The Fly Ash Revolution: Making Better Concrete with Less Cement
Two and a half months ago, a ship left Thailand loaded with Portland cement. It spent three weeks crossing the ocean and another week going through the Panama Canal before docking in New Orleans, where its cargo was transferred to barges. These barges then spent six weeks traveling up the Mississippi to Minneapolis, where the cement is now arriving at batch plants for use in concrete. Over 80 million metric tons of cement were produced last year in the U.S., but another 20 million were imported to meet the demands of our booming economy.
In spite of these imports, there are still areas of the country where suppliers cannot fill all the orders. The good news is that coal fly ash and other industrial by-products can help to stretch cement supplies while improving the durability and reducing the environmental impacts of concrete. This article explains the advantages of, and risks posed by, the use of these materials in general, and fly ash in particular.
The Cast of Characters
Known collectively as
mineral admixtures, industrial by-products from coal power plants, steel mills, and other sources have long been used in relatively small quantities to impart specific properties to concrete. Other recognized mineral admixtures include certain types of diatomaceous earth and calcined (fired) clays. Our focus here is on industrial and agricultural waste products.
Coal fly ash comes from mineral impurities in the coal that evaporate in the boiler and then condense into tiny glass spheres, 1 to 100 microns (millionths of a meter) in size, that are trapped by baghouses or electrostatic precipitators on the smokestacks. The primary ingredients of fly ash are silica, alumina, iron, and calcium. Other impurities end up in the
bottom ash, which is widely used to make granules for many functions including roofing shingles and sandblasting. Fly ash is designated as Class C or Class F, based on its chemical composition. As described in detail below, each type performs differently in concrete. Unburned carbon in fly ash can cause problems in the concrete, so fly ash for concrete use either comes from particularly clean-burning sources or is purified of its carbon.
According to the American Coal Ash Association, 55 million tonnes of fly ash were produced in the U.S. in 1997, and about 8.5 million tonnes were utilized in concrete. Another 9 million tonnes were used for a range of other applications, including stabilization or solidification of waste materials, structural fill, and road base. The remaining 37 million tonnes were landfilled or stockpiled by the power plants.
Blast furnace slag consists of limestone and other impurities that float to the surface as iron ore is melted to make pig iron. Cooled quickly with water, the slag becomes granulated. These granules can then be ground into a powder that behaves like cement, as described in ASTM Standard C 989. Ground granulated blast furnace slag is used extensively in Europe, but its availability in the U.S. has been limited, until recently, to the Eastern Seaboard. North America’s largest cement producer, Holnam, Inc., has contracted for slag with several large steel mills in the Midwest and Southeast, and is looking to import ground slag to the West Coast from Japan and Thailand. “By sometime during 2001 Holnam will be the largest supplier of ground granulated blast furnace slag in North America,” claims John Fisher, manager of mineral components for the company. Globally, about 20 million tonnes of granulated slag are produced annually.
Silica fume, also called
microsilica, is a by-product of the manufacture of silicon or ferrosilicon alloys. It is a more reactive silica source than fly ash, with particles about 100 times smaller than fly ash and cement particles. Silica fume is readily recognized as a useful additive for reducing the permeability of concrete while also increasing its strength. The amount of silica fume produced is relatively small, it is difficult to handle, and it is quite expensive to use—concrete with silica fume can cost two to three times as much as conventional concrete.
Rice hull ash, now being trademarked under the name
Agrosilica, is a newer entry into this field. If rice hulls, which consist primarily of silica, are burned in the right conditions, the resulting ash can be ground into particles that behave somewhat like silica fume in concrete. The potential quantities of rice hull ash worldwide are, like blast furnace slag, about 20 million tonnes per year. The use of rice hull ash got a bad name after proving ineffective in United Nations projects in some developing countries, but those failures were due to the use of field-burnt hulls. Unfortunately, getting the power plants that burn rice hulls to control the combustion for optimum ash production has not been easy for the executives at the company developing this technology, RHA Technology, Inc. of El Cerrito, California. In fact, they have given up on getting the ash locally in California for the time being and are negotiating to commercialize Agrosilica with a source in the Southeast.
Because of the relatively small quantities and special handling required, Agrosilica will cost up to twice as much as cement, according to Richard Peterson, CEO of RHA Technology. Agrosilica complements fly ash nicely in a mix, however, so RHA hopes to provide concrete mixes in which the higher cost of the rice hull ash is offset by the lower cost of the fly ash, resulting in high-performance concrete for the same cost as standard concrete. This approach should be tested this fall in a new Environmental Technology Center at Sonoma State University designed by A. George Beeler of AIM Associates in Petaluma, California. Much of the foundation and structure for this 2,700 ft2 (250 m2) building is to be made from concrete in which 45% of the cement is replaced by fly ash and 10% by rice hull ash.
Most of these materials, with the exception of the ground granulated blast furnace slag, are
pozzolans. To understand pozzolans, we first have to explain how cement works. Cements consist of oxides of calcium, silica and aluminum that react with water (hydrate) to create a very alkaline gel of calcium silicate hydrate. This gel then cures by absorbing CO
2 from water and from the air, which reduces its alkalinity, allowing the minerals to solidify. As it hardens, the cement binds the aggregates (typically sand and crushed stone) together, creating concrete. Blast furnace slag and some Class C fly ashes have cementitious properties, meaning that they can hydrate and cure into a binder, like cement.
As Portland cement hydrates, it also creates hydrated lime, or calcium hydroxide (Ca(OH)2), which does not contribute to the strength of the material. Pozzolanic materials represent a source of silica that can react with the lime to create additional cement gel: calcium silicate hydrate. In this process, material that had represented a weakness in the concrete is converted into additional binding agent. The still-standing edifices of ancient Rome are a testament to the durability of structures made from lime and natural pozzolans (volcanic rock).
What’s Wrong With Cement?
Standard concrete, made with Portland cement as its only cementing agent, is a miraculous building material, on which we’ve relied to build much of the urban world. It does have shortcomings, however, that may, over time, begin to constrain its use. First, more than most other building materials, the manufacture of cement is contributing to global warming. While inertia and entrenched interests have prevented serious action to address this problem so far, it may be only a matter of time before the realities of global warming become dire enough to overcome these forces.
Second, although concrete is quite strong, it is not always as durable as we’d like. The widespread premature deterioration of bridges and parking garages is a well-recognized problem. There are many reasons for these failures, and most of them can be addressed, in whole or in part, with the use of mineral admixtures.
Vol. 2, No. 2, the production of cement releases carbon dioxide (CO
2) in two ways: first, as an emission from the fuels used to create the process energy, and second, from the chemical changes in the minerals as limestone in converted into cement. The amount of CO2 released from each of these processes is about the same, for a total of one tonne CO2 per tonne of cement produced. Some of this CO2 is eventually reabsorbed into the concrete as the cement cures—but the total amount is not likely to exceed 10% of the original emissions (see
The amount of CO2 from fuel combustion is somewhat lower than we reported in 1993. This is due, in part, to newer data available from the U.S. EPA that has reduced our estimates of the amount of CO2 released from the combustion of various fuels. It is also a result of the gradual increase in energy efficiency of cement production as older wet-process kilns are retired or converted into dry-process kilns. In terms of on-site fossil fuel use, the dry-process kilns are, on average, 30% more efficient than wet kilns. No new wet-process kilns have been constructed since the mid-1970s, but as of 1997 such kilns still produced roughly 27% of U.S. cement, so the industry’s average energy efficiency still has lots of room to improve.
U.S. demand for Portland cement and masonry cement exceeded 100 million tonnes in 1998 for the first time. Even if building construction slows down, the cement industry expects demand to stay well above domestic production capacity due to projected federal spending on highways. As a result, many new cement kilns are now planned or under construction in the U.S. In spite of the relative efficiency of these newer facilities, they represent a large overall increase in CO2 emissions.
U.S. cement production represents less than 6% of world production. With 84,000 tonnes of capacity in 1997, the U.S. is the world’s third largest producer of cement, close behind Japan but only at one-sixth of China’s nearly 500,000-tonne capacity. At 80,000 tonnes, India is just behind the U.S.
In terms of greenhouse gas emissions, the 100 million tonnes of cement used annually in the U.S. represent 100 million tonnes of CO2 emissions, or about 1.9% of carbon emissions from human sources, and about 1.6% of total greenhouse gas emissions. To put this in perspective, an average car getting 25 miles per gallon (10.6 km/l) and traveling 12,500 miles (20,100 km) in a year emits about 5 tons (4.5 tonnes) of CO2. U.S. cement consumption, therefore, is equivalent in terms of global warming to 22 million passenger cars (not sport utility or light trucks). Closer to home, the impacts on CO2 emission associated with the construction of a specific building are also notable. Going from 85% Portland cement to 50%, and making up the difference with fly ash, in a typical 3,000 psi (20 Mpa) mix results in a reduction of 175 lbs of cement per cubic yard (104 kg/m3). This translates to one ton of CO2 emissions reduced for each 11.4 cubic yards of concrete (or one tonne for each 9.6 m3).
The 1.5 billion metric tons of cement produced worldwide in 1997 account for over 6% of carbon emissions, based on data from the Carbon Dioxide Information Analysis Center at Oak Ridge National Lab. The steady increase in cement production nationally and globally represents a barrier to efforts to stabilize—and reduce—carbon emissions. Using large quantities of readily available industrial by-products to reduce the amount of cement in concrete is one promising solution. Increasing the fraction of mineral admixtures in all concrete from 15% of the cementing materials to 50% would eliminate up to 600 million tonnes of CO2 emissions—equivalent to removing one quarter of all cars in the world. This level of replacement is the maximum possible, because it would require utilization of all fly ash produced.
Over time, standard cements and concrete mixes have become optimized for the performance characteristic that is most often demanded by building contractors in a hurry: early strength. High early strength has typically been achieved by grinding cement more finely, according to John Fisher of Holnam, and by using more of it in the mix. The resulting concrete, however, is more susceptible to some types of failure than concrete that has matured more slowly.
For example, the hydration of cement releases heat, so concrete with high levels of cement tends to experience significant heat buildup, which can lead to cracking due to the temperature differences between the interior and the surface of the concrete. Even in small concrete elements, this cracking may result in imperceptible weaknesses in the concrete. In very massive structures, such as dams, this heat buildup is a problem even without excess cement, and the use of pozzolans, as explained below, is commonplace.
Another more recent factor affecting durability is that cement tends to be more alkaline now than it used to be, according to Dean Golden of the Electric Power Research Institute (EPRI). This change, Golden explains, is due to the fact that cement producers, under pressure to reduce their emissions, are now trapping very alkaline compounds such as chlorides from their flue gases and mixing them into the cement. More alkaline (higher pH) cement is more likely to expand due to a reaction with high-silica aggregates in the concrete—a problem known as the alkali-silica reaction (ASR).
Among the different options for addressing these problems, the use of fly ash to replace some of the cement stands out as a low-cost solution with multiple benefits, although the possibilities and benefits vary according to the type of fly ash used. In its Standard C 618, the American Society for Testing and Materials (ASTM) recognizes Class F and Class C fly ashes, based on their chemistry. In terms of its performance in concrete, the amount of calcium in fly ash is an important indicator, and Class F fly ash is lower in calcium than Class C. Class C fly ash that is particularly high in calcium contains some of that calcium in crystalline form, in addition to the calcium in the glass spheres. This crystalline calcium gives Class C fly ash cementitious properties, in addition to its pozzolanic action.
The class of fly ash depends primarily on the coal it comes from. Bituminous coal, found primarily in the Appalachian Basin and Illinois Basin, produces Class F fly ash. Subbituminous coal tends to produce Class C fly ash, but the type produced varies, depending on the calcium content in the coal. Lignite coals also vary—Texas lignite produces a Class F fly ash, while North Dakota lignite creates Class C. The largest source of high-calcium Class C ash is coal from the Powder River Basin in Wyoming. This coal is both relatively inexpensive and very low in sulfur (which allows existing power plants to meet emissions requirements without adding expensive scrubbers), so it is widely used throughout the central and mountain states of the U.S.
The glass spheres from either type of fly ash improve the workability of the concrete, both by their mechanical action and by defusing the electrical charge that causes cement particles to clump together. As a result, fly ash allows for reductions in the amount of water needed in the mix, which makes the concrete stronger and more durable. By reducing the amount of cement and gaining strength more gradually, high-volume fly ash mixes generate much less heat—and this heat is less likely to cause cracking if the concrete has not yet solidified.
The small particles of fly ash, rice hull ash, or silica fume also improve concrete by filling the small voids around the cement particles where water can collect. By filling these pores, they greatly reduce the bleeding of water to the surface as the concrete cures. This lack of bleed water makes for stronger concrete, but it also makes concrete slabs less forgiving in terms of finish. More significant, by filling the voids, these materials can make concrete virtually impermeable to intrusion of elements that can cause damage.
Damage of concrete due to rust on the reinforcing steel is generally attributed to intrusion of chloride ions. By preventing such intrusion, high-volume fly ash concrete is less susceptible to this damage. Sulfate attack, another threat to concrete, comes from certain soils or waters. It is reduced both by the lower permeability and by a change in the internal chemistry of the concrete: sulfates attack concrete by reacting with calcium hydroxide, but pozzolans—especially Class F fly ash—eliminate much of this chemical. Concrete made with 60% or more ground granulated blast furnace slag is widely recognized for its resistance to sulfate attack.
The alkali-silica reaction described above is minimized by pozzolans that reduce the alkalinity. Class F fly ash is very effective in this regard, while the performance of Class C ash varies. In response to this benefit, the California Department of Transportation now routinely requires at least 25% Class F fly ash in its projects, and will allow up to 35%.
The most commonly raised concern about high-fly-ash concrete is that it gains strength more slowly. In general this is true, because the pozzolanic reactions take place only after much of the cement has cured, and because these mixes have much less cement. Class C fly ash, because it has cementitious properties of its own, will generally retard strength gain less than Class F fly ash. There are other measures that can be taken to minimize this effect if necessary.
The low early strength can be minimized by keeping the water content very low. Dr. V. M. Malhotra of Natural Resources Canada has done extensive testing with up to 65% Class F fly ash combined with superplasticizer additives to minimize water use. Another researcher, Dr. P. K. Mehta, Professor Emeritus at the University of California at Berkeley, is pioneering the use of high-volume fly ash mixes without such additives. His mixes are less expensive, but they also don’t flow quite as well and take longer to gain strength than those of Malhotra.
Because the small fly ash or other pozzolanic particles fill voids in the concrete, more air-entraining chemicals may be needed to ensure adequate freeze-thaw protection. Malhotra has determined that 56% Class F fly ash is a maximum reasonable level when air entrainment is needed to prevent freeze-thaw damage, while up to 65% is workable when freeze-thaw is not an issue. Rice hull ash may not have this problem, because the ash particles themselves contain many voids.
Residual carbon in fly ash also tends to reduce the necessary air pockets, so carbon in fly ash used in areas subject to freezing should be minimized. ASTM C 618 restricts carbon content (referred to as “loss on ignition”) to 6% of the fly ash by weight, though 4% is the most the market will accept, according to EPRI’s Dean Golden. Modern, efficient coal-burning plants now produce fly ash with 1% carbon or less, but some older plants are not so good. In addition, restrictions on nitrogen oxide emissions are forcing some plants to operate at suboptimal temperatures, further increasing carbon content. Many fly ash brokers now have processes to remove excess carbon, which they can sell separately, so low-carbon fly ash is available even from less efficient power plants.
The ultimate strength gain of concrete with pozzolans depends on water being locked up within the material for later reactions, so proper curing, with adequate protection against drying, is even more important than for typical concrete. Laboratory tests show significant damage to high-fly-ash concrete from de-icing chemicals. Test sections of sidewalk in Halifax, Nova Scotia do not exhibit similar problems, so this issue is currently unresolved.
Fly ash makes its way into concrete products in three ways: 1) as an ingredient (silica source) in the cement itself through being fed into the cement kiln (this route does not save much by way of energy or CO
2 emissions); 2) as a component of a
blended cement—a cement that is sold with the pozzolanic fly ash mixed in (these are designated by a “P” in the type, as in “Type 1P”); or 3) as a separate ingredient added to the concrete at the ready-mix or batch plant. Most advocates of increased fly ash use in concrete discourage the use of blended cements, because these tend to include only 5% to 25% fly ash, and they may discourage batch plants from stocking fly ash separately and providing high-volume fly ash mixes when appropriate. Cements blended with blast furnace slag are not similarly limited—they often contain 50% or more of the slag.
One might think that cement companies would be resisting the move towards increased use of mineral admixtures apart from their inclusion in blended cements, but that is not true for some of the biggest companies. “Whether or not you agree that CO2 is causing global warming is immaterial,” says Holnam’s John Fisher, “We can use more mineral components right away.” Discussing the barriers to the use of mineral admixtures, Fisher acknowledges that their stance was not always as proactive: “We’re partly responsible for this attitude—in the 1970s we lobbied against the use of mineral components.”
Coal power plants are not as widely distributed, nor as close to most construction sites, as cement plants. As a result, it is likely that fly ash will have to be transported a longer distance than cement, which could offset some of the benefits. This drawback is not all that significant, however, because coal plants are all served by rail, which is a much more efficient means of transportation than trucking. An analysis by Jon Madderom of ISG Resources, based on actual energy use in hauling, shows that moving fly ash 1,000 miles (1,600 km) by train requires about 310,000 Btus per ton (360 MJ/tonne), or about 40% more than moving an equivalent amount of cement 100 miles (160 km) by truck. This transportation energy use amounts to only about 5% of the 5,600,000 Btus/ton (6,500 MJ/tonne) required to make the cement.
Using fly ash has another environmental benefit, beyond the durability and avoided cement use. The huge quantities of fly ash that are stockpiled and ponded at power plants or landfilled nearby not only take up space, they also present a risk in terms of leaching of trace heavy metals. Once in the concrete, these metals are effectively locked up in the cementitious matrix and pose little threat. In fact, given the common practice of burning waste materials such as tires and solvents in cement kilns, it is possible that the fly ash is less of a health threat than the cement.
A more reasonable potential health threat is a possible increase in exposure to gamma radiation from radium-226, because fly ash has, on average, somewhat more radium-226 than cement, sand, or soil. An analysis of this risk by the EPA published in the January 28, 1983 Federal Register concludes that there is a slight increased risk of exposure to gamma radiation due to this difference, but that this increase is offset by a reduced likelihood of exposure to radon gas, since the radon gas is less likely to escape from the glass fly ash spheres and from the relatively impermeable fly ash concrete. Since inhalation of radon daughter products is considered the most significant exposure threat, it’s possible that the use of fly ash actually makes the concrete safer.
Yet another possible concern with the use of high-volume fly ash mixes is the possibility of offgassing of chemicals, such as formaldehyde, from any superplasticizers used. These chemicals are described generically as sulfonated melamine-formaldehyde and sulphonated napthalene formaldehyde condensates. These admixtures are widely used in the concrete industry and are generally considered safe, but their use could pose a problem to occupants with chemical sensitivities. As with other materials, it is best to test a sample of such concrete on any sensitive individuals before using it in a building. If the construction can be managed to avoid the need for high early strength, both money and possible chemical exposures will be reduced by avoiding these admixtures.
The Civil Engineering Research Foundation is preparing a major performance verification initiative on the use of industrial waste materials in concrete. If these tests corroborate the performance benefits that many advocates of fly ash and the other pozzolans describe, they should go a long way in helping to interest the engineering community in these materials. Along similar lines, fly ash supplier ISG Resources, Inc., is funding testing of high-fly-ash mixes in the San Francisco Bay area to increase the comfort level of engineers with this approach.
Meanwhile, several companies, including ISG and an Atlanta company called Mineral Resource Technologies (MRT), are advertising blended cements that are as much as 90% Class C fly ash. On a similar front, fly ash expert and concrete supplier Ramon Carrasquillo of the University of Texas at Austin is currently testing concrete made with 100% replacement of cement by Class C fly ash for possible use in the Biomedical Sciences building at the University of Texas at Houston. According to Carrasquillo, the University has endorsed this approach as part of its commitment to radically reducing the environmental impacts of the building.
The construction industry has never been quick to change, and there are many in the engineering community who will continue to specify pure Portland cement in their mixes until they are forced to do otherwise. For the rest, however, the increasing availability and track record of these mineral admixtures represents a great opportunity to provide a better product while also protecting the environment. In the words of ecologically inclined structural engineer Bruce King of Sausalito, California, “It’s one of the few clean, simple no-brainers in green building, because the technology is worked out, the cost is usually equal or less, and the quality of the product is far higher. That’s pretty easy to sell to clients.”
– Nadav Malin
For more information:
Proceedings from a December 8, 1998 Forum entitled
Concrete, Flyash, and the Environment are available on
EBN’s Web site:
www.buildinggreen.com. This forum was sponsored by EHDD Architects and the Pacific Energy Center in San Francisco, California.