Reducing Environmental Impacts of Cement and Concrete

Reducing the carbon emissions of concrete means replacing its portland cement content. But is fly ash from coal power plants a safe substitute?

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We make more concrete than any other material in the world. It is used in our roads, dams, bridges, and buildings because of its versatility, strength, and durability. Yet producing the portland cement that binds concrete together is energy intensive and emits enormous amounts of carbon dioxide (CO₂) as well as numerous other pollutants.

Fly ash can replace a percentage of portland cement in concrete, thus reducing the impacts of concrete while using a material that would otherwise be landfilled. The mercury and other heavy metals contained in fly ash have made the design and construction industry nervous about its use, however. This article explores the environmental footprint of portland cement production and future emissions regulations and looks at the issues surrounding its most common replacement, fly ash.

Portland Cement Manufacturing

Concrete is typically made up of 41% crushed rock, 26% sand, 16% water, 11% portland cement, and 6% entrained air. When combined, the cement and water form a slurry that flows between the aggregate and cures through a “hydration” process into a solid, rock-like mass.

Portland cement, the key ingredient, was patented in 1824 by British bricklayer Joseph Aspdin and now accounts for about 95% of the cement market. It is made from calcium carbonate (primarily from locally quarried limestone or chalk), silicon, aluminum, and iron (from clay, sand, and a variety of other materials). About 1.6 tons of these raw materials are required to make 1 ton of cement. In 2009, over 78 million tons (71 million metric tons or mmt) of portland cement were produced in the U.S., and 3 billion tons (2.8 billion metric tons) were produced in the rest of the world. That’s about 900 pounds (400 kg) of cement for every person on the planet.

Kiln operation

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The raw materials for portland cement are typically mined locally, crushed, sorted, analyzed for chemical composition, and carefully combined before entering a rotary cement kiln. These kilns are the world’s largest pieces of moving industrial equipment, with some reaching 25 feet (7.6 m) in diameter and 1,000 feet (305 m) in length; they rotate one to three times per minute and slope gently toward a heat source. There are two main types of kilns in use today. Older, inefficient “wet” kilns combine raw materials into a slurry before processing, while shorter dry kilns combine raw ingredients in powder form and use about 25% less energy than wet kilns. Of the 153 cement kilns currently operating in the U.S., 118 are dry and 35 are wet, according to Bruce McCarthy, president and CEO of the Portland Cement Association (PCA).

These kilns heat the raw materials to about 1,650ºF (900ºC), transforming the limestone (calcium carbonate) into lime (calcium oxide) and carbon dioxide through a reaction called calcination. Then, as the materials reach about 2,700ºF (1,480ºC), the calcium oxide reacts with the other raw materials. Carefully controlled temperatures ensure that the compounds combine, melt, and cool properly to form marble-sized pellets called clinker. After cooling, this clinker is ground into a fine powder and gypsum is added (typically 5% by weight) to aid the setting time, storage, and workability of the cement. The portland cement is then bagged at the plant or sent to precast or ready-mix facilities, where it is mixed with aggregates and water to form concrete.

Energy consumption

The Inventory of Carbon and Energy at the University of Bath in the U.K. estimates that it takes 4 million Btu to produce one ton (0.9 metric tons) of portland cement. According to the U.S. Department of Energy, portland cement production accounts for about 0.33% of the annual energy consumed in the U.S, roughly equivalent to the amount of energy in 13 million tons (12 mmt) of coal.

As high as these numbers are, it’s important to keep them in perspective. According to the University of Bath, cement has lower embodied energy per pound than steel, aluminum, fiberglass insulation, and many other building materials, and concrete’s embodied energy is significantly less—about the same as cellulose insulation. While these “cradle-to-gate” figures are not directly comparable because the materials have different properties and functions, the numbers offer some perspective.

Carbon emissions

Manufacturing portland cement accounts for approximately 5% of the anthropogenic CO₂ emissions worldwide and about 2% of total CO₂ emissions in the U.S. As construction materials go, cement production is one of the largest CO₂ emitters. Some of the CO₂ released during cement production comes from burning coal and other carbon-intensive fuels as a heat source in kilns that can run non-stop for over a year at a time. But portland cement is unique in that about half of its carbon emissions comes from calcining limestone, which emits CO₂ during the chemical reaction. This calcination process alone generated approximately 45 million tons (41 mmt) of CO₂ emissions in 2008. According to the EPA’s report, "Quantifying Greenhouse Gas Emissions from Key Industrial Sectors in the United States," energy-intensive iron and steel production generates more CO₂ than cement (114 vs 83 mmt, respectively), but less than 50% of that steel is used for construction compared with 95% of cement, making construction-based emissions approximately 57 mmt for iron and steel and 79 mmt for cement.

Mercury and Other Toxins in Cement

Cement kilns emit other hazardous substances, including chromium, arsenic, and mercury—a bioaccumulative toxin that can permanently damage the kidneys and nervous system, especially in children. Yet cement kiln emissions have been mostly exempt from regulations in the U.S., and self-reported data on emissions from the cement companies have been inconsistent. According to the environmental nonprofit EarthJustice, the estimate from the U.S. Environmental Protection Agency (EPA) for national mercury emissions from cement kilns in 2008 was 23,000 lbs per year, nearly double its own 2006 estimate.

Contributing to our lack of knowledge, some plants have been found to be underreporting emissions by significant amounts, and Cemex—one of the nation’s largest cement producers—did not report any data at all, suggesting the final amount is probably much higher. A discrepancy in data reporting at Ash Grove’s Durkee, Oregon plant led to the discovery that the plant’s mercury emissions were at least 2,582 pounds per year, making it one of the nation’s worst single-source emitters of mercury. The plant was shut down and the company has since installed mercury-control equipment, but even with these controls it has not been able to meet mercury limits imposed by the state, and the plant’s future is unclear.

The air quality around cement kilns is finally going to improve, however. In August 2010, EPA set the first serious limits for cement kiln emissions, a ruling that will have a profound impact on the environment, public health, and the cement industry. EPA estimates the new National Emission Standards for Hazardous Air Pollutants (NESHAP) rules will reduce annual emissions of mercury by 16,000 pounds (92%), total hydrocarbons by 10,600 tons (83%), particulate matter by 11,500 tons (92%), hydrochloric acid gases by 5,800 tons (97%), sulfur dioxide (SO₂) by 110,000 tons (78%), and nitrogen oxides (Nox) by 6,600 tons (5%). In addition to limiting their emissions, kilns will have to install continuous emissions monitoring systems.

EPA estimates that implementing the new limits will provide $7–$18 billion in health and environmental benefits but will cost the cement industry up to $950 million annually in 2013. PCA claims costs to the industry could be as high as $4.7 billion and result in the closure of 30 plants and the loss of 22 million tons (20 mmt) of capacity—much of which would be made up by imports whose emissions may not be regulated. The association claims that additional costs to the construction industry could total $2.3 billion annually by 2020.

Mercury sources and managing emissions

So where do all these hazardous compounds come from? People assume that the mercury in cement comes from the fuel, particularly coal, but that is only one potential source. The amount of mercury and other toxins in soils, coal, and limestone can vary substantially. The average amount of mercury in coal, for instance, is approximately 0.1 microgram per gram (0.1 part per million or ppm) but can be higher than 1.0 microgram per gram. And fly ash from high-mercury-content coal sources used as a raw material in cement kilns can also contribute to mercury emissions. But in the case of the Durkee, Oregon, facility, the majority of the mercury came from the local limestone. Whether it comes from the fuel or the raw materials, mercury is volatilized by the high temperatures in the kiln and carried out the flue along with other toxins, with a small amount collecting on the cement kiln dust. The mercury carried into the atmosphere is more dangerous than that captured in the cement kiln dust since it breaks down into bioaccumulative methyl and dimethyl mercury. Recycling the dust, a common practice in the industry, feeds it back into the kiln, which could volatilize the mercury again. Minimizing the emissions may be difficult for some plants.

Switching from coal to natural gas will eliminate the fuel-based source of mercury, and reducing the use of fly ash as a raw material will help, but in locations where limestone contains naturally occurring mercury, controlling the source may not be possible.

Portland Cement Substitutes

For now, the best option for minimizing the environmental impact of portland cement is to not use it. However, with no commercially viable alternatives, the green building community is trying to replace as much as possible with a number of alternative materials: either supplementary cementitious materials (SCMs), mineral admixtures, or pozzolans. Use of these materials lowers the embodied energy of the concrete and offsets almost one ton of carbon emissions for every ton of portland cement replaced.

There are two main types of SCMs: hydraulic materials that react with water to form cementitious compounds, and pozzolanic materials that react with calcium hydroxide in cement mixes to form additional cementitious compounds. All pozzolans have to conform to ASTM standard C618 for particle size, chemical requirements, and other attributes.

In general, these materials improve concrete’s workability and strength and make concrete less permeable, which adds to its durability and helps protect steel reinforcements from corrosion, a major source of concrete failure. They are available from natural sources, such as clay or rice husks, or are generated as waste products from industrial processes, as in the case of fly ash or slag. Using industrial waste products keeps them out of landfills, but the environmental cost of their production is significant.

Ground, granulated blast furnace slag

Ground, granulated blast furnace slag is a by-product of iron smelting. The molten slag is separated from the iron, cooled, and ground into a fine powder. Its use can prevent early temperature rise in concrete so large pours don’t overheat and crack, and can reduce the chance of an alkali silica reaction, which weakens concrete. Slag also makes concrete resistant to chemicals in the environment, such as chlorine (which corrodes steel) and sulfates (which are found in groundwater and which penetrate the concrete, weakening the bonds between cement and aggregate). Because it has a similar chemical makeup, this hydraulic slag can replace 20%–70% of portland cement in a concrete mix, depending on application.

Silica fume

Silica fume is a hydraulic material collected from the flue gases of silicon and ferro-silicon metal production. Its particles are about one-hundredth the size of a cement grain (which is about 20 microns in diamater) and are extremely reactive, creating a dense concrete capable of providing compressive strengths up to 10,000 psi. Silica fume dramatically reduces concrete’s permeability and improves chemical resistance but can only replace up to 12% of portland cement. It can be used in lightweight concrete or high-performance concrete to improve compressive strength. Silica fume requires special handling and the use of a superplasticizer (water-reducing) admixture to avoid drying out the concrete mix.

Natural pozzolans including rice husk ash

Natural pozzolans include metakaolin (made from kaolin-containing clay that is mined and then calcined at high temperatures), diatomaceous earth, and volcanic ash (the pozzolan used by the Romans to create buildings like the Pantheon). Rice husk ash is also used and has several environmental advantages over other choices: it’s a renewable, agricultural waste product, is a product of combustion for heat recovery, has low heavy-metal content, and works as a pozzolan similar to silica fume. According to structural engineer Helena Meryman, rice husk pozzolans could replace up to 20% of portland cement, and they’re particularly good when used with fly ash. Since rice husk ash accelerates early strength gain, solving a problem with fly ash concretes, using it allows more fly ash to be substituted for portland cement. Rice husk ash has great potential, but it is still a niche product and availability is limited.

Limestone

The fines from limestone processing can also be used as a direct replacement for up to 5% of portland cement content in the U.S. Canadian rules now allow levels up to 15%, and European standards allow 35%. Performance depends on the quality of the limestone, and these pozzolans are vulnerable to sulfate exposure.

Fly ash

Fly ash consists of fine spherical particulates removed from flue gases by emissions-control equipment in coal-fired power plants and is the most widely used portland cement replacement. Fly ash should not be confused with heavier bottom ash that settles out of the flue gases and does not work as a pozzolan. According to Tom Adams, executive director of the American Coal Ash Association (ACAA), a total of 136 million tons (124 mmt) of coal combustion products were produced in 2008 (that includes fly ash, synthetic gypsum, and bottom ash), with 12.5 million tons (11.3 mmt) of fly ash used as a pozzolan in concrete and 3.1 million tons (2.8 mmt) as raw materials in cement production.

Fly ash can replace up to 40% of portland cement in a standard concrete mix but is capable of replacing up to 70% in foundations, dams, and other large structures. It not only improves concrete’s performance, it also lowers the amount of water required to produce the concrete. Like other pozzolans, fly ash has to conform to ASTM standard C618 for chemical requirements that include carbon. Too much carbon, typically from incomplete coal combustion, can reduce the amount of air contained in concrete (air entrainment) and reduce freeze-thaw resistance. There are two types of fly ash: class F fly ash is pozzolanic and comes from burning anthracite or bituminous coal found in the eastern U.S., whereas class C fly ash is both cementitious and pozzolanic and comes from burning lignite or subbituminous coal mined in the West. Both work in concrete and have benefits depending on end use. For example, class C fly ash provides earlier strength gain.

The Fly Ash Conundrum

Fly ash was thrust onto the public stage by the 2008 collapse of an earthen dam at the Kingston Fossil Plant in Tennessee, which released over a billion gallons (3.8 million m³) of water and coal combustion waste into the Emory River. The tailings, a mix, of fly ash, bottom ash, and other materials, showed elevated arsenic levels in preliminary testing. Although the waste was not the same as the higher-quality fly ash used in concrete, the disaster either sullied the reputation of fly ash or raised awareness of it, depending on one’s perspective.

Since the disaster, the coal industry, environmentalists, and EPA have been struggling over whether or not fly ash should be considered toxic waste—which would dramatically alter how it is disposed of—and how “beneficial uses,” such as in concrete, will be regulated. A ruling under the Resource Conservation and Recovery Act (RCRA) ruling is expected by the end of 2010 (see “EPA Proposes Disposal Rules for Coal Ash,” EBN June 2010) and will have a profound impact on the coal industry, the environment, and the construction industry. The issue is so politically charged that the EPA has taken down its Coal Combustion Products Partnership (C2P2) website, and representatives from EPA and the Federal Highway Administration who have studied leaching from concrete aggregate told EBN that they are not allowed to talk about fly ash.

According to ACAA’s Adams, the RCRA ruling could go in one of two directions. Under Subtitle C of RCRA, fly ash would have to be disposed of as a hazardous waste with EPA enforcement. Fly ash used in concrete would get a “special waste” exemption. Adams thinks the stigma of being hazardous material may keep engineers and architects from specifying its use (although the U.S. Green Building Council has indicated it would not alter its recognition of fly ash as a recycled-content material, encouraging its use in LEED projects). Under Subtitle D, fly ash would keep its “non-hazardous” designation and disposal would be enforced only by the states. Adams says that scenario is unlikely, since it would take away EPA’s enforcement authority.

What’s in fly ash?

Fly ash contains trace amounts of various heavy metals—including arsenic, cadmium, chromium, mercury, and selenium. The chemical makeup of the fly ash is directly determined by the chemical makeup of the coal that was burned to produce it, and differences in power plant design and operation also impact the ash, which can differ from hour to hour at the same utility. And emissions-control technology, such as activated carbon injection used by some utilities, removes metals from the waste gas stream. The activated carbon and metals can then end up in the fly ash. As a result of all these factors, levels of metals such as mercury in fly ash can vary dramatically. Landfilling is the most common disposal method—although the amount of landfill space needed and the locations of these landfills remain environmental issues. When considering fly ash as a “beneficial use,” the issue of whether this somehow justifies the combustion of coal also comes into play.

Studies show little leaching

The danger of mercury leaching from concrete due to fly ash content has become a major concern in the green building community. Observers have been looking for data on how much mercury leaches from concrete in various settings. Academic research into the topic is complex and ongoing, but key peer-reviewed studies conducted to date indicate that concrete seems to do a good job of binding heavy metals—and that fly ash may even help in that process. EBN is not aware of any peer-reviewed studies that indicate leaching of mercury from concrete at unsafe levels. Below is a quick summary of some past and current research regarding leaching.

• A 1997 study, conducted in Germany by Hohberg and Schissl, showed that leaching of heavy metals from concrete products, with or without fly ash, is independent of the amount of those metals in the cement. (An industry-supported study in Hawaii had similar findings, which led to an increase in the amount of mercury allowed in fly ash used for concrete in the state.)

• A 2008 study by researchers at The Ohio State University found that fly ash concrete exposed to heat through steam curing retained 99% of its mercury content and showed final emissions similar to those of common soil.

• A follow-up study at Ohio State in 2009 that looked at both gas emissions and liquid leaching showed the amount of mercury emitted from fly ash concrete was independent of the amount of mercury in the cement. Interestingly, leached levels of mercury from fly ash concrete were lower than from concrete made solely with portland cement. Also, the addition of activated carbon lowered mercury gas emissions, suggesting that activated carbon aids in binding mercury in the concrete as well.

The latter finding is particularly significant, since higher levels of mercury found in the ash from utilities using activated carbon emission controls could, ironically, limit its use in concrete. On the negative side, carbon effects air entrainment in concrete, making this fly ash less desirable as a pozzolan. New activated carbon compounds have been developed, however, that do not impact air entrainment in concrete.

While studies like these may ease the minds of those specifying fly ash in concrete, further research on other metals, end-of-life disposal concerns, and potable water applications should be explored further.

Should We Use Fly Ash?

Despite the positive indications around mercury leaching and the benefits of using recycled content to offset portland cement, green designers remain conflicted about the use of fly ash. The pending EPA ruling has only added to the confusion.

“Replacing portland cement is a high priority for all of us,” said Russell Perry, FAIA at SmithGroup, a firm that regularly specifies fly ash and slag concretes. He hopes the leaching data is correct and that fly ash concrete turns out to be safe, but, he confided, “The thought of enabling the coal industry hurts.” In other words, by finding a use for tons of fly ash from coal combustion, are we contributing to a dependence on this dirty fuel? Perry is not alone with these sentiments. The engineering firm KPFF Structural in Portland, Oregon has gone further and stopped using fly ash altogether, replacing it with slag in its concrete because of concerns about coal as a fuel as well as metals leaching from fly ash.

Fly ash—at least when used in concrete products—has at least one prominent environmental supporter. Earthjustice, the nonprofit legal firm leading the fight for tougher coal waste disposal regulations, favors the use of fly ash in concrete. “The use of fly ash as a substitute for portland cement seems to be a beneficial reuse that avoids portland cement production and offsets greenhouse gases,” Earthjustice lawyer Lisa Evans told EBN. The alternative is placing it in the landfill where toxins may concentrate and leach into groundwater. Structural engineer Helena Meryman adds, “My preference is to use fly ash and other waste materials. Ultimately,” she said, “it is a stepping stone because we don’t want to be burning coal 100 years from now.”

From the field

Regarding fly ash and other issues, EBN asked members of the design and construction community how they minimize the environmental footprint of concrete.

• Mark Webster, P.E., project manager (and structural engineer) at Simpson, Gumpertz & Heger: “In my practice I try to get as much portland cement out of the mix as I can by substituting fly ash and blast furnace slag where possible. Replacement of 35%–40% in foundations is easily achievable where strength gain is not as critical. We structural engineers have to get away from always specifying 4000+ psi concrete when 3000 psi concrete suffices, for instance in slabs.”

• Andrew Gayer, P.E., vice president and director of operations at HOK: “A properly designed mix could replace up to 80% of cement. With the initiative in the ready-mix industry towards performance-based specifications, specifying a requirement to replace a minimum amount of cement with a variety of supplementary cementitious materials allows the local ready-mix guy the ability to use his expertise to design and produce an economical mix that is sustainable and still meets the dual requirements of the designer and the contractor.”

• Scott Shell, FAIA, EHDD Architecture: “The construction industry in the Bay Area really prefers to use slag, which acts more like cement. It’s been a struggle to get high percentages of fly ash and low cement in practice, because so many players are affected and aren’t fluent in designing or using these mixes. It affects the mix design, structural properties, placement, and finishing, and any one player can balk at the uncertainty. There are solutions, but too often this uncertainty and the requirements for new testing fall victim to the time pressures of a project. This is an area that we desperately need more work on.”

A Stepping Stone

Concrete is a uniquely useful building material well suited to our vast infrastructure and building needs. The material carries a large carbon footprint, though, and efforts to reduce that footprint using supplemental cementitious materials like fly ash are important. At the same time, providing safe buildings must also remain a priority. The recent flare-up of concern about mercury and other toxins in coal fly ash contaminating concrete may be misplaced, because those same contaminants can occur in portland cement—and there’s some evidence to suggest that contaminants are less likely to leach out of concrete that contains fly ash in the mix than from concrete that doesn’t. More research needs to be done, but if we can reduce concrete’s carbon footprint using fly ash in a way that locks up contaminants and keeps them out of landfills—where leaching is much more likely—then concrete should remain a key material for creating durable, green buildings, at least until something better comes along.

– Brent Ehrlich

For more information:

American Coal Ash Association
www.acaa-usa.org

National Slag Association
www.nationalslag.org

PCA
www.cement.org

Silica Fume Association
www.silicafume.org

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IMAGE CREDITS:
1. Photo: Bonnie Kirkpatrick
2. Source: Cradle-to-gate measurements of embodied energy taken from University of Bath Inventory of Carbon and Energy (ICE), Version 1.6a, 2008.
3. Source: Portland Cement Association
4. * Type I/II meets Type II as well as Type I criteria for compressive strength. A: Air entrained, improves resistance to freeze thaw cycles. MH: Moderate heat of hydration, adjusts amount of heat given off during curing.
5. Photo: National Slag Association
6. Photo: Headwaters Resources
7. Photo: Simpson Gumpertz & Heger
8. Photo: Michael David Rose Photography

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