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Greening the Mix:
Recycled Materials Make Concrete
More Sustainable
Mary Burger
maryburg@sbcglobal.net
UC Berkeley Landscape Architecture and Environmental Planning
LA 121: Design in Detail
May 2008
Concrete Tiles with Recycled Aluminum Aggregate
Greening the Mix
Mary Burger
Concrete, with its unique combination of compressive strength and plasticity, is an essential
material for the built landscape. Innovations of the past few decades in coloring, stamping,
stenciling, engraving, and otherwise working the surface of concrete have exploded the design
possibilities. Yet for all its usefulness, concrete hasn’t always been especially sustainable. The
quarrying, processing, and transport of heavy raw materials like sand, gravel, and rock, the high
water requirements, and the energy-intensive production of Portland cement have made concrete
one of the more environmentally costly materials.
Now that there’s overwhelming motivation among landscape designers to mitigate the
environmental impacts of our designs, how does concrete fit into the picture? Can it contribute to
the search for new, more ecologically conscious practices?
As it turns out, yes. Concrete offers a wide range of options for using recycled materials, all with
environmental benefits. Industrial by-products and post-consumer wastes are used in concrete
production as alternative materials, and even as alternative fuel source. These options can reduce
the need for raw materials, divert wastes from landfills, and decrease the embodied energy in
concrete. What’s more, some recycled content can actually improve the strength and durability of
the concrete.
Even more exciting from a design standpoint, recycled content can create visual effects not possible
with traditional concrete. Innovative designers are using recycled aggregates to create colors,
patterns, and graphic qualities that are unique and compelling. Recycled materials bring concrete
into the 21st century by supporting environmentally responsible practices and enabling designers to
push expectations and experiences of the landscape. I’ll give an overview of the kinds of recycled
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materials used in making concrete, and then highlight two designers who are leading the charge in
innovations with ‘green’ concrete.
Recycled Materials in Concrete
Options for using recycled material in concrete include supplementary cement materials (SCMs) to
replace Portland cement, recycled aggregates to replace newly-quarried sand, gravel, and rock,
waste water to replace the use of potable water, and high-energy wastes such as used tires to power
Portland cement kilns. In some cases, recycled material in concrete can earn LEED credits for a
project, for Resource Reuse, Recycled Content, Heat Island Effect, Innovation and Design Process,
and other categories.
Supplementary Cement Materials (SCMs)
In a basic concrete mix, cement reacts with water to bind aggregate in a tight, hard matrix. Cement
is the most chemically complex and energy-intensive part of the mix. Portland cement, the most
common cement, is made from limestone, chalk, and shale, in high-temperature kilns that heat the
materials to over 2500 degrees Fahrenheit. Making Portland cement releases about 1 ton of CO2 for
every ton of cement produced. Cement is around 10% by volume of concrete, but accounts for over
90% of the embodied energy. Recycled alternatives to Portland cement can mean a big reduction in
the use of energy and raw materials. Using SCMs can earn LEED Recycled Content credits.
A variety of industrial waste products can be used as SCMs. Fly ash is a by-product of coal-fired
electric power plants, consisting of captured particles that are left over after coal is burned. Blast
furnace slag is created during steel production, when iron is extracted from ore in a blast furnace.
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Silica fume results from reducing quartz in an electric-arc furnace to create silicon. Rice hull ash
uses discarded waste products from rice production.
SCMs all act as pozzolan cements, interacting with Portland cement in the curing process to cause
chemical reactions that can actually improve the strength and performance of concrete.
A little
chemistry helps to understand this: as concrete cures, Portland cement reacts with water to produce
calcium silicate hydrate, the basic chemical compound that makes concrete hard and impermeable.
Calcium hydroxide also forms in the process. Pozzolan cements react with calcium hydroxide to
create more calcium silicate hydrate, making concrete that is harder, more dense, and less porous.
(Increased porosity is actually preferred for new permeable concretes, but that’s a subject for
another article.)
Pozzolans have been used since the earliest days of concrete. Ancient Romans in the Vesuvius
region discovered that a sandy volcanic ash, called pozzolana after the nearby town of Pozzuoli,
made cement when they mixed it with powdered lime and water. Modern industrial furnaces are
like mini volcanoes, producing synthetic pozzolanic ash.
How much SCM to use varies with each project. Some designers call for maximum SCMs both to
increase the performance benefits from these materials and to qualify for more LEED credits. Fly
ash, the most common SCM, has been used to replace as much as 60% of the Portland cement in a
mix. But more isn’t always better. Since SCMs are industrial by-products, their properties vary
among different sources. SCMs change the chemical properties of concrete, and the wrong
proportion can degrade the mix.
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David Shepherd of the Portland Cement Association explains: “What many architects assume is
that fly ash is a filler material, but it actually has a significant chemical influence on concrete. And
because fly ash is a by-product, performance characteristics can vary from plant to plant. It’s not as
simple as picking an amount like 40 percent and throwing it into the mix. Within one job site, there
may be applications where no SCMs are appropriate, while in others, high volume replacement is
an ideal solution.”
The best strategy is to choose optimization, not maximization: know the properties of finished
concrete that you need, know the types of materials available, and do field testing to adjust
proportions and customize an ideal mix. A customized ternary mix, such as Portland cement, fly
ash, and slag, could work better than a prescribed formula. This approach requires designers to be
more actively involved in developing the mix, but is more effective in the long run than using one
that doesn’t perform.
SCMs have subtle effects on the color of concrete. Fly ash and silica fume create shades of gray
slightly darker than regular Portland cement. Blast furnace slag creates a lighter gray. This can be
a benefit: lighter colors have a higher albedo (reflectivity), reflecting more light and heat back into
the atmosphere and reducing the heat island effect of the concrete structure. Lighter-colored
concrete can earn LEED Heat Island Effect credits. In the past, getting lighter concrete meant using
the more expensive white Portland cement. Silica fume reduces that cost while bringing all the
benefits of recycling a waste material.
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Recycled Aggregates
Aggregates make up about 60 to 75% of a concrete mix. Aggregates are categorized as fine (less
than 3/8 inch in diameter) or coarse (3/8 to 1-1/2 inch in diameter.) Traditionally, aggregates are
newly quarried sand, gravel, or crushed rock. But many options exist for using recycled aggregates
in both structural and non-structural concrete. Depending on the source, recycled aggregates can
earn LEED credits for Resource Reuse or Recycled Content.
Crushed salvaged concrete is a readily available recycled aggregate. On some large projects a
concrete crusher can be set up on site to process old concrete for the new mix. In other cases,
demolished concrete can be hauled from a site, crushed elsewhere, and returned, or crushed
concrete can simply be bought from a recycler. Crushed concrete can be used for up to 100% of
coarse aggregates, but because of various physical properties it should replace only 10 to 20%
of fines. Crushed concrete aggregate has little effect on the appearance of concrete. But other
recycled aggregates can have dramatic visual impact, and can be used for interesting and innovative
design.
Recycled glass is a popular material for designers interested in developing effects in concrete.
Glass aggregates create surfaces that are like more exotic versions of terrazzo. Glass is salvaged
from many discarded materials, including bottles, mirrors, windows, light bulbs, and so on.
Aggregates can be transparent, opaque, frosted, textured, and just about any color imaginable.
Surfaces can sparkle or shimmer, with depth, layering, or complex patterns. (It’s worth noting that
pulverized waste glass can be used as fines, but won’t be visible.)
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Other recycled aggregates include plastics, wood chips, and even metals. In the final section of this
article I’ll highlight some interesting work by designers using recycled aggregates. But first, a few
words about water and fuel.
Waste Water in Concrete
Water makes up over 15% of a basic concrete mix. Typically this water comes from the municipal
water supply or other potable water sources. Waste water can replaced potable water and save it for
human consumption.
Waste water from concrete plants, including wash water used to clean mixers and storm water
collected on site, is well suited for reuse in concrete. The waste water has very high pH from the
lime in Portland cement and high levels of heavy metals and other contaminants. Releasing the
water into the environment harms soil, vegetation, and aquatic life. Reusing the water solves the
problem of safe disposal, and reduces the draw on potable water. The waste water has no adverse
effects on the curing process or quality of the concrete. In fact, it can decrease the porosity and
capillary action and improve the concrete’s durability.
Some types of concrete can use non-potable water from wells, streams, lakes, and even seawater,
though the water must be tested for contaminants that could interfere with the curing process.
High-Energy Wastes to Fuel Cement Kilns
Fuel costs for operating kilns are one of the biggest expenses for Portland cement manufacturers.
The cement industry has long sought alternatives to fossil fuels even before recent conditions made
this a widespread goal in the construction industry. Cement manufacturers now commonly fire
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kilns with high-energy waste products such as used tires, used motor oil, spent solvents, printing
inks, paints, and cleaning fluids.
The kilns fire at such a high temperature that the waste is actually completely destroyed. Cement
plants using waste fuels provide an effective way to dispose of hazardous materials and to extract
additional energy that would otherwise go to waste. A few innovators are even moving beyond the
use of waste materials and seeking completely renewable energy. The California Portland Cement
Company in Mojave, California has invested in wind power as part of its program for powering
kilns.
It’s not clear whether alternatively fueled cement sources can earn LEED credits for a project, but
cement plants themselves sometimes earn environmental credits. The Mojave plant was recently
awarded an EnergyStar rating from the EPA, a credential that would probably reflect well on any
building project that contracted with them.
Design Innovators in Sustainable Concrete
Mike Miller
Mike Miller, principle at The Concretist in Benicia, California, creates customized concrete
pavements, floors, walls, and furnishings. Miller and his team are continually evolving new
techniques to create color, imagery, and patterns that are beyond the ordinary. Miller doesn’t
particularly regard himself as an activist for environmentally-friendly concrete, but he’s done some
things with recycled aggregates that push the possibilities for this field.
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Miller created the pavement at the BP Helios House, a showcase gas station in Beverly Hills that
highlights green building practices. Materials, lighting, water, and energy use on the site were all
designed to maximize sustainability. For the pavement, Miller used recycled glass aggregate
including tumbled white glass, crushed green glass, and mirrors. Glass fragments cast a range of
soft and bright reflections from the pavement surface. Other elements on site reflect the recycling
ethos; the restroom wash basin (by another artisan) uses recycled aluminum.
Top left, Helios House pavement sample.
Bottom left, pavement in two shades of gray.
All Concretist photos courtesy Mike Miller
except lower left, losanjealous.com
Top right, Helios House station with solar panels.
Bottom right, sink surface in restroom.
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Miller has also used recycled aggregates in residential applications. He created an indoor/outdoor
floor at a private pool house in Orinda, using broken blue glass that the owners accented with
furnishings.
Pool house has indoor/outdoor concrete floor.
Recycled blue glass is visible in the floor.
Miller and his team walk their talk, and finish their own offices with recycled-aggregate concrete.
The floor in Miller’s partner’s office reveals a complex texture of salvaged glass chunks and even
razor blades.
Concretist office floor paving, with razor blades and glass chunks.
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Miller achieves his effects by seeding glass aggregate onto fresh concrete and then grinding the
surface with diamond tools, finishing with an acrylic sealer to give the concrete a translucent
surface. Always exploring new techniques, he likes collaborating with artists for fresh ideas.
“Most artists are people who are explorers by nature, and they’re really good problem-solvers. I’ve
continually teamed up with people like that.”
David Hertz
Architect David Hertz, an outspoken advocate for environmental building practices, has long been a
leader in using recycled materials in concrete. In the 1980s he developed a composite concrete
product called Syndecrete that uses a variety of recycled aggregates, as well as fly ash and waste
water. Syndecrete is engineered for both performance and design qualities. Hertz’ Santa Monicabased company Syndesis advertises that Syndecrete has “less than half the weight and twice the
compressive strength of ordinary concrete.” Products include tiles, slabs, countertops, and
furnishings, with custom items possible. Finishes call attention to the recycled content, and Hertz
likes to push the limits on what can be mixed into concrete. Past installations have used broken
CDs, razor blades, and even teeth. Most Syndecrete products are a little more tame than that, but
Syndesis offers a wide range of colors and aggregate patterns, using glass, wood chips, plastic,
and metal. Syndecrete can earn LEED credits for Materials and Resources, Innovation and Design
Process, and other categories.
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A sample of colors and textures available
with Syndecrete. All photos from
Syndecrete.com.
Syndecrete tiles containing recycled aluminum
with stamped text.
Syndecrete tiles used for patio and pool coping.
Custom tub cast in Syndecrete.
As a designer and an entrepreneur, Hertz is a proponent for sustainability. Remarks he made in
1994 are a touchstone for all designers committed to green building practices, and a good place to
wrap up this look at sustainable concrete:
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Balancing society's expenditure of natural and human resources should be a first priority for
everyone in the building industry. The key is 'sustainability', a word we hear more and more
often. It means providing for the needs of the present without detracting from our ability to
fulfill the needs of the future…While we are unlikely to achieve zero impact on the natural
environment, the new measure will be how much we can minimize the depletion of our
natural resources. We must look at the embodied energy of materials through their lifecycles—a cradle-to-the-grave analysis.
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Sources
A Material for a Finite Planet. David Hertz. http://www.syndesisinc.com/global/concrete.html.
1994.
BP Helios House web site. www.thegreencurve.com. 2008.
Concrete Aggregate Substitutes. ToolBase Services. http://www.toolbase.org/TechnologyInventory/Foundations/concrete-aggregate-substitutes. 2008.
Concrete Basics. Portland Cement Association.
http://www.cement.org/basics/concretebasics_concretebasics.asp. 2008.
Concrete Decor Archives — Artisan in Concrete. http://concretedecor.net/All_Access/701/CD701artisan_in_concrete.cfm. 2007.
Concrete: Scientific Principles. Materials Science and Technology Teacher’s Workshop,
Department of Materials Science and Technology, University of Illinois Urbana-Champagne.
http://matse1.mse.uiuc.edu/concrete/prin.html. Not dated.
Even When It’s Gray, Concrete Is Green. National Ready Mixed Concrete Association.
http://www.nrmca.org/GreenConcrete/default.asp. 2007.
Foundation for a Sustainable Future: Concrete Innovates. Portland Cement Association.
http://www.cement.org/buildings/sustainable_design.asp. 2007.
Helios House: Gas Station of the Future? Posted by Sung on Losanjealous, independent cultural
web site. http://www.losanjealous.com/2007/11/29/helios-house-gas-station-of-the-future/. 2007.
Miller, Mike. Interview with the author. April 2008.
New Concretes Are Designed to Deliver. Portland Cement Association.
http://www.cement.org/buildings/sustainable_mix.asp. 2008.
Ready Mixed Concrete LEED Reference Guide. Ready Mixed Concrete Research Foundation.
www.rmc-foundation.org. 2006.
Sustainable Developments with Concrete. Portland Cement Association.
http://www.concretethinker.com/Benefits.aspx. 2008.
Syndecrete web site. www.syndecrete.com. 2008.
The Concretist. www.theconcretist.com. 2007.
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