Setting Additives Influence on the Thermomechanical Properties of

REFRACTORY CERAMICS
Setting Additives Influence on the
Thermomechanical Properties of
Wet Shotcrete Refractory Castable
Matrices
The effect of coagulants and setting admixtures on the thermomechanical properties of wet shotcrete refractory castable
matrices was evaluated and discussed.
Y.A. Marques, R.G. Pileggi,
F.A.O. Valenzuela, M.A.L. Braulio and
V.C. Pandolfelli
Dept. of Materials Engineering, Federal
University of São Carlos, São Carlos, S.P., Brazil
prayed concretes were
originally developed for
civil construction in the
early 20th century.1 The
advances and benefits that
have been attained since then are
responsible for the current widespread
use of this placing technique. High installation rates, lower costs, automation
capabilities and performances similar to
preshaped refractories are some of the
benefits of this technique.1–4
S
Wet shotcrete consists of pumping the
castable suspension out of the mixer and
onto the target surface (Figure 1).1,5
Compressed air is injected into the tip of
the pipeline nozzle to generate the
castable spray that lines the surface. The
sprayed castable is consolidated by an
additive, also injected at the nozzle tip,
which causes sudden loss of fluidity in
the concrete. This technique results in
low rebound and does not require molds.
This process also is characterized by low
porosity of the placed material because
of the high shearing rates imposed by
the process.
Wet shotcrete has evolved as a result of
the advances in pumpable castables,
©The American Ceramic Society
American Ceramic Society Bulletin
rheometric analyses,2,5 equipment and
additives.2 These developments have led
to the introduction of wet shotcrete in
the refractory industry.1
Despite its simple concept, the task of
preparing refractory shotcrete is complex. It involves particle-size design, dispersion, mixing, pumping, spraying and
setting.2,6–8
Refractory castables can be prepared in
a broad range of particle sizes.5,8 They
consist of a matrix (particle size of <100
µm, controlled by surface forces and
interactions with the aqueous media)
and aggregates (particle size of >100 µm,
controlled by mass forces). For better
sprayability, the pumped castable should
be homogeneous, dispersed and
segregation-free (matrix/aggregate
separation).1,2
The design of pumping castables has
been mastered to a considerable extent.
However, other aspects of wet shotcrete
that require research are the generation
of spray at the conical nozzle (Figures 1
and 2) and the proper selection and
quantity of setting additive. The granules
formed during spraying can become brittle or plastic during the short trajectory
from the nozzle to the target. These
results depend on the setting additive
and its interaction with castable
constituents.
www.ceramicbulletin.org
August 2005
9201
Figure 1 Schematic description of the wet shotcrete process,
showing mixing, pumping and spraying stages.5
Figure 2
The formation of spray is influenced
by the equipment (pump, pipeline
and nozzle design), operational proceedings (compressed air pressure,
additive injection, placing direction
and spray opening angle), castable
rheological behavior and matrix
composition. The setting additives
directly affect the rheology of the
material.4 Therefore, spraying and
consolidation of the castable also
depend on how the additive interacts with the matrix.
Cement accelerators are normally
used in wet shotcrete applications to
make the castable set instantaneously.1,2,4 Most of these additives are
based on alkaline compounds. Their
performance depends on their
chemical composition and cement
particle-size distribution.3 However, a
disadvantage of this class of additives is that they decrease castable
mechanical strength. Moreover, they
can decrease material refractoriness
in high-temperature applications.
These effects scale with the amount
of additive.2,3
To minimize these effects, alkali-free
additives have been developed.2,3
They also decrease the risks related
with the toxic nature of alkaline substances. The setting mechanism of
admixtures usually derives from an
increase in the ionic strength and a
change in suspension pH levels. This
©The American Ceramic Society
Schematic illustration of the wet-shotcrete spraying technique of refractory
castables.7
promotes greater attracting forces
between the particles and, thus,
directly affects material packing
structure.3,4,9
A novel class of organic viscosityenhancing admixtures has been
developed recently for hydraulically
bound materials (cement based).10
The admixtures in this castable generate water-trapping gels that
increase viscosity of the solution,
cohesion and material adhesion.
Nevertheless, these additives may
retard the drying of refractory castables, which extends their processing
time. Other admixtures form 3-D network gels as a result of crosslinked
bonds with cement calcium ions.10,11
This mechanism decreases water
retention, because cohesion also is
promoted by the newly generated
chemical bonds.
Organic polyelectrolytes that have
long polymeric chains also have been
proposed as admixtures for wet shotcrete refractory castables.4,9 Although
the long polymeric molecule chain
establishes bridges between the particles,12 the steric effect prevents the
particles from approaching each
other too closely.Therefore, the permeability and drying time of the
castable is not overly affected.9,12
Setting additives also can affect
other castable properties. Several
American Ceramic Society Bulletin
recent studies4,5,7 have focused on
the rheological aspects of wet shotcrete applications. Another study
reports the influence of various
coagulation mechanisms on the permeability and drying behavior of
refractories.9 However, the impact of
coagulant admixtures on the thermomechanical properties of refractory castables remains unclear.
The major problems involved in
studies of the postsetting properties
of shotcrete are associated with sampling. In the field, the panel waterdrilling technique can damage the
samples. In the laboratory, the rapidly decreasing fluidity of the castable
makes shaping a difficult task. This
inconvenience has been overcome
by pressing the material.
Nevertheless, the particle-packing
structure may undergo alterations
and, therefore, not reproduce the
actual process.3,9 Moreover, the various coagulation mechanisms of
additives can directly influence particle packing, which affects material
microstructure.
Previous research8 has shown that
the creep behavior and elastic modulus of refractory castables, based on
different Andreasen’s packing coefficients, are directly related to the
nature and content of the matrix.
Moreover, the maximum deformation
during the creep test is only slightly
www.ceramicbulletin.org
August 2005
9202
setting additives on the thermomechanical properties of wet shotcrete
refractory castable matrices.
Design/Evaluation Tests
Castables for shotcrete applications
are pumped before they are sprayed.
Therefore, a high-alumina, ultra-lowcement refractory composition that
had pumpable characteristics was
first formulated (Figure 3(A)).5 It was
based on the Andreasen model and
had a packing coefficient of q = 0.26.
The formulation had an original
composition of 78.9 wt% white fused
alumina (Elfusa, Brazil), 20.6 wt%
calcined alumina (A1000-SG and
A3000-FL, Almatis, U.S.) and 1 wt%
aluminous cement (CA-14M, Almatis,
U.S.). The castable was reformulated
so that the entire particle-size distribution was equivalent to that of
the castable matrix (<100 µm)
(Figure 3(B)).
The matrix formulation also had an
Andreasen packing coefficient of q =
0.26.The interparticle spacing (IPS)5
was 0.077 µm, similar to that of the
castable (0.088 µm).The matrix suspension was prepared with 60 vol%
of solids and 40 vol% of water.This
was equivalent to a water content of
18 vol% in the concrete. A polycarboxylate ether (0.15 wt%) (SKW,
Germany) was used as the dispersant.
Figure 4 Castable coagulation mechanisms:
(A) castable without additives;
(B) agglomeration caused by attraction
forces among the particles; (C) bridging
effect caused by PAS molecules; and
(D) bridging effect and gel formation.9
dependent on the maximum particle
diameter.
The chemical and structural effects
of the coagulation mechanisms on
refractory castables can be isolated
by characterizing the fine fraction of
the particle-size distribution that corresponds to the castable matrix (<100
µm).Therefore, the main purpose of
this work is to evaluate the impact of
©The American Ceramic Society
The matrix was mixed in a lab mixer
(Etica SA, Brazil), and the additive
was incorporated as follows: powder
dispersion in water at a constant
mixing speed; injection of setting
additive; 20 s of homogenization;
molding of samples for the mechanical strength and creep tests; curing
in a saturated atmosphere (100% relative humidity) at 50°C for 72 h; and
further drying for another 72 h,
embedded in silica gel (50°C).
Additives
Three distinct classes of setting additives were used: inorganic (sodium
silicate (SS) (Aldrich, Brazil)); calcium
chloride (CC) (Synth, Brazil); organic
polyelectrolyte (sodium polyacrilate
American Ceramic Society Bulletin
(PAS) (BASF, Germany)); and viscosityenhancing polymer (hydroxyethyl
cellulose QP90 (HEC) (Union Carbide,
Brazil)). Two systems were studied:
0.6 wt% of each of these additives
added separately; and combined
0.075 wt% of alginic acid salt (Alg)
(Fluka, Switzerland) and 0.6 wt%
sodium polyacrilate.
Various additive coagulation mechanisms were chosen to consolidate the
castables (Figure 4). SS and CC are
inorganic admixtures commercially
used. Basically, they increase system
ionic strength, alter the potential
energy balance, and promote particle
attraction and agglomeration.4,9,13
The PAS used in this work is a highmolecular-weight organic polyelectrolyte (MW = 15,000 g/mol). It flocculates/coagulates the particles in suspension by promoting bridging,
depletion and ionic strength
increase.12 However, the steric effect
associated with its molecules keeps
the particles apart, which preserves
their original positions.
Alg is a high-molecular-weight polymer (MW = 48–186 kg/mol) derived
from brown seaweed.4,9,10 This additive gels in water by crosslinking its
molecules with the calcium ions in
the cement. Consolidation promoted
by Alg does not alter particle positions in the matrix.
HEC is a water-soluble, nonionic,
semisynthetic organic polymer that
increases liquid viscosity and yield
stress by generating a thixotropic
lubricant gel. The gel does not affect
system pH or ionic strength.4,10
Porosity Tests
The total matrix porosity of the samples cured and dried at 50°C and
prefired at 500°C for 5 h was evaluated. Kerosene was used as the immersion liquid (ASTM C 20-87). The mean
pore-size diameters of these samples
also were evaluated using mercury
porosimetry (Model EUA,
Aminco–Winslow).
www.ceramicbulletin.org
August 2005
9203
The splitting strength technique
(ASTM C 496-90) was used to evaluate the mechanical strength of samples (40 mm in diameter and height).
The splitting strength was evaluated
for five samples of each experimental set after they were cured (72 h)
and dried (50°C) or prefired (500°C
for 5 h after they were dried).
Pore diameter (µm)
Figure 6 Influence of shotcrete additives on pore-size distribution of castable matrix after thermal treatment at 500°C for 5 h (CPI Hg is cumulative percentage of intruded mercury).
σf (MPa)
Refractoriness under load (RUL) was
evaluated for cylindrical specimens
(50 50 mm) that had a central hole
(12.4 mm in diameter). Tests were
conducted on prefired samples
(500°C for 5 h) to ensure that all possible residual hydrates were eliminated. This decreased the likelihood of
explosion. For RUL and creep evaluations, the samples were heated to
1500°C in 5°C/min steps, under a
compressive load of 0.2 MPa (Model
421, Netzsch). For the creep tests, the
load was maintained for 12 h at
1500°C.8,14
CPI Hg (%)
Strength
Shotcrete Additive Selection
Apparent porosity and density
results (Figure 5) showed that these
properties of the castable matrix
were not greatly affected by the setting additives, particularly after they
were prefired at 500°C for 5 h.
The total pore volume and pore
mean diameter were similar in all the
compositions. Therefore, the cumulative percentage of intruded mercury
(CPIHg) as a function of the pore
diameter for the various setting
additives showed no significant
influence (Figure 6).
These results are congruent with
previous research15 that shows these
characteristics are governed by the
amount of water in the matrix.
Therefore, the mechanical strength,
RUL and creep values in this study
are less likely to be influenced by the
apparent porosity of the matrix.
The present results demonstrate
that shotcrete admixtures affect the
©The American Ceramic Society
Figure 7 Influence of shotcrete additives on mechanical strength of castable matrix.
mechanical strength of the castable
matrix in various ways (Figure 7). The
influence of organic and inorganic
additives is clearly distinguished.
Inorganic additives SS and CC
decreased the mechanical strength
of the matrix dried at 50°C. The combination of SS and high-alumina
cement (HAC) increased the pH level
of the matrix,4,12 which led to intense
particle agglomeration. As a result,
the bonding force of the matrix was
directly affected, which weakened
the structure of the matrix after drying at 50°C.4 Furthermore, SS retarded HAC setting, which promoted the
formation of compounds, such as
American Ceramic Society Bulletin
2CaO·Al2O3·SiO2·8H2O, that prevented
cement hydration.13
The low mechanical strength of the
castable matrix that contained CC is
attributed to its agglomerating
effect. CC is a cement hydration
accelerator2,3 that promotes a lesspacked structure.
The organic additives PAS, HEC and
the combination of PAS and Alg
increased castable mechanical
strength after drying at 50°C. This
effect was caused mainly by the formation of polymeric chains (drying
at 50°C) and gel drying (at 50°C and
thermal treatment at 500°C). These
www.ceramicbulletin.org
August 2005
9204
dL/L0 (%)
T (°C)
admixtures. PAS and its combination
with Alg showed a similar behavior
in the RUL and creep tests. Both
additives shifted the onset of deformation to higher temperatures
(Figure 9). Moreover, these organic
admixtures decreased the maximum
creep deformation attained by the
castable matrix, which improved the
performance of the material.
A comparison of the mechanical
strength of the samples dried at
50°C and the creep results (Figure
10) revealed a definite correlation.
Figure 9 Starting temperature deformation of matrix, T, with various additives, and maximum percentual
deformation, dL/L0 , attained by matrix during creep test.
dL/L0 (%)
The amounts of additive in the
matrix can be greater under terms of
field performance.1,3 Therefore, their
effect on the properties of the
castable can be intensified.
Moreover, a distinct criterion for the
selection of setting additives—
besides rebound loss2–4 and drying
behavior9—should influence the
thermomechanical properties of the
castable matrix.
σf (MPa)
Figure 10 Maximum creep strain, dL/L0 , vs mechanical strength, σf , of unfired castables cured and
dried at 50°C.
resulted in a 3-D particle-bound
structure.
The refractoriness under load and
creep behavior of the matrix can be
evaluated by the deformation (dL/L0)
as a function of temperature (up to
1500°C) and time (Figure 8). The
results indicated that the shotcrete
additives used here influenced these
properties significantly.
The addition of SS resulted in the
greatest deformation of the castable
matrix. Moreover, the SS-containing
samples presented the lowest start-
©The American Ceramic Society
ing deformation temperature (Figure
9). This result may have been caused
by the lower-temperature melting
phases in the Na2O–SiO2–Al2O3–CaO
system.
CC led to greater deformation than
did the material without additives.
However, its effect was less intense
than the SS-containing samples.
Based on these results, the inorganic
admixtures yielded the poorest
results in the RUL and creep tests.
HEC, a gelling additive, resulted in a
lower creep than the inorganic
American Ceramic Society Bulletin
Particle packing and pore-size distribution did not substantially affect
the mechanical strength after drying.
Therefore, the different values
obtained were related to the additive binding property and the RUL
and creep results to its chemistry.
Admixtures Influence Properties
The introduction of coagulant
admixtures greatly influenced the
thermomechanical properties of wet
shotcrete refractory castable matrix.
Organic additives that contained
sodium PAS and PAS + Alg improved
matrix performance in all properties
evaluated.
Commercial inorganic additives
that promote particle agglomeration
(SS and CC) decreased the mechanical strength of green samples.
Moreover, CC and, to a greater
extent, SS resulted in a higher creep
and decreased the starting temperature deformation.
www.ceramicbulletin.org
August 2005
9205
PAS and PAS + Alg increased the
mechanical strength of green and
prefired matrix samples. These additives also promoted less creep and
higher starting deformation temperatures. Additives with these coagulant/flocculation mechanisms should
be taken into greater consideration
for wet shotcreting. ■
Acknowledgments
The authors are grateful to the Brazilian
research funding agencies FAPESP and
CNPq. They also are grateful to ALCOABrazil and Magnesita for supporting this
research and to D. Vasques Filho for helping on the experimental procedure.
Refract. Appl., 8 [3] 15–20 (2003).
7D. Vasques Filho, Y.A. Marques, R.G.
Pileggi and V.C. Pandolfelli,“Influence of
the Polymeric Fibers on Shotcrete
Refractory Castables” (in Portuguese),
Ceramica, 50, 69–74 (2004).
8R.G. Pileggi, F.T. Ramal Jr., A.E. Paiva and
V.C. Pandolfelli,“High-Performance
Refractory Castables: Particle Size
Design,” Refract. Appl., 8 [5] 17–21 (2003).
9Y.A. Marques, D. Vasques Filho, R.G.
Pileggi and V.C. Pandolfelli,“Influence of
Additives on the Permeability and Drying
Behavior of Wet Shotcrete Refractory
Castables” (in Portuguese), Ceramica, 50,
7–11 (2004).
10K.H. Khayat,“Viscosity-Enhancing
References
Admixtures for Cement-Based Materials:
An Overview,” Cem. Concr. Res., 20,
171–88 (1998).
1I.L. Glassgol,“Refractory
11A.R. Studart, V.C. Pandolfelli, E. Tervoot
Shotcrete—The
Current State of Art,” Shotcrete Magazine,
[Summer] 24–32 (2002).
2M. Jolin, D. Beaupré and S. Mindess,
“Tests to Characterize Properties of Fresh
Dry-Mix Shotcrete,” Cem. Concr. Res., 29,
753–60 (1999).
and L.J. Gauckler,“Gelling of Alumina
Suspensions Using Alginic Acid Salt and
Hydroxyaluminum Diacetate,” J. Am.
Ceram. Soc., 85 [11] 2711–18 (2002).
12I.R. Oliveira, A.R. Studart, R.G. Pileggi
3L.R. Prudêncio Jr., "Accelerating
Admixtures for Shotcrete," Cem. Concr.
Res., 20, 213–19 (1998).
and
V.C. Pandolfelli,“Dispersion and Particles
Packing: Fundamental Aspects and the
Application in Ceramic Processing” (in
Portuguese), Fazendo Arte Editorial, São
Paulo, 2000; 224 pages.
4R.G. Pileggi, Y.A. Marques, D. Vasques
13J. Ding, Y. Fu
and J.J. Beaudoin,“Study of
Hydration Mechanisms in High-Alumina
Cement—Sodium Silicate System,” Cem.
Concr. Res., 26 [5] 799–804 (1996).
Filho, A.R. Studart and V.C. Pandolfelli,
“Wet-Shotcrete Additives,” Am. Ceram.
Soc. Bull., 81 [6] 51–57 (2002).
5R.G. Pileggi and V.C. Pandolfelli,
“Rheology and Particle-Size Distribution
of Pumpable Refractory Castables,” Am.
Ceram. Soc. Bull., 80 [10] 52–57 (2001).
6R.G. Pileggi,Y.A. Marques, D. Vasques
Filho,
A.R. Studart and V.C. Pandolfelli,“Shotcrete
Performance of Refractory Castables,”
14D.J. Bray, J.R. Smyth
and T.D. McGee,
“Creep of 90+% Alumina Concrete,” Am.
Ceram. Soc. Bull., 59 [7] 706–10 (1980).
15F.T. Ramal Jr., R. Salomão and V.C.
Pandolfelli,“Water Content and Its Effect
on Refractory Castables’ Drying Behavior,”
Refract. Appl., 10 [3] 10–13 (2005).
Equations
Andreasen Packing Model5,12 Equation
CPFT = 100 (D/DL)q
where CPFT is the cumulative percentage of particles smaller than diameter D, DL
the maximum diameter (CPFT = 100% when D = DL) and q the distribution
coefficient.
Splitting Tensile Strength Equation
σf = 2F/πDh
where σf is the splitting tensile strength, F the maximum force (in newtons) applied,
D the sample diameter and h the sample height.
©The American Ceramic Society
American Ceramic Society Bulletin
www.ceramicbulletin.org
August 2005
9206