitrate had an increasing effect on the
10 seconds gel strength in case of CEM I 52.5 R LA cement paste and a decreasing effect in
case of CEM I 42.5 RR cement paste.
CEM I 52.5 R LA - w/c = 0.40
35
Reference
0.10% PA
30
25
20
15
10
5
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.42 Gel strength after 10 seconds of rest for CEM I 52.5 R LA cement pastes
70
CEM I 42.5 RR - w/c = 0.50
35
Reference
0.10% PA
30
25
20
15
10
5
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.43 Gel strength after 10 seconds of rest for CEM I 42.5 RR cement pastes
The 10 minutes gel strengths are depicted in Figure 4.44 and Figure 4.45. An increasing effect
of calcium nitrate on the gel strength can be observed in case of CEM I 52.5 R LA cement
paste. In case of CEM I 42.5 RR cement paste, the gel strength remained unaffected by the
addition of calcium nitrate. The tested polyacrylate led to less gelling than the
lignosulphonates.
CEM I 52.5 R LA - w/c = 0.40
300
Reference
0.10% PA
250
200
150
100
50
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.44 Gel strength after 10 minutes of rest for CEM I 52.5 R LA cement pastes
71
CEM I 42.5 RR - w/c = 0.50
120
Reference
0.10% PA
100
80
60
40
20
0
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.45 Gel strength after 10 minutes of rest for CEM I 42.5 RR cement pastes
The heat of hydration curves are given in Figure 4.46 and Figure 4.47. It can be seen that
polyacrylate retarded far less than the lignosulphonates and even resulted in higher
workability at much lower dosages. Still, especially in case of CEM I 52.5 R LA cement
paste, a considerable retarding effect can be observed. Calcium nitrate, however, was able to
counteract the retardation almost entirely.
72
CEM I 52.5 R LA - w/c = 0.40 - 0.10% PA
3
1.00 % CN
0.75 % CN
2.5
0.50 % CN
2
0.25 % CN
0.00 % CN
1.5
1
0.5
0
1
5
9
13
17
21
25
29
33
37
41
45
Time (hours)
0.10% polyacrylate for different dosages of calcium nitrate.
CEM I 42.5 RR - w/c = 0.50 - 0.10 % PA
4.5
0.00 % CN
4
0.25 % CN
3.5
0.50 % CN
3
0.75 % CN
1.00 % CN
2.5
2
1.5
1
0.5
0
1
4
7
10
13
16
Time (hours)
0.10% polyacrylate for different dosages of calcium nitrate.
73
Figure 4.48 shows the total organic content left in pore solutions extracted from the cement
pastes. When one doesn’t consider the fifth measurement in case of the CEM I 52.5 R LA, as
it probably might not be reproducible, both curves show a similar trend: when 0.25% calcium
nitrate was added, a small increase of the TOC was denoted; at higher dosages, calcium
nitrate appeared to have a decreasing effect on the TOC and thus an increasing effect on
plasticizer consumption.
Total Organic Carbon
700
TOC (mg C/l)
650
600
550
500
CEM I 52.5 R LA
450
CEM I 42.5 RR
400
0.00
0.25
0.50
0.75
1.00
Calcium nitrate (%)
Figure 4.48 Total organic carbon content left in pore solutions extracted from
CEM I 52.5 R LA cement pastes (w/c = 0.40) and CEM I 42.5 RR cement pastes (w/c = 0.50)
both containing 0.10% PA.
4.2.6. Conclusions
The general trends for the flow resistance are that the flow resistance decreases with
increasing dosage of lignosulphonates, NLS is a marginally better plasticizer than CLS, PA is
much more effective plasticizer than NLS and CLS and that the addition of calcium nitrate
increases the flow resistance to variable extent.
The general trends for the static gel strength are that gelling decreases with increasing dosages
of lignosulphonates, the tested PA leads to less gelling than the lignosulphonates and that
gelling tendency increases with increasing dosage of calcium nitrate.
74
The general trends for the heat of hydration curves are increased retardation of cement setting
with increasing dosage of lignosulphonate, PA retards far less than the lignosulphonates and
addition of calcium nitrate decreases retardation with increasing dosage.
The flow resistance of the CEM I 42.5 RR cement paste (w/c = 0.50) is higher than the CEM I
52.5 R LA paste (w/c = 0.40) in spite of the higher water-to-cement ratio. This is due to the
higher specific surface and the content of cubic C3A. When comparing CEM I 42.5 RR
cement paste (w/c = 0.50) and CEM I 52.5 R LA paste (w/c = 0.40), one can see that higher
dosages of plasticizer were required to achieve comparable reductions in flow resistance. The
according to the mineralogy: CEM I 42.5 RR cement has a lower belite and a higher alkali
content than CEM I 52.5 LA cement (see section 2.5 and Table 3.2).
75
4.3 Measurements on mortar
4.3.1. Experimental
Two admixture combinations were tried out on mortar. The first admixture blend consisted of
0.50% sodium lignosulphonate (NLS) and 0.75% calcium nitrate (CN), the second consisted
of 0.10% polyacrylate (PA) and 0.75% calcium nitrate. To investigate the effect of calcium
nitrate, references prepared with the same dosages of plasticizer but without calcium nitrate
were also made. Granulated calcium nitrate was used for the mortar mixes.
The mortars (w/c = 0.40) were prepared with CEM I 52.5 R LA cement and had a
cement:aggregate ratio of 1:3. The dmax of the aggregate was 8 mm. Total mortar volume was
approximately 5 liters.
The blending was performed in a Hobart mixer. Cement and aggregate were dry mixed for 1
minute at speed I. Then water containing plasticizer and/or accelerator was added while
mixing at speed I for 1 minute. Addition of water took place during the first 30 seconds. After
5 minutes of rest, the mortar was again blended at speed I for ½ minute to stir up any false
setting, followed by 1 ½ minute of blending at speed II.
For each mortar two 100x100x100 mm cubes were cast in a double 17 mm thick Styrofoam
mould with glass parts on two counterpart walls to give smooth surfaces for compressive
strength test. They were cured at 20°C in a climate room with a relative humidity of 60%. The
temperature of one of these was logged to monitor the rate of hydration in a semi-adiabatic
case resembling higher volumes in formwork in practice. The compressive strength of the
other cube was tested after 1 day of curing. The testing speed was 8 kN/s.
Twelve 40x40x160 mm prisms were cast in steel moulds for each of the mortars. Six mortar
prisms were cured at 5°C to determine 2 and 28 day strength at low temperature. The six
remaining prisms were cured at 20°C. Their strength was determined at 1 and 28 days. During
the first day the prisms were covered with wet clothes and plastic foil, where after they were
demoulded and placed in water baths.
The flexural strength of three of the prisms and the compressive strength on the resulting six
end pieces were tested at each terminus in accordance with EN 196-1. Testing speed was
20 N/s while determining flexural strength and 160 N/s while determining compressive
strength.
76
A smaller amount (based on 1 kg of cement) of each of the mortars was also prepared to
determine air content and slump. The air content was determined using an Air Content Tester
(see Figure 4.49) by FORM+TEST, Riedlingen, Germany which operates in accordance with
EN 459. The slump was determined 15 minutes after water addition by use of a mini-slump
cone (dimensions: upper φ: 40 mm, lower φ: 80 mm, height: 120 mm).
Figure 4.49 Air Content Tester by FORM+TEST, Riedlingen, Germany.
4.3.2. Results and discussion
Figure 4.50 shows the temperature profiles of the 100x100x100 mm cubes for the mortars
prepared with 0.50% sodium lignosulphonate. Addition of 0.75% calcium nitrate clearly
speeded up hydration as the peak in the hydration curve appeared about 12 hours earlier
compared to the mortar without calcium nitrate. The compressive strength measured on the
cube prepared with calcium nitrate was 15.4 MPa (see Table 4.22), whereas no strength could
be measured on the reference cube as it was too weak for demoulding.
The temperature profiles for the mortars prepared with 0.10% polyacrylate are depicted in
Figure 4.51. Again calcium nitrate, added in a 0.75% dosage, was able to counteract
retardation as the peak in hydration curves shifted about 3 hours to earlier times compared to
the reference mortar. As the area beneath both curves was more or less the same after 24
hours of curing, the difference in compressive strength was rather small (see Table 4.22). The
values for the compressive strength in case of the mortars prepared with 0.10% polyacrylate
were much higher than those of the mortars prepared with 0.50% sodium lignosulphonate as
the lignosulphonates had a much stronger retarding effect on cement hydration.
77
Table 4.22 Compressive strength after 1 day of curing at 20°C of insulated
100x100x100 mm mortar cubes.
Compressive strength (MPa)
CUBES
0.00% CN
0.75% CN
0.50% NLS
15.4
0.10% PA
37.8
38.8
Temperature (degrees Celsius)
CEM I 52.5 R LA - w/c = 0.40 - 0.50% NLS
45
NLS + CN
NLS
40
35
30
25
20
0
12
24
36
48
Time (hours)
Figure 4.50 Temperature (°C) profile of 100x100x100 mm cube cured at 20°C for CEM I
52.5 R LA mortar (w/c = 0.40) prepared with 0.50% sodium lignosulphonate and 0.75%
calcium nitrate and for CEM I 52.5 R LA mortar (w/c = 0.40) prepared with 0.50% sodium
lignosulphonate and no calcium nitrate.
CEM I 52.5 R LA - w/c = 0.40 - 0.10% PA
45
PA + CN
PA
40
35
30
25
20
0
6
12
18
24
Time (hours)
52.5 R LA mortar (w/c = 0.40) prepared with 0.10% polyacrylate and 0.75% calcium nitrate
and for CEM I 52.5 R LA mortar (w/c = 0.40) prepared with 0.10% polyacrylate and no
calcium nitrate.
78
Table 4.23 shows the compressive and flexural strength as measured on prisms after 1 and 28
days of curing at 20°C. In case of sodium lignosulphonate, no 1 day strength could be
measured for the mortar prisms without calcium nitrate as they broke upon demoulding. The
prisms containing 0.75% calcium nitrate however gain sufficient strength for removal of
formwork in practice after 1 day. After 28 days of curing there was no significant difference
in strength between the prisms with calcium nitrate and those without calcium nitrate. The
differences in 1 day strength for mortar prisms prepared with 0.10% polyacrylate are less
pronounced. The 28 day strength of the prisms prepared with 0.10% polyacrylate appeared to
be significantly higher than those of the prisms prepared with 0.50% sodium lignosulphonate.
Similar conclusions can be drawn for the prisms cured at 5°C (see Table 4.24). The difference
in 2 day strength for the prisms prepared with 0.10% polyacrylate, however, was much more
pronounced compared to the prisms cured at 20°C at 1 day as it was more than doubled by
calcium nitrate and raised to a level where it is considered frost resistant (> 5 MPa) after 2
days when 0.75% was added.
Table 4.23 Compressive strength (MPa) (upper) and flexural strength (MPa) (lower) for 1:3
mortar prisms cured at 20°C.
PRISMS
0.50% NLS
0.50% NLS + 0.75% CN
0.10% PA
0.10% PA + 0.75% CN
Strength (MPa)
1 day
28 days
52 ± 3
6.7 ± 0.2
8.1 ± 0.3
50 ± 2
2.04 ± 0.03
6.75 ± 0.04
27.4 ± 0.5
69 ± 2
4.9 ± 0.5
8.2 ± 0.3
29.5 ± 0.5
72.9 ± 0.6
5.4 ± 0.3
8.4 ± 0.1
mortar prisms cured at 5°C.
PRISMS
0.50% NLS
0.50% NLS + 0.75% CN
0.10% PA
0.10% PA + 0.75% CN
Strength (MPa)
2 day
28 days
55 ± 1
6.6 ± 0.5
1.8 ± 0.1
47 ± 2
0.64 ± 0.03
5.9 ± 0.2
2.4 ± 0.2
70 ± 3
0.74 ± 0.07
7.9 ± 0.2
5.5 ± 0.3
73 ± 2
1.49 ± 0.06
8.3 ± 0.6
79
Table 4.25 shows the air content and the density of the four mortars. It can be seen that the
mortars prepared with 0.50% sodium lignosulphonate contained much more air than those
prepared with 0.10% polyacrylate. This is caused by the lignosulphonates being pure factory
products rather than ready commercial products usually added a defoaming agent and
explains the large difference in 28 day strength between the mortar prisms prepared with
0.50% sodium lignosulphonate and 0.10% polyacrylate as the measurements comply well
with the rule of thumb of 5% reduced strength per volume percent of air (in this case around
30% reduction).
The mini-slump measured on the mortars 15 minutes after water addition are given in
Table 4.26. In case of 0.50% sodium lignosulphonate a much lower value for the slump was
found when 0.75% calcium nitrate was added. However, in case of 0.10% polyacrylate,
addition of 0.75% calcium nitrate had no effect on the slump. These figures indicate that it is
difficult to correlate slump tests made on mortar with earlier measurements (flow resistance,
gel strength) made on cement paste.
Table 4.25 Air content (%) and density (kg/dm3) of the mortars.
Air content (%)
MORTAR
Density (kg/dm3)
0.00% CN
0.75% CN
12.4 %
11.3 %
0.50% NLS
2.11 kg/dm3
2.16 kg/dm3
5.8 %
5.6 %
0.10% PA
2.31 kg/dm3
2.31 kg/dm3
Table 4.26 Mini-slump (mm) of the mortars, measured 15 minutes after water addition.
Mini-slump (mm)
MORTAR
0.00% CN
0.75% CN
71 mm
25 mm
0.50% NLS
18
mm
18 mm
0.10% PA
4.3.3. Conclusions
The strength data of CEM I 52.5 R LA mortars showed that both sodium lignosulphonate and
polyacrylate delayed hydration considerably at 5°C and that calcium nitrate to a certain extent
was able to counteract that. Mortar with only 0.50% sodium lignosulphonate had no strength
after 1 day at 20°C and 2 days 5°C, but gained sufficient strength for removal of formwork in
practice after 1 day at 20°C when 0.75% calcium nitrate was included and even some strength
after 2 days at 5°C. The compressive strength of mortar with 0.10% polyacrylate was raised to
a level where it is considered frost resistant after 2 days when 0.75% calcium nitrate was
included (calcium nitrate more than doubled the compressive strength).
80
4.4 General conclusion
The following trends have been observed for the investigation of flow resistance, static gel
strength and the heat of hydration curves for pastes based on two Portland cements plasticized
with sodium and calcium lignosulphonates (NLS and CLS) in dosages ranging from 0.15% to
1.00%, as well as polyether grafted polyacrylate (PA) in a 0.10% dosage. The setting
accelerator calcium nitrate was added in dosages ranging from 0.00% to 1.00%.
The general trends for the gel strength are that gelling decreases with increasing dosages for
lignosulphonates, the tested PA leads to less gelling than the lignosulphonates and that gelling
tendency increases with increasing dosage of calcium nitrate.
The flow resistance of the CEM I 42.5 RR cement paste (w/c = 0.50) is higher than the CEM I
52.5 R LA paste (w/c = 0.40) in spite of the higher water-to-cement ratio. This is due to the
higher specific surface and the content of cubic C3A. When comparing CEM I 42.5 RR
cement paste (w/c = 0.50) and CEM I 52.5 R LA paste (w/c = 0.40), one can see that higher
dosages of plasticizer were required to achieve comparable reductions in flow resistance. The
according to the mineralogy.
Two admixture blends were also tried out in mortar. The strength data of CEM I 52.5 R LA
mortars plasticized by 0.50% NLS and 0.10% PA showed that both NLS and PA delayed
hydration considerably at 5°C and that calcium nitrate to a certain extent was able to
counteract that. Mortar with only 0.50% NLS had no strength after 1 day at 20°C and 2 days
at 5°C, but gained sufficient strength for removal of formwork in practice after 1 day at 20°C
when 0.75% calcium nitrate was included and even some strength after 2 days at 5°C. The
compressive strength of mortar with 0.10% PA was raised to a level where it is considered
frost resistant after 2 days when 0.75% calcium nitrate was included (calcium nitrate more
than doubled the compressive strength).
Chapter 5
Long transport of fresh concrete
5.1 Introduction
This chapter discusses long transport of fresh concrete. The preliminary study was largely
carried out on paste. It was investigated if a concrete mix from a ready mix plant after being
deliberately over-retarded for long transport in for instance hot climate or cities with
unpredictable traffic (e.g. traffic jam) could be activated by adding an accelerator in the
revolving drum close to the construction site before pumping the concrete in place. Four
different retarders/plasticizers were studied: sodium lignosulphonate (section 5.2), citric acid
(section 5.3), lead nitrate (section 5.4) and sodium gluconate (section 5.5). The accelerator
was chosen to be calcium nitrate.
5.2 Sodium lignosulphonate
5.2.1. Introduction
In this section it was investigated if the retardation of sodium lignosulphonate on the setting
of Portland cement, which has already been studied in the previous chapter, can be
counteracted by a delayed addition of calcium nitrate. Based on results from the previous
chapter, a sodium lignosulphonate dosage which retards setting for at least 8 hours was
chosen for each of the two studied cement types. Calcium nitrate was then added 2, 4, 6 and 8
hours after the start of the initial blending after which both calorimetric and rheological
measurements were performed on the cement pastes. Strength measurements on mortar were
also carried out.
81
Chapter 5: Long transport of fresh concrete
82
5.2.2. Experimental
The cement pastes were made with distilled water. Sodium lignosulphonate (NLS) was added
to the water before mixing. Both CEM I 52.5 R LA and CEM I 42.5 RR Portland cement
were used. Cement pastes prepared with CEM I 52.5 R LA cement had a w/c ratio of 0.40 and
a NLS dosage of 0.40% by weight. Those prepared with CEM I 42.5 RR cement had a w/c
ratio of 0.50 and a NLS dosage of 1.00 % by weight. Total paste volume was approximately
450 ml.
The blending was performed in a high shear mixer of Braun by adding the cement to the
water containing NLS and mixing for ½ minute, resting for 5 minutes and blending again for
1 minute.
The pastes were then each poured into a 500 ml glass beaker, covered with a plastic sheet to
avoid evaporation of water and kept at laboratory conditions at a temperature of
approximately 22°C. Every 2 hours approximately 100 ml of paste was sampled out of the
beaker after it was mixed up again for 1 minute using the hand blender end-piece of the Braun
mixer.
A 0.50% or a 1.00% dosage of calcium nitrate in the form of granules was then added to the
sample after which blending was performed by mixing for ½ minute, resting for 5 minutes
and mixing again for 1 minute in the Braun mixer. A reference, without calcium nitrate, was
also studied for each of the two cement types. In case of the references a sample was also
taken and investigated immediately after the initial blending.
The hydration heat was measured by accurately weighing 6 to 7 grams of cement paste into a
glass ampoule after which the ampoule was sealed and loaded into the calorimeter.
The rheological properties (flow resistance, gel strength after 10 seconds and 10 minutes of
rest) were studied 15 minutes after the addition of calcium nitrate (measurement sequence is
given in Chapter 3).
The adsorbed amount of plasticizer on the cement was also determined.
Mortar (w/c = 0.40) was prepared with CEM I 52.5 R LA cement and a cement:aggregate
ratio of 1:3. The dmax of the aggregate was 8 mm. The mortar was plasticized by adding
0.40% sodium lignosulphonate to the water in a 30% aqueous solution. Total mortal volume
was approximately 5 liters.
83
minute at speed I. Then water, containing plasticizer, was added while mixing at speed I for 1
minute. Addition of water took place during the first 30 seconds. After 5 minutes of rest, the
mortar was again blended at speed I for ½ minute to stir up any false setting, followed by 1 ½
minute of blending at speed II.
Fifteen minutes after water addition the slump was determined using a mini-slump cone after
which the mortar was then sealed and kept at a constant temperature of 20°C for about 4
hours. The slump was also determined 2 hours and 15 minutes after water addition and 15
minutes before activation after mixing the mortar for the 1 minute at speed I.
After 4 hours, the mortar was mixed for 1 minute at speed I after which six 40x40x160 mm
prisms and one 100×100×100 mm cube were cast to serve as retarded, but not activated
references. Then, 1.00% calcium nitrate was added in a 50% aqueous solution while mixing at
speed I for 1 minute. Addition of calcium nitrate took place during the first 30 seconds. After
5 minutes of rest, the mortar was again blended at speed I for ½ minute, followed by 1 ½
minute of blending at speed II. Again, six 40x40x160 mm prisms and one 100×100×100 mm
cube were cast. The slump of the mortar was determined 15 minutes after activation.
The 100×100×100 mm cubes were cast in a 17 mm thick Styrofoam mould with glass parts on
two counterpart walls to give smooth surfaces for compressive strength test. They were cured
at 20°C in a climate room with a relative humidity of 60%. The temperature of these cubes
was logged to monitor the rate of hydration in a semi-adiabatic case resembling higher
volumes in formwork in practice. The cubes were used to determine the compressive strength
after 1 day. The testing speed was 8 kN/s.
The 40×40×160 mm prisms were cast in steel moulds. The prisms were cured at 20°C and
60% relative humidity. Their strength was determined after 1 and 28 days of curing. During
the first day the prisms were covered with wet clothes and plastic foil, where after they were
demoulded and placed in water baths.
strength.
84
Figure 5.1, 5.2, 5.3 and 5.4 show the heat of hydration curves for CEM I 52.5 R LA cement
pastes after addition of 0.50% and 1.00% calcium nitrate 2, 4, 6 and 8 hours after initial
mixing respectively. The reference is also shown in each graph.
CEM I 52.5 R LA - w/c=0.40 - 0.40% NLS - 2 hours after initial mixing
2.5
1.00% CN 0.50% CN Ref.
2
1.5
1
0.5
0
1
5
9
13
17
21
25
29
33
37
41
45
Tim e (hours)
Figure 5.1 Heat of hydration curves for CEM I 52.5 R LA cement paste (w/c=0.40) with
0.40% NLS. 0.50% and 1.00% dosages of calcium nitrate were added 2 hours after initial
mixing. The reference is also shown.
2.5
1.00% CN 0.50% CN Ref.
2
1.5
1
0.5
0
1
5
9
13
17
21
25
29
33
37
41
45
Time (hours)
85
2.5
1.00% CN 0.50% CN Ref.
2
1.5
1
0.5
0
1
5
9
13
17
21
25
29
33
37
41
45
Time (hours)
2.5
1.00% CN
0.50% CN
Ref.
2
1.5
1
0.5
0
1
5
9
13
17
21
25
29
33
37
41
45
Time (hours)
86
The heat of hydration curves for CEM I 52.5 R LA cement paste show that a 0.40% dosage of
NLS was effective in retarding hydration to around 8 hours after mixing. The graphs also
show that calcium nitrate had an accelerating effect on hydration independently of the time of
addition. However, the later the time of addition, the less pronounced the accelerating effect
tended to be. Furthermore, the 1.00% calcium nitrate dosage appeared to be more effective in
counteracting the retardation than the 0.50% dosage.
Figure 5.5, 5.6, 5.7 and 5.8 show the heat of hydration curves for CEM I 42.5 RR cement
pastes after addition of 0.50% and 1.00% calcium nitrate 2, 4, 6 and 8 hours after initial
mixing respectively. The reference is also shown in each graph.
CEM I 42.5 RR - w/c=0.50 - 1.00% NLS - 2 hours after initial mixing
4.5
Ref.
4
0.50% CN
3.5
3
1.00% CN
2.5
2
1.5
1
0.5
0
1
5
9
13
17
21
25
29
33
37
41
45
Time (hours)
Figure 5.5 Heat of hydration curves for CEM I 42.5 RR cement paste (w/c=0.50) with 1.00%
NLS. 0.50% and 1.00% dosages of calcium nitrate were added 2 hours after initial mixing.
The reference is also shown.
87
4.5
Ref.
4
0.50% CN
3.5
1.00% CN
3
2.5
2
1.5
1
0.5
0
1
5
9
13
17
21
25
29
33
37
41
45
Time (hours)
4.5
0.50% CN
4
Ref.
1.00% CN
3.5
3
2.5
2
1.5
1
0.5
0
1
5
9
13
17
21
25
29
33
37
41
45
Time (hours)
88
CEM I 42.5 RR- w/c=0.50 - 1.00% NLS - 8 hours after initial mixing
4.5
0.50% CN
4
Ref.
1.00% CN
3.5
3
2.5
2
1.5
1
0.5
0
1
5
9
13
17
21
25
29
33
37
41
45
Time (hours)
It can be seen from the heat of hydration curves for CEM I 42.5 RR cement paste that a
1.00% dosage of NLS was able to retard hydration to around 8 hours after mixing. The graphs
also show that calcium nitrate had an accelerating effect on hydration independently of the
time of addition and that the later the time of addition the less pronounced the accelerating
effect tended to be. The 1.00% calcium nitrate dosage did not appear to be significantly more
effective in counteracting the retardation than the 0.50% dosage for the following reasons:
- the maxima in the hydration curves occurred around the same time
- the maxima for the 0.50% dosages was higher than for the 1.00% dosage
- the area under the hydration curve (usually correlated with strength development)
appeared to be smaller in case of the 1.00% calcium nitrate dosage compared to both
the reference and the 0.50% dosage.
The rheological data has been transformed into flow resistance, gel strength after 10 seconds
and after 10 minutes of rest. Table 5.1 shows the flow resistance, Table 5.2 the 10 seconds gel
strength and Table 5.3 the 10 minutes gel strength for CEM I 52.5 R LA cement pastes.
89
Table 5.1 Flow resistance (Pa/s) for CEM I 52.5 R LA cement paste (w/c=0.40) with 0.40%
NLS. 0.50% and 1.00% dosages of calcium nitrate were added 2, 4, 6 or 8 hours after initial
mixing. The results for the reference are also given.
Flow resistance
[Pa/s]
0.00% CN
0.50% CN
1.00% CN
0h
274
2h
330
664
832
Time of addition
4h
558
785
1019
6h
854
1060
1349
8h
1223
1385
1695
(w/c=0.40) with 0.40% NLS. 0.50% and 1.00% dosages of calcium nitrate were added 2, 4, 6
or 8 hours after initial mixing. The results for the reference are also given.
10 sec. gel
strength [Pa]
0.00% CN
0.50% CN
1.00% CN
0h
<1
2h
4.5
8.6
10
Time of addition
4h
10
10
16
6h
19
22
26
8h
22
22
26
10 min. gel
strength [Pa]
0.00% CN
0.50% CN
1.00% CN
0h
6.5
2h
3.8
40
34
Time of addition
4h
11
26
37
6h
24
52
62
8h
37
44
74
Looking at the reference, one can see a gradual increase in flow resistance and gel strength
over time. However, even after 8 hours, there was still a considerable plasticizing effect as the
value of 1223 Pa/s for the flow resistance after 8 hours is still much lower than the flow
resistance of a CEM I 52.5 R LA cement paste without any admixtures immediately after
mixing being around 2300 Pa/s (see Chapter 4).
The addition of calcium nitrate had an increasing effect on both flow resistance and gel
strength resulting in a loss of workability. Nevertheless, the measured flow resistances were
again still far below the value of 2300 Pa/s associated with the flow resistance of a CEM I
52.5 R LA cement paste without any admixtures measured immediately after mixing.
90
Table 5.4 shows the flow resistance, Table 5.5 the 10 seconds gel strength and Table 5.6 the
10 minutes gel strength for CEM I 42.5 RR cement pastes.
Table 5.4 Flow resistance (Pa/s) for CEM I 42.5 RR cement paste (w/c=0.50) with 1.00%
NLS. 0.50% and 1.00% dosages of calcium nitrate were added 2, 4, 6 or 8 hours after initial
mixing. The results for the reference are also given.
Flow resistance
[Pa/s]
0.00% CN
0.50% CN
1.00% CN
0h
252
2h
145
130
150
Time of addition
4h
132
142
165
6h
167
174
188
8h
203
220
257
10 sec. gel
strength [Pa]
0.00% CN
0.50% CN
1.00% CN
0h
<1
2h
<1
<1
<1
Time of addition
4h
<1
<1
1.1
6h
<1
1.1
4.5
8h
4.5
6.2
8.6
10 min. gel
strength [Pa]
0.00% CN
0.50% CN
1.00% CN
0h
6.5
2h
1.9
2.7
5.0
Time of addition
4h
3.5
3.8
4.6
6h
5.0
5.9
7.7
8h
6.5
9.2
11
The reference shows a decline in flow resistance (i.e. an increase in workability) up till 4
hours after water addition followed by a gradual increase. After 8 hours, however, the flow
resistance is still lower than immediately after water addition, and thus, considerably lower
than the value of 2788 Pa/s (see Chapter 4) associated with the flow resistance of a CEM I
42.5 RR cement paste without any admixtures measured immediately after mixing (i.e. there
is still a good plasticizing effect 8 hours after water addition). A trend can be denoted when
looking at the gel strengths after 10 minutes of rest, where a decline was measured up till 2
hours after water addition.
91
The addition of calcium nitrate had an increasing effect on both flow resistance and gel
strength resulting in a loss of workability. Nevertheless, the measured flow resistances were
again still far below the value of 2788 Pa/s associated with unplasticized CEM I 42.5 RR
cement paste immediately after mixing.
Figure 5.9 and Figure 5.10 show the consumed amounts of sodium lignosulphonate based on
the absorption measurements for CEM I 52.5 R LA and CEM I 42.5 RR cement paste
respectively.
CEM I 52.5 R LA - w/c=0.40 - 0.40% NLS
Consumed LNS (% of added)
95
90
85
80
Ref.
0.50% CN
75
1.00% CN
70
0
1
2
3
4
5
6
7
8
Time of addition (hours)
Figure 5.9 Consumed amounts (% of added) of NLS for CEM I 52.5 R LA cement paste
or 8 hours after initial mixing. The results for the reference are also shown.
92
CEM I 42.5 RR - w/c=0.50 - 1.00% NLS
80
Consumed LNS (% of added)
78
76
74
72
70
68
66
Ref.
64
0.50% CN
62
1.00% CN
60
0
1
2
3
4
5
6
7
8
Time of addition (hours)
Figure 5.10 Consumed amounts (% of added) of NLS for CEM I 42.5 RR cement paste
or 8 hours after initial mixing. The results for the reference are also shown.
It can be seen from Figure 5.9 and Figure 5.10 that as time went on more sodium
lignosulphonate was consumed. The measurements also show that addition of calcium nitrate
had an increasing effect on the consumed amount of NLS. An increased NLS consumption,
however, was not always matched by a similar increase in workability. This might be
explained by the fact that, as time went on, more hydration products were formed, resulting in
a loss of workability due to intercalation of NLS in the hydration products rather then being
absorbed on the hydration products where they can fulfil their role as plasticizing agent. The
increasing effect of calcium nitrate on plasticizer consumption might be explained by the fact
that addition of calcium nitrate results in increased hydration rate and increased plasticizer
consumption due to both increased intercalation and increased adsorption of plasticizer (due
to the increased number of adsorption sites).
93
Figure 5.11 shows the temperature profiles of the 100x100x100 mm cubes for CEM I 52.5 R
LA cement mortar. It can be seen that 0.40% NLS was able to retard hydration for at least 8
hours and that addition of 1.00% calcium nitrate was able to initiate hydration. The
compressive strength (see Table 5.7) measured 1 day after water addition on the activated
mortar cubes was 21 MPa, whereas the compressive strength of the reference cube was only
9.2 MPa.
CEM I 52.5 R LA - w/c = 0.40 - 0.40% NLS
45
NLS + CN
NLS
40
35
30
25
20
0
6
12
18
24
Time after water addition (hours)
52.5 R LA mortar (w/c = 0.40).
Table 5.7 Compressive strength (MPa) for 1:3 mortar cubes for a reference and an activated
mortar.
CUBES
Reference
Activated
1 day after water addition
9.2
21
Table 5.8 shows the compressive and flexural strength as measured on 1 day and 28 days old
mortar prisms cured at 20°C. The measurements again show that calcium nitrate was able to
speed up hydration as a 250% higher 1 day compressive strength was found for the activated
mortar prisms. The 28 days compressive strength of the activated mortar prisms was about
25% higher. This might be attributed to lower binder porosity (only observable by gentle
drying techniques) caused by calcium nitrate as seen before in mixes containing calcium
nitrate (Justnes, H., [16]).
94
mortar prisms for a reference and an activated mortar cured at 20°C.
PRISMS
Reference
Activated
Strength (MPa)
1 day
28 days
4.0 ± 0.1
52 ± 2
1.20 ± 0.02
7.6 ± 0.2
14.1 ± 0.2
66 ± 1
3.3 ± 0.2
6.9 ± 0.4
The slumps measured on mortar are listed in Table 5.9. The measurements show the same
trends as the rheology of the CEM I 52.5 R LA cement past: a gradual decline in workability
over time and a decreasing effect of addition of calcium nitrate on the workability (compare
slump before and after activation).
Table 5.9 Slump (mm) for 1:3 mortar (w/c = 0.40) made with CEM I 52.5 R LA cement.
Slump (mm)
CEM I 52.5 R LA (w/c=0.50)
15 min.
after water
addition
57
Time of measurement
2h 15min.
3h 45min.
before
activation
28
25
4h 15 min.
after
activation
20
5.2.4. Conclusion
It was investigated if the retardation of sodium lignosulphonate on the setting of Portland
cement can be counteracted by a delayed addition of calcium nitrate for use as a system for
long transport of fresh concrete. CEM I 52.5 R LA and CEM I 42.5 RR cement paste were
retarded using sodium lignosulphonate in dosages of 0.40% and 1.00% respectively. Calcium
nitrate was added in 0.50% and 1.00% dosages 2, 4 6 and 8 hours after start of initial
blending. Strength measurements were also carried out on CEM I 52.5 R LA cement mortar.
It has been found that:
- sodium lignosulphonate was effective in retarding hydration to around 8 hours after
mixing,
- calcium nitrate had an accelerating effect on hydration,
- for CEM I 52.5 R LA cement, there was a gradual decrease in workability over time,
- for CEM I 42.5 RR cement paste, an initial increase in workability (up till 4 hours
after water addition) was followed by a gradual decline in workability.
- addition of calcium nitrate decreased workability,
- for CEM I 52.5 R LA cement mortar, the 1 day compressive strength more than
doubled when 1.00% calcium nitrated was included, and,
- the 28 days compressive strength was considerably (+25%) higher when calcium
nitrate was included.
95
5.3 Citric acid
5.3.1. Introduction
It was in principle studied if cement paste could be over-retarded for long transport using
citric acid, a hydrocarboxylic acid with strong retarding effects, and activated again (upon
arrival at the construction site) by addition of calcium nitrate. Based on earlier calorimetric
measurements, to be discussed in the Chapter 6 (see 6.3.2. and Figure 6.2), a dosage of 0.20%
citric acid was used to over-retard CEM I 52.5 R LA cement paste. Calcium nitrate was added
2, 4, 6 and 8 hours after water addition.
5.3.2. Experimental
Cement paste was made with distilled water. CEM I 52.5 R LA Portland cement was used. A
dosage of 0.20% citric acid powder was added to the water before mixing. The paste had a
w/c ratio of 0.40. Total paste volume was approximately 450 ml.
water and mixing for ½ minute, resting for 5 minutes and blending again for 1 minute.
The paste was then poured into a 500 ml glass beaker, covered with a plastic sheet to avoid
evaporation of water and kept at laboratory conditions at a temperature of approximately
22°C. Every 2 hours approximately 100 ml of paste was sampled out of the beaker after it was
mixed up again for 1 minute using the hand blender end-piece of the Braun mixer.
A 1.00% dosage of calcium nitrate in the form of granules was then added to the sample after
which blending was performed by mixing for ½ minute, resting for 5 minutes and mixing
again for 1 minute in the Braun mixer. A reference sample was also taken and investigated
immediately after the initial blending.
96
Figure 5.12 shows the heat of hydration curves for CEM I 52.5 R LA cement pastes overretarded using 0.20% citric acid powder after addition of 1.00% calcium nitrate 2, 4, 6 and 8
hours after initial mixing. A reference without calcium nitrate is also shown. It can be seen
that citric acid was able to retard hydration. A small peak, however, was seen about 1 ½ hour
after water addition. Addition of 1.00% calcium nitrate resulted in exothermal reactions but
was not able to initiate hydration as no significant heat release was recorded within the first
45 hours after water addition.
CEM I 52.5 R LA - w/c = 0.40 - 0.20% Citric Acid
1.6
1.4
Reference
1.2
1
0.8
8 hours
6 hours
0.6
4 hours
2 hours
0.4
0.2
0
1
5
9
13
17
21
25
29
33
37
41
45
Time (hours)
0.20% citric. 1.00% calcium nitrate was added 2, 4, 6 and 8 hours after initial mixing. The
reference is also shown.
and after 10 minutes of rest. Table 5.10 shows the flow resistance, Table 5.11 the 10 seconds
gel strength and Table 5.12 the 10 minutes gel strength.
97
citric acid. 1.00% calcium nitrate was added 2, 4, 6 and 8 hours after initial mixing. A
reference without calcium nitrate was also investigated.
Flow resistance
[Pa/s]
Reference (no CN)
1.00% CN
0h
390
2h
2078
Time of addition
4h
6h
3214
3959
8h
4749
(w/c=0.40) with 0.20% citric acid. 1.00% calcium nitrate was added 2, 4, 6 and 8 hours after
initial mixing. A reference without calcium nitrate was also investigated.
10 sec. gel
Time of addition
strength [Pa]
0h
2h
4h
6h
8h
Reference (no CN)
<1
1.00% CN
30.5
35.8
41.9
49.1
(w/c=0.40) with 0.20% citric acid. 1.00% calcium nitrate was added 2, 4, 6 and 8 hours after
10 min. gel
Time of addition
strength [Pa]
0h
2h
4h
6h
8h
Reference (no CN)
28.4
1.00% CN
95.6
80.3
87.6
87.6
One can observe an important loss in workability (i.e. increased flow resistance and increased
gelling tendency) after addition of calcium nitrate 2 hours after water addition which might be
explained by the hydration reactions that took place about 1 ½ hours after water addition
(peak in the hydration curve for the reference). After two hours, one can see a gradual
increase in flow resistance and gel strength over time.
5.3.4. Conclusion
It was investigated if the system citric acid/calcium nitrate could be used for long-transport
purposes. CEM I 52.5 R LA cement paste was retarded through the addition of 0.20% citric
acid and 1.00% calcium nitrate was added 2, 4, 6 and 8 hours after water addition. It was
found that calcium nitrate was suitable to initiate hydration and that important workability
losses occurred over time.
98
5.4 Lead nitrate
5.4.1. Introduction
It was investigated if cement paste over-retarded by lead nitrate could be activated by calcium
nitrate. Based on earlier calorimetric measurements, to be discussed in the Chapter 6 (see
6.2.2. and Figure 6.1), a dosage of 0.75% lead nitrate was used to over-retard CEM I 52.5 R
LA cement paste. Calcium nitrate was added 2, 4 and 6 hours after water addition.
5.4.2. Experimental
Cement paste was made with distilled water. CEM I 52.5 R LA Portland cement was used. A
dosage of 0.75% lead nitrate was added to the water before mixing. The paste had a w/c ratio
of 0.40. Total paste volume was approximately 450 ml.
The paste was then poured into a 500 ml glass beaker, covered with a plastic sheet to avoid
evaporation of water and kept at laboratory conditions at a temperature of approximately
22°C. Every 2 hours approximately 100 ml of paste was sampled out of the beaker after it was
mixed up again for 1 minute using the hand blender end-piece of the Braun mixer.
99
Figure 5.13 shows the heat of hydration curves for CEM I 52.5 R LA cement pastes overretarded using 0.75% lead nitrate after addition of 1.00% calcium nitrate 2, 4 and 6 hours after
initial mixing. A reference without calcium nitrate is also shown. It can be seen that lead
nitrate was able to retard hydration. Addition of 1.00% calcium nitrate was able to accelerate
hydration with about 10 hours. The time between addition of calcium nitrate and hydration,
however, appeared to be too long in order to use this combination as for long transport
purposes.
CEM I 52.5 R LA - w/c = 0.40 - 0.75% Lead Nitrate
2
Reference
1.8
6 hours
1.6
4 hours
1.4
2 hours
1.2
1
0.8
0.6
0.4
0.2
0
1
5
9
13
17
21
25
29
33
37
41
Time (hours)
0.75% lead nitrate. 1.00% calcium nitrate was added 2, 4 and 6 hours after initial mixing. The
reference is also shown.
and after 10 minutes of rest. Table 5.13 shows the flow resistance, Table 5.14 the 10 seconds
gel strength and Table 5.15 the 10 minutes gel strength.
100
lead nitrate. 1.00% calcium nitrate was added 2, 4 and 6 hours after initial mixing. A
reference without calcium nitrate was also investigated.
Flow resistance
Time of addition
[Pa/s]
0h
2h
4h
6h
0.00% CN
1159
1.00% CN
714
982
1009
(w/c=0.40) with 0.75% lead nitrate. 1.00% calcium nitrate was added 2, 4 and 6 hours after
10 sec. gel
Time of addition
strength [Pa]
0h
2h
4h
6h
0.00% CN
7.3
1.00% CN
3.3
6.2
6.2
(w/c=0.40) with 0.75% lead nitrate. 1.00% calcium nitrate was added 2, 4 and 6 hours after
10 min. gel
Time of addition
strength [Pa]
0h
2h
4h
6h
0.00% CN
73.6
1.00% CN
13.0
18.4
18.4
One can see a decrease in flow resistance and gel strength (i.e. an increase in workability) two
hours after water addition. After two hours, one can observe a gradual decrease in
workability. However, even 6 hours after water addition, there is still a clear plasticizing
effect.
5.4.4. Conclusion
For the system lead nitrate/calcium nitrate, it was found, for CEM I 52.5 R LA cement paste
retarded using 0.75% lead nitrate, that 1.00% calcium nitrate was not able to accelerate
hydration to an extent which would allow the system to be used for long transport of fresh
concrete. It was also found that lead nitrate acted as a good plasticizer up till at least 6 hours
after water addition.
101
5.5 Sodium gluconate
5.5.1. Introduction
Sodium gluconate belongs to a first generation of plasticizers, the salts of carboxylic acids,
which have strong retarding effects and are mostly used for their dominating retarding
behaviour. In this section it was investigated if sodium gluconate can be used as a retarder for
long-term transport purposes. First, it was determined which dosage would retard hydration
for about 8 hours. Then, it was investigated if calcium nitrate could activate hydration.
Measurements were carried out on cement paste.
5.5.2. Experimental
Cement pastes were made with distilled water and sodium gluconate was added to the water.
Both CEM I 52.5 R LA and CEM I 42.5 RR Portland cement were used. Cement pastes
prepared with CEM I 52.5 R LA cement had a w/c ratio of 0.40. Those prepared with CEM I
42.5 RR cement had a w/c ratio of 0.50. Four different sodium gluconate dosages were
investigated: 0.10, 0.20, 0.30 and 0.40% by weight. Total paste volume was approximately
250 ml.
water containing sodium gluconate and mixing for ½ minute, resting for 5 minutes and
blending again for 1 minute.
The hydration heat was measured by weighing 6 to 7 grams of cement paste into a glass
ampoule after which the ampoule was sealed and loaded into the calorimeter.
The rheological properties of the pastes were studied 15 minutes after the start of the
blending.
Then, cement pastes were made with distilled water. Both CEM I 52.5 R LA (w/c = 0.40) and
CEM I 42.5 RR (w/c = 0.50) Portland cement were used. A dosage of 0.10% sodium
gluconate (based on above mentioned experiments) was added to the water before mixing in
case of CEM I 52.5 R LA cement paste. In case of CEM I 42.5 RR cement paste, 0.20%
sodium gluconate was added. Total paste volume was approximately 450 ml.
102
The pastes were then each poured into a 500 ml glass beaker, covered with a plastic sheet to
avoid evaporation of water and kept at laboratory conditions at a temperature of
approximately 22°C. Every 2 hours approximately 100 ml of paste was sampled out of a
beaker after it was mixed up again for 1 minute using the hand blender end-piece of the Braun
mixer.
rest) were studied 15 minutes after the addition of calcium nitrate.
103
The heat of hydration curves for the pastes made with CEM I 52.5 R LA cement are shown in
Figure 5.14 and Figure 5.15. The heat of hydration curves for the pastes made with CEM I
42.5 RR cement are shown in Figure 5.16 and Figure 5.17. The rheological data has been
transformed into flow resistance (area under the flow curve in the range from 2 to 50 1/s), gel
strength after 10 seconds of rest and gel strength after 10 minutes of rest. The results are given
in Table 5.16 for CEM I 52.5 R LA cement and Table 5.17 for CEM I 42.5 RR cement.
Table 5.16 Rheological measurements for CEM I 52.5 R LA cement paste (w/c=0.40) and
different dosages of sodium gluconate.
CEM I 52.5 R LA
sodium gluconate dose
0.00 %
0.10 %
0.20 %
0.30 %
0.40 %
Flow resistance
[Pa/s]
2283
1471
385
362
342
Gel strength [Pa]
10 sec.
10 min.
19
74
10
74
<1
296
2.8
44
2.4
37
Table 5.17 Rheological measurements for CEM I 42.5 RR cement paste (w/c=0.50) and
different dosages of sodium gluconate.
CEM I 42.5 RR
sodium gluconate dose
0.00 %
0.10 %
0.20 %
0.30 %
0.40 %
Flow resistance
[Pa/s]
2788
2961
3108
2312
461
Gel strength [Pa]
10 sec.
10 min.
22
68
31
74
31
88
> 100
> 400
14
> 400
104
CEM I 52.5 R LA - w/c = 0.40 - sodium gluconate
4
3.5
3
0.20%
2.5
2
0.30%
1.5
0.10%
1
0.40%
0.5
0
0
1
2
3
4
5
6
Time (hours)
0.10%, 0.20%, 0.30% and 0.40% sodium gluconate during the first 6 hours after mixing.
CEM I 52.5 R LA - w/c = 0.40 - sodium gluconate
2.5
2
1.5
0.20%
0.30%
0.10%
1
0.40%
0.5
0
1
5
9
13
17
21
25
29
33
Time (hours)
0.10%, 0.20%, 0.30% and 0.40% sodium gluconate from 1 to 36 hours after mixing.
105
CEM I 42.5 RR - w/c = 0.50 - sodium gluconate
14
12
10
8
0.10%
6
0.20%
4
0.30%
0.40%
2
0
0
1
2
3
4
Time (hours)
Figure 5.16 Heat of hydration curves for CEM I 42.5 RR cement paste (w/c=0.50) with
0.10%, 0.20%, 0.30% and 0.40% sodium gluconate during the first 4 hours after mixing.
CEM I 42.5 RR - w/c = 0.50 - sodium gluconate
4
3.5
0.10%
0.20%
3
2.5
2
0.30%
1.5
0.40%
1
0.5
0
1
5
9
13
17
21
25
29
33
Time (hours)
0.10%, 0.20%, 0.30% and 0.40% sodium gluconate from 1 to 36 hours after mixing.
106
Looking at the rheological measurements in Table 5.16 and Table 5.17, one can observe a
strong decline in flow resistance from a dosage of 0.20 % sodium gluconate for CEM I 52.5 R
LA cement, and from a dosage of 0.40 % for CEM I 42.5 RR cement. This favourable
plasticizing effect was, however, overshadowed by higher gel strengths measured for a
number of mixes. These higher gel strengths can be explained by the heat of hydration curves
during the first hours after mixing (Figure 5.14 and Figure 5.16). In the case of CEM I 52.5 R
LA cement and a sodium gluconate dosage of 0.20 %, one can observe the start of a strong
early hydration reaction around 30 minutes after the start of mixing. As the rheological
measurement sequence was only started 15 minutes after the start of blending, the start of this
strong early hydration reaction took place in the 10 minutes waiting period proceeding the 10
minutes gel strength measurement, and thus giving rise to the higher 10 min. gel strength of
296 MPa. Similar hydration peaks can be observed for 0.30 % and 0.40 % dosages of sodium
gluconate at two and five hours after mixing respectively. In the case of CEM I 42.5 RR
cement high gel strengths were measured for 0.30 % and 0.40 % dosages of sodium
gluconate. A peak was not observed in case of a 0.30 % dosage, but the higher 10 sec. gel
strength and the higher specific heat measured during the first hour after mixing might
indicate that a very early hydration took place in the time that was needed to prepare and
transfer the sample to the calorimeter.
Ramachandran ([9], p. 168) mentions that in the presence of carbohydrate type admixtures the
consumption of SO3 is accelerated, so that insufficient SO3 remains in the liquid phase for
properly controlling the C3A hydration which in turn promotes quick setting. As CEM I 52.5
R LA cement has a lower SO3 content than CEM I 42.5 RR cement (see Chapter 3), this
might explain why higher sodium gluconate dosages were needed for quick setting to occur
for the latter cement type.
Low dosages of sodium gluconate, 0.10 % for CEM I 52.5 R LA cement and 0.10 % and
0.20 % for CEM I 42.5 RR cement, did not show this early hydration peaks. In the case of
CEM I 52.5 R LA the 0.10 % dose was able to postpone setting to 13 hours after mixing. The
same 0.10 % dose was not effective in retarding setting of the CEM I 42.5 RR cement paste
for a long enough period. The 0.20 % sodium gluconate dose, however, was able to retard
setting of CEM I 42.5 RR cement paste up to 13 hours after mixing. Ramachandran ([9], p.
167) found that hydroxycarboxylic acids retard setting times of cement pastes, containing
low-alkali, low-C3A cements more effectively than those of cement pastes with higher alkali
and C3A contents. This was confirmed here as CEM I 52.5 R LA cement has a lower alkali
and C3A content than CEM I 42.5 RR cement (see Chapter 3).
107
Figure 5.18 and Figure 5.19 show the heat of hydration curves for CEM I 52.5 R LA and
CEM I 42.5 RR cement paste after addition of 1.00% calcium nitrate 2, 4 and 6 hours after
initial mixing. CEM I 52.5 R LA paste was prepared with 0.10% sodium gluconate, CEM I
42.5 RR paste with 0.20% sodium gluconate. The curves clearly show that calcium nitrate
was very effective in re-activating the deliberately over-retarded cement pastes.
CEM I 52.5 R LA - w/c = 0.40 - 0.10% sodium gluconate
2.50
6 hours
2.00
Reference
4 hours
2 hours
1.50
1.00
0.50
0.00
1
5
9
13
17
21
25
29
33
37
Time (hours)
0.10% sodium gluconate. 1.00% calcium nitrate was added 2, 4 and 6 hours after initial
CEM I 42.5 RR - w/c=0.50 - 0.20% sodium gluconate
3.5
6 hours
3
Ref.
4 hours
2 hours
2.5
2
1.5
1
0.5
0
1
5
9
13
17
21
25
29
33
37
Time (hours)
0.20% sodium gluconate. 1.00% calcium nitrate was added 2, 4 and 6 hours after initial
108
Rheological data is given in Table 5.18, Table 5.19 and Table 5.20 for CEM I 52.5 R LA
cement paste and in Table 5.21, Table 5.22 and Table 5.23 for CEM I 42.5 RR paste.
sodium gluconate.
Flow resistance
Time of addition
[Pa/s]
0h
2h
4h
6h
0.00% CN
1551
1.00% CN
2380
2813
3376
(w/c=0.40) with 0.10% sodium gluconate.
10 sec. gel
Time of addition
strength [Pa]
0h
2h
4h
6h
0.00% CN
16.2
1.00% CN
26.1
30.5
35.8
10 min. gel
Time of addition
strength [Pa]
0h
2h
4h
6h
0.00% CN
61.9
1.00% CN
95.6
114
135
Table 5.21 Flow resistance (Pa/s) for CEM I 42.5 RR cement paste (w/c=0.50) with 0.20%
sodium gluconate.
Time of addition
Flow resistance
[Pa/s]
0h
2h
4h
6h
0.00% CN
2977
1.00% CN
4580
5622
5884
Time of addition
10 sec. gel
strength [Pa]
0h
2h
4h
6h
0.00% CN
35.8
1.00% CN
41.9
41.9
49.1
10 min. gel
Time of addition
strength [Pa]
0h
2h
4h
6h
0.00% CN
73.6
1.00% CN
67.5
80.3
80.3
109
The rheological data show that workability decreases over time and that the workability of the
pastes is low (i.e. sodium gluconate has no significant plasticizing effect). In practice, the
system will therefore have to be combined with a plasticizer in order to obtain sufficient
workability.
5.5.4. Conclusion
CEM I 52.5 R LA and CEM I 42.5 RR cement paste were over-retarded with 0.10% and
0.20% sodium gluconate respectively. It was found that 1.00% calcium nitrate, added 2, 4 and
6 hours after water addition, was able to re-activate the deliberately over-retarded cement
pastes to an extent which would allow the system sodium gluconate/calcium nitrate to be used
for long transport of fresh concrete. In practice, the system will have to be combined with a
plasticizer in order to obtain sufficient workability as the workability of the pastes was low.
110
It was in principle studied if a concrete mix from a ready mix plant after being deliberately
over-retarded for long transport could be activated by adding an accelerator in the revolver
drum close to the construction site before pumping the concrete in place. Four different
retarder/accelerator systems were studied:
(1) sodium lignosulphonate/calcium nitrate
(2) citric acid/calcium nitrate
(3) lead nitrate/calcium nitrate
(4) sodium gluconate/calcium nitrate
For the system sodium lignosulphonate/calcium nitrate, it was found that:
- sodium lignosulphonate was effective in retarding hydration to around 8 hours after
-
mixing,
calcium nitrate had an accelerating effect on hydration,
for CEM I 52.5 R LA cement mortar, the 1 day compressive strength more than
doubled and the 28 days compressive strength was considerably (+20%) higher when
1.00% calcium nitrate was included.
For the system citric acid/calcium nitrate, it was found, for CEM I 52.5 R LA cement paste
retarded through the addition of 0.20% citric acid, that 1.00% calcium nitrate was not able to
initiate hydration.
For the system lead nitrate/calcium nitrate, it was found, for CEM I 52.5 R LA cement paste
retarded using 0.75% lead nitrate, that 1.00% calcium nitrate was not able to accelerate
hydration to an extent which would allow the system to be used for long transport of fresh
concrete.
Finally, for the system sodium gluconate/calcium nitrate, it was found, for both CEM I 52.5 R
LA and CEM I 42.5 RR cement paste, that 1.00% calcium nitrate was able to re-activate the
deliberately over-retarded cement pastes to an extent which would allow the system to be
used for long transport purposes. In practice, the system will have to be combined with a
plasticizer in order to obtain sufficient workability.
Chapter 6
Reutilizing residual fresh concrete
6.1 Introduction
This chapter deals with a third potential application concerning the search for a system to
preserve residual fresh concrete for a few days (e.g. over a weekend) followed by activation
before use. However, it may also be used as an overnight concept. Whereas recently
(Koshikawa, S. et al., [17]) a freezing preservation technique has been proposed as method for
reutilizing left-over concrete, this study concentrated on a technique consisting of overretardation of residual fresh concrete followed by later activation by use of an accelerator.
The problem was studied in four phases. First a number of retarders were screened to
investigate if they were able to retard hydration for about three days in moderate dosages. For
the strong retarders found in the first phase a number of dosages were tested in order to
determine the required dosage. In a third phase it was investigated if calcium nitrate could
activate hydration of over-retarded cement paste. Mortar measurements were made in phase
four to investigate strength build-up.
6.2 Phase I - Screening of retarders
6.2.1 Experimental
The investigated cement pastes were made with distilled water. CEM I 52.5 R LA Portland
cement was used. The pastes had a w/c ratio of 0.40. The retarders were added to the water
before mixing. Total paste volume was approximately 250 ml. Paste recipes are listed in
Table 6.1.
111
Chapter 6: Reutilizing residual fresh concrete
112
water containing the retarder and mixing for ½ minute, resting for 5 minutes and blending
again for 1 minute.
The heat of hydration versus time curves were measured by accurately weighing 6 to 7 grams
of cement paste into a glass ampoule after which the ampoule was sealed and loaded into the
calorimeter.
Table 6.1 Paste recipes for CEM I 52.5 R LA pastes (w/c = 0.40)
Retarder
sodium phosphate
zinc acetate
lead nitrate
sodium gluconate
citric acid
(sodium salt of) tartaric acid
household sugar
sucrose
Dosage
1.00%
1.00%
1.00%
1.00%
1.00%
1.00%
0.30%
0.30%
6.2.2 Results and discussion
Figure 6.1 shows the heat of hydration versus time curves recorded for the investigated
retarders.
CEM I 52.5 R LA - w/c=0.40 - series of different retarders
3
zinc acetate
2,5
sodium phosphate
2
lead nitrate
sucrose
1,5
sugar
sodium gluconate
1
sodium salt of tartaric acid
citric acid
0,5
0
0
1
2
3
4
5
Time (days)
Figure 6.1 Heat of hydration curves for CEM I 52.5 R LA cement pastes (w/c = 0.40).
113
Sodium phosphate appeared to have only minor retarding properties as the peak in the
hydration curve already occurred after less than 24 hours. Zinc acetate and lead nitrate were
stronger retarders than sodium phosphate but retarded still less than 3 days: the peaks in the
hydration curves occurred around 30 and 50 hours after water addition respectively. It can
also be noted that these (inorganic) retarders did not show any important plasticizing effects.
The organic retarders, however, also acted as plasticizers. Glucose and household sugar, both
known to have particular strong retarding capacities, were added in smaller dosages (0.30%).
Although the peaks in the hydration curves for these retarders were only seen 5 ½ days after
water addition, heat liberation already commenced after 2 days. Sodium gluconate showed a
small peak about 4 days after water addition (see also Chapter 5). Citric acid and tartaric acid
both acted as strong retarders as no heat release was observed during the recording which
lasted 6 days.
6.2.3. Conclusion
Citric acid and tartaric acid both showed strong retarding capacities in a 1.00% dosage. No
heat liberation was observed during the recording of the heat of hydration curves which lasted
6 days. Together with the fact that they acted as good plasticizing agents, it was decided to
select them as strong retarders for use in the remainder of this study.
114
6.3. Phase II – Determination of required retarder dosage
6.3.1 Experimental
Cement paste was made with distilled water. CEM I 52.5 R LA Portland cement was used.
The paste had a w/c ratio of 0.40. Total paste volume was approximately 500 ml.
The cement paste was divided over seven plastic 200 ml cups. The amount of cement paste in
each of the cups was weighed accurately. The cups were covered with plastic foil to prevent
water evaporation and kept in laboratory conditions at a temperature of approximately 22°C
for two hours. Before dividing the cement paste over the beakers a not retarded reference
sample was taken by accurately weighing 6 to 7 grams of paste into an ampoule after which
the ampoule was sealed and kept under the same conditions as the beakers.
Two hours after water addition, dosages of 0.10%, 0.20%, 0.30% and 0.40% citric acid (CA)
and 0.10%, 0.20% and 0.30% sodium salt of tartaric acid (NT, short for natrium tartaricum)
by weight of cement were added (in powder form) to the different beakers. Blending was
performed by stirring up the cement paste with a plastic spoon for 1 minute, resting for 5
minutes and stirring again for 1 minute.
Cement paste was sampled out of each of the cups and 6 to 7 grams of cement paste was
accurately weighed into a glass ampoule. The ampoules were sealed and loaded, together with
the not retarded reference, into the calorimeter.
The heat of hydration curves for the pastes prepared with citric acid and tartaric acid (NT) are
shown in Figure 6.2 and Figure 6.3 respectively. The not retarded reference is also shown.
It can be seen that for both retarders a 0.40% dosage will retard hydration for more than 68
hours which is sufficient in order to preserve the paste over a weekend.
6.3.3 Conclusion
A dosage of 0.40% of citric acid or tartaric acid appeared to be sufficient in order to retard
hydration for three days.
115
CEM I 52.5 R LA - w/c=0.40 - citric acid
2,5
Ref.
0.10% CA
2
0.20% CA
1,5
1
0.30% CA
0.30% CA
0.40% CA
0.30% CA
0,5
0
3
7
11
15
19
23
27
31
35
39
43
47
51
55
59
63
67
71
75
79
83
87
91
Time (hours)
Figure 6.2 Heat of hydration curves for CEM I 52.5 R LA cement pastes (w/c = 0.40)
retarded through addition of 0.10%, 0.20%, 0.30% and 0.40% citric acid (CA) two hours after
water addition. A not retarded reference is also shown.
CEM I 52.5 R LA - w/c=0.40 - sodium salt of tartaric acid (NT)
2,5
Ref.
0.10% NT
0.20% NT
2
1,5
0.30% NT
1
0,5
0
3
7
11
15
19
23
27
31
35
39
43
47
51
55
59
63
67
71
75
79
83
87
91
Time (hours)
Figure 6.3 Heat of hydration curves for CEM I 52.5 R LA cement pastes (w/c = 0.40)
retarded through addition of 0.10%, 0.20% and 0.30% sodium salt of tartaric acid (NT) two
hours after water addition. A not retarded reference is also shown.
116
6.4. Phase III – Activation using calcium nitrate
6.4.1 Experimental
The cement pastes were made with distilled water and had a w/c ratio of 0.40. CEM I 52.5 R
LA Portland cement was used. Two volumes of 400 ml cement paste were prepared.
The pastes were then each poured into a 500 ml glass beaker and covered by plastic film to
avoid evaporation of water. They were kept at laboratory conditions at a temperature of
approximately 22 °C for two hours.
Two hours after water addition, the content of one beaker was poured into the high shear
mixer and a dosage of 0.40% citric acid was added in powder form to simulate
over-retardation of fresh concrete. The paste was then blended by mixing for ½ minute,
resting for 5 minutes and blending again for 1 minute. A dosage of 0.40% sodium salt of
tartaric acid was added to the other beaker the same way. Both cement pastes were then each
poured into a plastic bag and put in glass beakers covered by plastic film (see Figure 6.4).
They were kept at laboratory conditions for 68 hours. A temperature sensor was inserted in
each of the bags and the temperature was logged using a Squirrel (type: 1203) data logger.
Figure 6.4 Beakers containing over-retarded cement paste.
117
A not activated reference sample was taken out of each of the plastic bags 68 hours after the
retardation by accurately weighing 6 to 7 grams of the cement paste into a glass ampoule after
which the ampoule was sealed.
The over-retarded cement pastes were each divided over three plastic 200 ml cups. Granular
calcium nitrate was then, 68 hours after addition of the retarders, added in 1.00%, 1.50% and
2.00% dosages. Blending was performed by stirring up the cement paste with a plastic spoon
for 1 minute, resting for 5 minutes and stirring again for 1 minute. Samples out of each of the
cups, together with the reference, were loaded into the calorimeter.
Figure 6.5 and Figure 6.6 show the temperature profiles recorded for the cement pastes overretarded with 0.40% citric acid and 0.40% sodium salt of tartaric acid respectively. The
temperature in each of the beakers remained stable and it can therefore be assumed that no
hydration took place in the period between over-retardation and activation. It is once more
proven that a dosage of 0.40% citric acid or tartaric acid is sufficient in order to retard
hydration of cement paste over a weekend.
Temperature logging - 0.40% citric acid
Temperature [degrees C]
30
25
20
Ambient
15
Batch 1
10
5
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
Time [hours]
Figure 6.5 Temperature (°C) profile of CEM I 52.5 R LA cement paste (w/c = 0.40) retarded
through addition of 0.40% citric acid two hours after water addition. The ambient temperature
is also shown.
118
Temperature logging - 0.40% sodium salt of tartaric acid
Temperature [degrees C]
30
25
20
Ambient
15
Batch 2
10
5
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
Time [hours]
Figure 6.6 Temperature (°C) profile of CEM I 52.5 R LA cement paste (w/c = 0.40) retarded
through addition of 0.40% sodium salt of tartaric acid two hours after water addition. The
ambient temperature is also shown.
Figure 6.7 and Figure 6.8 show the heat of hydration curves for cement paste retarded through
the addition of 0.40% citric acid and tartaric acid, respectively, two hours after water addition
and activated through the addition of 1.00%, 1.50% and 2.00% calcium nitrate 68 hours after
retardation. The curves clearly show that calcium nitrate was able to initiate hydration a few
hours after addition. It can also be seen that, compared to the reference, hydration takes place
at a significantly lower rate and that in most cases more than one peak occurred in the heat of
hydration curves.
CEM I 52.5 R LA - w/c=0.40 - 0.40% citric acid
1,5
1,2
Ref.
0,9
0,6
2.00% CN
1.50% CN
1.00% CN
0,3
0
0
24
48
72
96
120
144
Time after activation (hours)
Figure 6.7 Heat of hydration curves for CEM I 52.5 R LA cement paste (w/c = 0.40) retarded
through the addition of 0.40% citric acid and activated through the addition of calcium nitrate
(CN) 68 hours after retardation. A not activated reference is also shown.
119
CEM I 52.5 R LA - w/c=0.40 - 0.40% tartaric acid
2,5
Ref.
2
1,5
1
1.50% CN
2.00% CN
0,5
1.00% CN
0
0
24
48
72
96
120
144
Figure 6.8 Heat of hydration curves for CEM I 52.5 R LA cement paste (w/c = 0.40) retarded
through the addition of 0.40% tartaric acid and activated through the addition of calcium
nitrate (CN) 68 hours after retardation. A not activated reference is also shown.
6.4.3 Conclusion
It has been found that CEM I 52.5 R LA cement paste can be over-retarded for at least 68
hours by 0.40% citric acid or sodium salt of tartaric acid added two hours after water addition.
Calcium nitrate, added in dosages between 1.00% and 2.00%, was able to initiate hydration a
few hours after addition.
120
6.5 Phase IV – Strength measurements
6.5.1. Experimental
The mortars were prepared with CEM I 52.5 R LA cement and had a cement:aggregate ratio
of 1:3. The dmax of the aggregate was 8 mm. A w/c ratio of 0.50 was chosen in order to obtain
sufficient workability. Total mortal volume was approximately 5 liters.
minute at speed I. Then water was added while mixing at speed I for 1 minute. Addition of
water took place during the first 30 seconds. After 5 minutes of rest, the mortar was again
blended at speed I for ½ minute to stir up any false setting, followed by 1 ½ minute of
blending at speed II.
The mortar was poured into a plastic bag, sealed and kept at a constant temperature of 20°C
for about two hours, after which 0.40% citric acid was mixed into the mortar in a 30%
aqueous solution. The retarder was added while mixing at speed I for 1 minute. Addition took
place during the first 30 seconds. After 5 minutes of rest, the mortar was again blended at
speed I for ½ minute, followed by 1 ½ minute of blending at speed II. The mortar was again
poured into a plastic bag, sealed and stored in a 20°C room for three days.
After 3 days, the mortar was mixed for 1 minute at speed I after which three 40x40x160 mm
prisms were cast in steel moulds to serve as retarded, but not activated references. Then,
1.50% calcium nitrate was added in a 50% aqueous solution while mixing at speed I for 1
minute. Addition of calcium nitrate took place during the first 30 seconds. After 5 minutes of
rest, the mortar was again blended at speed I for ½ minute, followed by 1 ½ minute of
blending at speed II.
Two 100×100×100 mm cubes were cast in a 17 mm thick Styrofoam mould with glass parts
on two counterpart walls to give smooth surfaces for compressive strength test. They were
cured at 20°C in a climate room with a relative humidity of 60%. The temperature of one of
these cubes was logged to monitor the rate of hydration in a semi-adiabatic case resembling
higher volumes in formwork in practice. The cubes were used to determine the compressive
strength after 2 and 7 days. The testing speed was 8 kN/s.
Nine 40×40×160 mm prisms were cast in steel moulds. The prisms were cured at 20°C and
60% relative humidity. Their strength was determined after 1 or 2 days, 3 days and 28 days of
curing. During the first days the prisms were covered with wet clothes and plastic sheets.
After that they were demoulded and immersed in water baths.
121
strength.
The rheology of the mortar was determined using a mini-slump cone at the following times:
15 minutes after water addition, 15 minutes before and after retardation and 15 minutes before
and after activation.
The complete procedure was repeated using 0.40% sodium salt of tartaric acid as a retarder
instead of citric acid.
Three prisms of not retarded mortar were also made to serve as a reference for the 28 day
strength.
CEM I 52.5 R LA cement paste (w/c = 0.40) was prepared and flow resistances and static gel
strengths after 10 seconds and 10 minutes of rest were determined in order to compare them
with the slumps measured on the mortar prepared with 0.40% citric acid. Paste volume was
approximately 400 ml. The blending was performed in a high shear mixer of Braun by adding
the cement to the water, mixing for ½ minute, resting for 5 minutes and blending again for 1
minute. The rheological properties were determined 15 minutes after water addition using the
rheometer.
The remaining cement paste was poured into a 500 ml glass beaker, covered with plastic foil
to prevent water evaporation and kept in laboratory conditions at a temperature of
approximately 22°C for 1 ½ hours. Then the cement paste was poured into the high shear
mixer and mixed for 1 minute. A sample was brought to the rheometer and its rheological
properties were determined 1 ¾ hours after water addition.
Two hours after water addition, 0.40% citric acid powder was added to the cement paste.
Blending was performed by mixing for ½ minute, resting for 5 minutes and mixing again for
1 minute. Again a sample was brought to the rheometer and the rheological properties were
measured 15 minutes after addition of citric acid.
The cement paste was again poured into a 500 ml glass beaker, covered with plastic foil and
kept in laboratory conditions for about 3 days. Then, 71 ½ hours after retardation, the cement
paste was poured in the high shear mixer a last time and blended for 1 minute. A sample was
taken to investigate the rheological properties 71 ¾ after retardation.
122
Finally, 1.50% granulated calcium nitrate was added to the retarded paste. The paste was
blended using the same blending sequence as before and a sample was brought to the
rheometer to determine the rheological properties 15 minutes after activation.
The temperature profiles recorded for the cubes cast in Styrofoam moulds are depicted in
Figure 6.9 and Figure 6.10. It can be seen that both profiles exhibited two peaks in accordance
with the isothermal heat of hydration curves for cement paste which also exhibited multiple
peaks (see Figure 6.7 and Figure 6.8). In case of citric acid, however, both occurred about one
day earlier compared to the calorimetric measurements on paste.
CEM I 52.5 R LA - w/c = 0.50 - 0.40% citric acid - 1.50% calcium nitrate
30
29
28
Temperature (°C)
27
26
25
24
23
22
21
20
0
24
48
72
96
120
144
Figure 6.9 Temperature (°C) profile of a 1:3 mortar cube cast in a styrofoam mould. The
mortar (w/c = 0.50) was prepared with CEM I 52.5 R LA cement, retarded through the
addition of 0.40% citric acid two hours after water addition and activated through the addition
of 1.50% calcium nitrate three days after retardation.
123
CEM I 52.5 R LA - w/c = 0.50 - 0.40% tartaric acid - 1.50% calcium nitrate
30
29
28
Temperature (°C)
27
26
25
24
23
22
21
20
0
24
48
72
96
120
144
Figure 6.10 Temperature (°C) profile of a 1:3 mortar cube cast in a styrofoam mould. The
mortar (w/c = 0.50) was prepared with CEM I 52.5 R LA cement, retarded through the
addition of 0.40% tartaric acid two hours after water addition and activated through the
addition of 1.50% calcium nitrate three days after retardation.
The compressive and flexural strength as measured on prisms retarded by addition of 0.40%
citric acid and 0.40% tartaric acid and activated by addition of 1.50% calcium nitrate are
listed in Table 6.2 and Table 6.3 respectively.
mortar prisms for not retarded, retarded but not activated and retarded and activated mortar.
Citric acid was used as retarder.
PRISMS
1
1 day
-
Not activated 2
no set
start of set
0.48 ± 0.01
0.130 ± 0.002
-
Not retarded
Retarded and activated 2
1
2
Strength (MPa)
2 days
3 days
-
time from water addition
time from activation
13.7 ± 0.2
3.0 ± 0.2
25.5 ± 0.4
4.4 ± 0.1
28 days
52 ± 2
7.6 ± 0.2
62 ± 2
6.8 ± 0.3
124
mortar prisms for not retarded, retarded but not activated and retarded and activated mortar.
Tartaric acid was used as retarder.
PRISMS
1
1 day
-
Not activated 2
no set
start of set
-
0.54 ± 0.01
0.16 ± 0.02
Not retarded
Retarded and activated 2
1
2
Strength (MPa)
2 days
3 days
19.8 ± 0.9
3.4 ± 0.3
0.65 ± 0.03
0.14 ± 0.02
28 days
52 ± 2
7.6 ± 0.2
66 ± 1
6.9 ± 0.4
time from water addition
time from activation
The strength measurement for the mortar retarded by addition of 0.40% citric acid (Table 6.2)
show that calcium nitrate was able to accelerate setting to the extent that after 1 day a low
strength could already be measured in case of the activated mortar whereas setting had not yet
occurred in case of the not activated reference. A similar clear difference in strength between
the activated and the not activated mortar prisms could be observed after 3 days of curing.
When comparing the strength at 28 days, one can see that the retarded and activated mortar
prisms had an approximately 20% higher compressive strength compared to the not retarded
reference mortar prisms. This might be attributed to calcium nitrate as compressive strength
increases have been seen before in mixes containing calcium nitrate (Justnes, H. (2003)).
In case of the mortar prepared with 0.40% tartaric acid (Table 6.3), calcium nitrate did not
appear to initiate strength build up as the not activated mortar prisms had a considerably
higher strength at 3 days, whereas only a marginal strength could be measured on the
activated mortar prisms. The combination tartaric acid – calcium nitrate should therefore be
rejected as a system to preserve and reutilize left-over concrete.
Table 6.4 shows the compressive strengths as measured 2 and 7 days after activation on
insulated cubes. The compressive strength of 11.4 MPa measured on two day old cubes
implies that, in practice, formwork could already be removed two days after activation. The
tests confirm that calcium nitrate was not suitable to initiate strength build up in case of the
mortar cubes retarded with tartaric acid.
Table 6.4 Compressive strength (MPa) for 1:3 mortar cubes for a retarded and activated
mortar.
CUBES
Citric acid
Tartaric acid
2 days
7 days
11.4
46.0
0.45
42.5
125
The slumps measured on mortar (w/c = 0.50) are listed in Table 6.5. Flow resistances, gel
strengths after 10 seconds and 10 minutes of rested measured on cement paste (w/c = 0.40)
are listed in Table 6.6. The measurements show that the rheology before retardation of both
cement paste and mortar was more or less maintained as it differed not much from the
rheology after activation. It can also be seen that both citric and tartaric acid, used as strong
retarders, also acted as good plasticizers.
Table 6.5 Slump (mm) for 1:3 mortar (w/c = 0.50) made with CEM I 52.5 R LA retarded
through the addition of 0.40% citric acid or 0.40% tartaric acid two hours after water addition.
Slump (mm)
Citric acid
Tartaric acid
15 min.
after water
addition
66
73
Time of measurement
1h 45min. 2h 15min.
3 days
before
after
before
retardation retardation activation
28
61
14
34
62
16
3 days
after
activation
23
24
Table 6.6 Flow resistances (Pa/s), gel strengths (Pa) after 10 seconds of rest and gel strengths
(Pa) after 10 minutes of rest for cement paste (w/c = 0.40) prepared with CEM I 52.5 R LA
cement and retarded through the addition of 0.40% citric acid.
RHEOLOGY
15 min.
after water
addition
2341
22
68
Time of measurement
1h 45min. 2h 15min.
3 days
before
after
before
retardation retardation activation
2899
795
2399
31
10
31
96
37
62
3 days
after
activation
2322
36
68
6.5.3. Conclusion
CEM I 52.5 R LA mortar (w/c = 0.50) was prepared and over-retarded by addition of 0.40%
citric or tartaric acid two hours after water addition. After three days 1.50% calcium nitrate
was added in order to initiate hydration and strength build-up.
It has been found that:
- both retarders were able to preserve the mortar for three days,
- calcium nitrate was able to activate hydration of the mortar retarded by citric acid to
an extent that would facilitate removal of formwork two days after activation,
- calcium nitrate was not suitable to activate mortar retarded by tartaric acid,
- considerably higher 28 day compressive strengths (+20%) were measured compared to
a not retarded reference,
- the rheology before retardation and after activation did not differ much.
126
The search for a method to preserve residual fresh concrete for a few days by a technique
consisting of over-retardation followed by later activation by use of an accelerator was
handled in four phases.
In the first phase eight retarders were screened to investigate if they were able to retard
hydration for about three days in moderate dosages. It has been found that citric acid and
(sodium salt of) tartaric acid both showed strong retarding capacities in a 1.00% dosage.
Phase two pointed out that 0.40% dosages of both citric and tartaric acid were sufficient in
order to retard hydration for three days.
In a third phase it was investigated if calcium nitrate could activate hydration of over-retarded
cement paste. It can be concluded that calcium nitrate, added in dosages between 1.00% and
2.00%, was able to initiate hydration a few hours after addition.
Finally, mortar measurements were carried out to investigate strength build-up. It has been
found that:
- both retarders were able to preserve the mortar for three days,
- calcium nitrate, added in a 1.50% dosage, was able to activate hydration of the mortar
retarded by citric acid to an extent that would facilitate removal of formwork two days
after activation,
- the same dosage of calcium nitrate was not able to activate mortar retarded by tartaric
acid,
- considerably higher compressive strengths (+20%) were measured compared to a not
retarded reference,
- the rheology before retardation and after activation did not differ much.
It can therefore be concluded that the system citric acid/calcium nitrate may facilitate storage
of fresh concrete over night or week-end for activation and use later on.
Chapter 7
Conclusions
The combination of plasticizers/retarders and accelerators has been investigated in view of
three different potential concrete applications.
The first application, making up the major part of this study, focused on the fact that
plasticizers that are used to increase flow for cementitious materials at equal water-to-cement
ratio also to a variable extent retard setting as a side effect. The objective was to find an
accelerator that at least partially would counteract this retardation without negatively affecting
rheology (studied on cement paste) too much. The plasticizers sodium (NLS) and calcium
lignosulphonate (CLS) were used in dosages ranging from 0.15% to 1.00%, polyether grafted
polyacrylate (PA) was used in a 0.10% dosage. The setting accelerator calcium nitrate was
added in dosages ranging from 0.00% to 1.00%. Both CEM I 52.5 R LA and CEM I 42.5 RR
Portland cement was used.
The general trends for the gel strength are that gelling decreases with increasing dosages for
lignosulphonates, the tested PA leads to less gelling than the lignosulphonates and that gelling
tendency increases with increasing dosage of calcium nitrate.
Two admixture blends were also tried out in mortar. The strength data of CEM I 52.5 R LA
mortars plasticized by 0.50% NLS and 0.10% PA showed that both NLS and PA delayed
hydration considerably at 5°C and that calcium nitrate to a certain extent was able to
127
Chapter 7: Conclusions
128
counteract that. Mortar with only 0.50% NLS had no strength after 1 day at 20°C and 2 days
at 5°C, but gained sufficient strength for removal of formwork in practice after 1 day at 20°C
when 0.75% calcium nitrate was included and even some strength after 2 days at 5°C. The
compressive strength of mortar with 0.10% PA was raised to a level where it is considered
frost resistant after 2 days when 0.75% calcium nitrate was included (calcium nitrate more
than doubled the compressive strength).
The second application concerns the search for a system for long transport of fresh concrete.
The experimental work was largely carried out on cement paste. It was investigated if a
concrete mix from a ready mix plant after being deliberately over-retarded for long transport
in for instance hot climate or cities with unpredictable traffic could be activated by adding an
accelerator in the revolving drum close to the construction site before pumping the concrete in
place.
Four
different
retarder/accelerator
systems
were
studied:
sodium
lignosulphonate/calcium nitrate, citric acid/calcium nitrate, lead nitrate/calcium nitrate and
sodium gluconate/calcium nitrate.
Our results pointed out that the system sodium gluconate/calcium nitrate might prove to be a
good system for long transport of concrete for both CEM I 52.5 R LA and CEM I 42.5 RR
cement. The system will, however, have to be combined with a plasticizer in order to obtain
sufficient workability.
In the third application a system to preserve residual fresh concrete for a few days followed
by activation before use was searched for.
Eight retarders were screened. Citric acid and sodium salt of tartaric acid were selected for
further investigation. CEM I 52.5 R LA cement paste and mortar could be retarded for several
days by citric and tartaric acid in 0.40% dosages. Hydration, however, could only be activated
by calcium nitrate in a 1.50% dosage in case of mortar retarded by citric acid. Calcium nitrate
was able to activate hydration to an extent which would facilitate removal of formwork within
two days after activation. The system citric acid/calcium nitrate may therefore be suitable for
storing of fresh concrete over night or week-end for activation and use later on.
References
[1]
Thys, A., Vanparijs, F., ‘Longterm Performance of Concrete with Calcium Nitrate
Admixture: Strength, Diffusivity and Microstructure’, Master of Science Thesis,
Katholieke Universiteit Leuven, 1996.
[2]
Ardoullie, B., Hendrix, E., ‘Chemical Shrinkage of Cementitious Pastes and Mortars’,
Master of Science Thesis, Katholieke Universiteit Leuven, 1997.
[3]
Clemmens, F., Depuydt, P., ‘Early Hydration of Portland Cements: Influence of
Accelerators, Measuring Methods’, Master of Science Thesis, Katholieke Universiteit
Leuven, 1999.
[4]
Van Dooren, M., ‘Factors Influencing the Workability of Fresh Concrete’, Master of
Science Thesis, Katholieke Universiteit Leuven, 2002.
[5]
Brouwers, K., ‘Cold Weather Accelerators: First Day Effects of Calcium Nitrate and
Sodium Thiocyanate on Two Kinds of Portland Cement’, Master of Science Thesis,
Katholieke Universiteit Leuven, 2005.
[6]
Justnes, H., ‘Rheology of Cement based Binders – State-of-the-Art’, SINTEF Civil and
Environmental Engineering, January 2003.
[7]
Hewlett, P.C., ‘ Lea’s Chemistry of Cement and Concrete’, Arnold, London, 1998.
[8]
Vikan, H. and Justnes, H., ‘Parameters Determining the Flow of Concrete Matrix’,
Proceedings 30th Our World in Concrete and Structures, Singapore, 23-24 August,
2005, 111-118.
[9]
Ramachandran, V.S., ‘Concrete Admixtures Handbook – Properties, Science and
Technology’, Noyes Publications, New Jersey, 1984.
[10] Ramachandran,V.S., Malhotra, V.M., Jolicoeur, C. and Spiratos, N., ‘Superplasticizers:
Properties and Applications in Concrete’, CANMET, Canada, 1998.
[11] Justnes, H., Nygaard, E.C., ’Setting Accelerator Calcium Nitrate – Fundamentals,
Performance and Applications’, Proceedings Third CANMET/ACI International
Conference, Auckland, New Zealand, 1997, 325-338.
[12] Mezger, T.G., ‘The Rheology Handbook’, Vincentz Verlag, Hannover, 2002.
[13] Taylor, H.F.W., ‘Cement Chemistry’, Academic Press, London, 1990.
[14] Justnes, H. and Petersen, B.G., ‘Counteracting Plasticizer Retardation of Cement Setting
with Calcium Nitrate’, Proceedings of the International Conference Innovations and
Developments In Concrete Materials and Construction, Dundee, Scotland, 9-11
September, 2002, 259-267.
[15] Justnes, H. and Petersen, B.G., ‘Counteracting Retardation of Cement Setting by Other
Admixtures with Calcium Nitrate’, Proceedings of 5th CANMET/ACI International
Symposium on Advances in Concrete Technology, July 29 – August 1, Singapore,
2001, 39-49.
[16] Justnes, H., ‘Explanation of Long-Term Compressive Strength of Concrete Caused by
the Set Accelerator Calcium Nitrate’, Proceedings of the 11th International Congress on
the Chemistry of Cement (ICCC), 11-16 May, 2003, Durban, South Africa, 475-484.
[17] Koshikawa, S., Itoh, Y. and Shintani, S., ‘A Proposal for Effective Utilization of LeftOver Concrete Using a Freezing Preservation Technique’, Cement Science and
Concrete Technology no. 54, 2000, 522-529.
[18] De Weerdt, K., Reynders, D., Justnes, H., Van Gemert, D., ‘Combining
Plasticizers/Retarders and Accelerators’, to be presented at the Eight CANMET/ACI
International Conference on Superplasticizers and Other Chemical Admixtures in
Concrete, October 29-November 1, 2006, Sorrento, Italy.
[19] Justnes, H., De Weerdt, K., Reynders, D., Van Gemert, D., ‘Counteracting Retardation
of Plasticizers by Calcium Nitrate’, to be presented at the Second International RILEM
Symposium on Advances in Concrete Through Science and Engineering, September 1113, 2006, Quebec City, Canada.

Combining Plasticizers/Retarders And Accelerators

Transcription

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