Is fly ash an inferior building and structural material

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Is fly ash an inferior building and structural material?

Viewpoint: Yes, fly ash cement is an inferior building and structural material in terms of durability, safety, and environmental effects.

Viewpoint: No, fly ash has proven to be an excellent building and structural material that actually can enhance the properties of concrete and other construction resources.

Most of us don't think that much about cement; we take it for granted in many ways. In general, we know it to be a strong and durable building material. Roads made of cement wind through our cities. Our homes and businesses are often built with cement foundations. But what if someone could come up with a way to make cement even stronger and better by changing its physical and chemical characteristics? How would we decide if the change was an improvement?

Fly ash is a fine, glass powder recovered from the gases of burning coal during the production of electricity. These micron-sized particles consist primarily of silica, alumina, and iron. Fly ash can be used to replace a portion of cement in concrete. One way to measure the quality of fly ash cement is to determine whether it is truly durable and strong. Comparisons of cement made with and without fly ash raise many questions. What type of cement will fill a space best? Will the material remain safe over time? Will it be moisture resistant? Will it remain strong?

Benefits such as cost reduction and energy savings are often part of the equation. Yet, the benefits are not clear cut, because each side has a different point of view. Whether product quality or cost savings are being discussed, opponents and proponents of fly ash cement are convincing in their assessment of the issue.

There are always risks associated with any building material. In the case of fly ash cement, the issue is critical, especially when one considers that it is often used for road construction and building foundations. No one wants to be driving along and have the road collapse, or find out that the foundation of a cherished home is faulty. Proponents of fly ash cement contend that, when mixed properly, it is less susceptible to environmental stresses than cement without fly ash. Critics say that the quality of fly ash cement is too varied to make that claim. Which side you choose to agree with will probably have a lot to do with the way you determine and weigh risk.

If you find environmental issues important, then you will probably find a discussion of the hazardous elements of fly ash cement interesting. This side of the debate is often addressed with passion. Are these hazardous elements a threat to the environment? Some think so. Others say that using fly ash is good for the environment, because it is a recycled waste product. With worldwide concerns about air pollution, water pollution, and overloaded landfills, recycling waste products is obviously important. However, opponents of fly ash cement consider that the health risks caused by hazardous elements in fly ash outweigh the benefits of recycling in this instance.

The debate over the use of fly ash cement will continue for some time. Further data on the durability, safety, health, and environmental concerns regarding this material must be determined and evaluated. Even then, the debate is likely to continue, as cost and risk management is an important component of this equation. For some, the benefits of fly ash cement will always outweigh the risks; others may never be comfortable with its use.

—LEE ANN PARADISE

Viewpoint: Yes, fly ash cement is an inferior building and structural material in terms of durability, safety, and environmental effects.

In the traditional story of the three little pigs, the house the pig built out of brick was the only one to survive the onslaught of the super-winded wolf, who easily blew down the house of straw and the house of sticks and ate the other two pigs. Safely ensconced in his brick house and protected from the voracious wolf, the third little pig thought he had it made. Unfortunately, he did not realize that his house was built with bricks and a floor cemented with concrete that contained a high fly ash content. As a result, the house showed signs of deterioration much earlier and often needed repairs. The little pig also became quite mysteriously ill as time went on. And after an earthquake racked the countryside, the house fell down.

This tale presents a note of caution. Despite its widespread use, the inclusion of fly ash in concrete mixtures and as a filler in other types of building materials is problematic at best. Touted as a safe and economical way to recycle coal incinerator ash, fly ash is most often used in cement and mortar and also in place of clay, soil, limestone, and gravel for roads and other construction. The proponents of fly ash say it conserves energy by reducing the need for standard materials such as cement, crushed stone, and lime, all of which require energy to be produced. They also propose that fly ash saves costs associated with obtaining construction materials such as the naturally occurring pozzolans (volcanic ash, opaline shale, and pumicite) traditionally used for making cement. Best of all, they claim, using fly ash allows recycling of a byproduct that could otherwise cause enormous disposal problems. Although all these points are true, they are not the whole truth.

Fly Ash in Cement Construction

Naturally occurring pozzolans have been used for at least 2,000 years to make cement-like products. The Romans used pozzolana cement from Pozzuoli, Italy, near Mt. Vesuvius, to build the Colosseum and the Pantheon in Rome. Fly ash is an artificial pozzolan, with glassy spherical particulates that contain the active pozzolanic ingredient. However, fly ash is inferior to natural pozzolan.

When coal powder burns, excess amounts of carbon dioxide and sulfur trioxide are trapped inside the spherical envelopes of fly ash, giving fly ash an inconsistent chemical composition. For example, the hydration of fly ash causes the envelope (the membrane that covers fly ash particles) too prevent or slow down its reaction with calcium hydroxide during cement curing. This slower process may lead to the envelope breaking at a later stage and causing the delayed formation of crystals of the mineral ettringite (DEF) in the concrete. DEF, sometimes referred to as an internal sulfate attack, results in gaps filled with ettringite crystals that can cause cracking and peeling in the concrete.

In addition to the problems of cracking and peeling, fly ash does not control alkali-aggregate reactions in cement as well as natural pozzolan. The fly-ash envelope slows down the reaction with calcium hydroxide, a product resulting from the hydration of Portland cement (the most common cement used in construction), and the silicate inside the fly ash particles reacts with alkali in the cement. As a result, silica gels are formed and expand, causing cracking and differential movements in structures, as well as other problems such as a reduction in durability in areas where there are freezes and thaws, as well as reductions in compressive and tensile strength. In contrast, natural pozzolan, quickly reacts with calcium hydroxide, trapping the alkali inside the cement paste to form a denser paste with almost no alkali-aggregate reaction.

One of the most touted advantages of fly ash concrete is that high-quality fly ash can reduce the permeability of concrete at a low cost. However, the quality of fly ash varies widely, often depending on how hot a coal plant is burning, which influences the ash's carbon content. Low nitrogen oxide (NOx) combustion technology used to burn coal in a manner that better controls pollution often increases the carbon content of the ash, resulting in low-quality fly ash with carbon content above 10%. (The American Society for Testing and Materials [ASTM] 618 standard for building codes sets a limit of 6% carbon content, and industry preferences are set at 3% or lower.) This low-quality product can actually increase permeability and interfere with the air-entrainment process, leading to unreliable pours. Many other variables also affect the quality of fly ash and its suitability for making concrete. For example, a low tri-calcium aluminate content of 1.3% and sodalite traces can result in a substantial lowering of sulfate resistance in mortar blends. Overall, fly ash is also typically linked with slower-setting concrete and low early strength.

In addition, the use of concrete containing fly ash cement in road construction is also associated with several cautionary measures. The Virginia Highway and Transportation Research Council (VHTRC) outlined several restraints concerning fly ash concrete used to construct highways and highway structures. The council noted that special precautions are often necessary to ensure that the proper amount of entrained air is present in the fly ash cement mixtures. They also noted that not all fly ashes have sufficient pozzolanic activity to provide good results in concrete. Finally, transporting fly ash to the construction site may nullify any other cost advantage of using fly ash, and the use of a superplasticizer admixture to make fly ash less reactive to water can also cancel out cost savings.

The Recycling of Fly Ash, Health, and the Environment

The growing concern over environmental pollution that began in the 1950s and the 1960s led to stronger regulations and new technologies to reduce air pollutants. The United States Environmental Protection Agency (EPA) now estimates that 95 to 99% of particulate and organic pollutants can be removed from air emissions resulting from coal combustion. Although they are removed from emissions, these pollutants are captured as part of the fly ash from the smoke stack. Approximately 50 to 60 million tons of this fly ash are produced each year in the United States as a byproduct of coal combustion, and disposing of this fly ash has caused concern. Why? Because fly ash can contain any number of more than 5,000 hazardous and/or toxic elements, including arsenic, cadmium, chromium, carbon monoxide, formaldehyde, hydrochloric acid, lead, and mercury. Fly ash also includes harmful organic compounds such as polychlorinated biphenyls (PCBs), dioxins, dimethyl and monomethyl sulfate, and benzene.

Many of the substances in fly ash are known to have carcinogenic and mutagenic effects; and some, such as dioxins, are so toxic that experts cannot agree on a safe level of exposure. In one study, a team of ecologists at the U.S. Department of Energy's Savannah River, SC, Ecology Laboratory linked fly ash with developmental abnormalities (both behavioral and physical) as a result of high levels of heavy metals leaching into the water. For example, affected bullfrog tadpoles and soft-shell turtles had elevated levels of arsenic, cadmium, selenium, strontium, and mercury.

Although recycling fly ash into building materials may seem to be a viable alternative to disposing fly ash into waste dumps where it can leach into the soil, using a hazardous material in building products is actually waste disposal masquerading as recycling. A fundamental rule of recycling is similar to that of medicine, that is, "First, do not harm." However, the use of fly ash in construction materials is far from safe. For example, some buildings in the United States, Europe, and Hong Kong have been found to have an increase in toxic indoor air contamination which is in direct relation to fly ash that has been used as an additive in concrete to make it more flowable. In a high rise building in Hong Kong, researchers suspect that the combination of fly ash and granite aggregation in concrete causes the building to be "hot" with the radioactive gas radon when the air-conditioning systems are shut down at night and on weekends. As a result, night and weekend workers may be exposed to higher and potentially dangerous radon levels.

One especially troubling component of fly ash is dioxin, one of the best-known contaminates of Agent Orange, the notorious defoliant used in the Vietnam War. On July 3, 2001, the British Broadcasting Corporation (BBC) featured a report on its Newsnight program about highly contaminated mixtures of fly ash and bottom ash (the ash left at the bottom of a flue during coal burning) that included heavy metals and dioxin. The mixtures had been used throughout several London areas to construct buildings and roads. Tests showed that the dioxin content of the fly ash was greater than 11,000 ng/kg (nanograms per kilogram), which is much greater than the 200 ng/kg left as a result of the use of Agent Orange. (In fact, 30 years after the end of the Vietnam War scientists still find elevated dioxin levels and birth defects in human tissues in Vietnam.)

In addition to the many hazardous compounds already contained in fly ash, the use of ammonia to condition fly ash adds another environmental/health problem. Ammonia can be adsorbed by the fly ash with the flue gas train in the form of both free ammonia and ammonium sulfate compounds. During later transport and use of this fly ash, the ammonia can desorb, which presents several concerns. The primary problem associated with ammonia in fly ash is connected with waste disposal, since moisture can cause the ammonia to leach into nearby rivers and streams. However, ammonia desorbing into the air from contaminated fly ash is also a concern with its use in concrete mixtures. During the mixing and pouring of concrete, ashes with high amounts of ammonia may create harmful odors that can affect workers' health. Fly ash also poses a potential health and environmental hazard during storage before mixing, since a strong wind can scatter the fly ash, and rain can cause it to leach into the ground.

Even if the fly ash were not causing immediate harm to people or the environment as part of a construction material, "disposing" fly ash in the concrete construction materials of a building is a temporary solution at best. Little is known about the leachability of materials made with fly ash. And, if the concrete in a building is a source of environmental health problems, replacement is often not an option. Once a building is constructed, little can be done since there is no proven method of encapsulation to control emissions. In the final analysis, it is bewildering for government regulations to require industries to spend millions of dollars on antipollution devices to capture deadly toxins, but then allow these toxins to be used via fly ash in the construction of office buildings, houses, roads, and even playgrounds.

Too Many Variables

Although numerous standards, regulations, and tests are in place or available concerning the use of fly ash cement mixtures in construction, the wide variability in the quality of fly ash and its potential negative effects on human health and the environment mark it as an inferior component for use in cement. Because of its inconsistent properties and particle size (fly ash particles can range from one to 100 microns in size), it is recommended that fly ash be obtained from consistent sources, and not just from one utility plant but also from one unit at one utility. Furthermore, prices can range anywhere from $13 to $28 a ton depending on the consistency. As a result, it is not only important to test and approve each source of fly ash but also the properties of a specific fly-ash-cement combination on a project-by project basis.

Construction companies trying to meet a deadline can easily overlook such detailed and consistent testing. In addition, to reduce costs, cement manufacturers have been known to use too much fly ash (typical construction specifications permit substituting fly ash for just 15% of the cement) in the production process. As a result of one such case, after an earthquake in Taiwan in 1999, many buildings collapsed. Problems with fly ash used as a fill material in cement construction has also been documented in the United States. In Chesterfield County, Virginia, at least 13 buildings built around 1997 developed problems, including floors heaving upward and cracking, because fly ash fill that had been exposed to moisture was used in their construction.

Although the Virginia example is not the norm, it points out that regulations concerning amounts and quality of fly ash used in cement can be and have been ignored. Such occurrences are compounded because of the many variables connected with fly ash quality. Considering the long-term health and environmental hazards that may occur by spreading fly ash—which absorbs 99% of any heavy-metal contamination in whatever is burned—in cement throughout the country in buildings and roads, fly ash further loses much of its luster as a safe and effective construction material. In the final analysis, difficulties in quality control that often lead to lower-grade cement products combined with potential health and environmental problems make fly ash cement an inferior building product.

—DAVID PETECHUK

Viewpoint: No, fly ash has proven to be an excellent building and structural material that actually can enhance the properties of concrete and other construction resources.

No matter how concrete is used—buildings, bridges, roads, sewers—it normally contains Portland cement, water, aggregate, and sometimes extra ingredients. One of the most common extra ingredients is fly ash. Fly ash falls under the classification of pozzolans, compounds that react with the lime in concrete to form the hard paste that holds together the aggregate. Both natural and synthetic pozzolans are available. The natural types include processed clays or shales, volcanic ash, and other powdery compounds. A synthetic pozzolan, fly ash is a byproduct of coal combustion, which is used to generate power.

Over the years, fly ash has become a welcome addition to concrete for many reasons. Fly ash in the concrete improves concrete flow, and furnishes a better-looking, stronger, longer-lasting, and more durable finished product. Fly ash is readily available and relatively inexpensive. From an environmental standpoint, using fly ash in concrete reduces the amount of this byproduct of the burning of coal, which would otherwise be destined for burial in landfills.

Increased Strength

The advantages of concrete made with fly ash (FA-concrete) grow from its physical and chemical characteristics. Fly ash particles are small, smooth, and round in shape, attributes that allow them to move readily around the aggregate to create an FA-concrete mixture with fewer voids. On the chemical side, fly ash makes a critical contribution by reacting with the lime (calcium hydroxide) that results when cement mixes with water. The fly ash reacts with the lime to make the same binder, called calcium silicate hydrate, that is created when cement and water mix. In other words, hydrated cement yields the concrete binder along with lime, and fly ash uses that lime to make more binder. Fly ash, therefore, can be used to replace at least some of the cement in a concrete mixture.

The fly ash-lime reaction is particularly important. In traditional concrete, lime continues to be produced during and well after the original placement. The only requirement is that moisture comes in contact with the cement. This occurs from the initial water in the mix, but also from water vapor that moves through voids in the concrete, traveling from moist to dry areas, as from the damp bottom of a concrete driveway slab to the dry surface. As the moisture moves, it picks up the excess, nondurable lime and transports it, frequently resulting in a white, chalky residue called efflorescence.

FA-concrete, on the other hand, combats the excess lime by reacting with it and making more cementitious paste, which fills the voids and curtails water flow through the concrete. Voids are a marked problem in traditional concrete because they allow moisture to move more easily through the concrete, which causes additional lime leaching. Voids can also diminish the structural strength of the concrete, and fly ash provides a remedy for this. First, it diminishes the number and size of voids during the initial pour due to its physical shape—essentially serving as a microaggregate. Second, it continues to react with lime and make additional paste to fill the remaining voids. This can generate a stronger concrete than is possible with the traditional mix.

Increased concrete strength is especially important in projects that require a high strength-to-weight ratio. High-rise buildings, for example, require strong but relatively light structural components that hold up well but put minimal stress on supporting structures. Traditional concrete has a strength ceiling at a mix of about seven bags of cement per cubic yard (each bag holds 94 lb/42 kg of cement). Because FA-concrete becomes stronger over time, FA-concrete can exceed this ceiling. In addition, a thinner, and therefore a lighter, FA-concrete component can confer the same strength properties as a larger, heavier traditional concrete component.

Numerous studies have confirmed the strength enhancement in concrete made with fly ash. One of the most compelling is a 2000 study of the long-term performance of FA-concrete. The study examined the compressive strength of test elements made from different concrete mixes after they had stood for 10 years in an outdoor environment. FA-concrete exhibited the highest compressive strength, followed by traditional concrete, and then an assortment of test specimens containing different additives. The study also verified the continuing action of fly ash on excess lime, and the rise in the strength of FA-concrete over time. An early test performed at 28 days indicated that the FA-concrete was slightly less strong than traditional concrete and the other test specimens. However, the study reports, "it attained the highest strength gain, of more than 120 percent between 28 days and 10 years."

Protective Qualities

FA-concrete is also less susceptible to many of the common environmental stressors that take a toll on traditional concrete. Decreased permeability is the primary reason. FA-concrete can fend off surface assaults and sustain considerably less damage because it has fewer voids and consequently blocks deleterious substances from freely penetrating the concrete. A major concrete corrosive is salt, and particularly the chloride it contains. Salt can also have negative effects on the steel bars and latticework often imbedded in concrete as reinforcement. Both seawater vapor in coastal areas and deicing salt in colder regions can quickly ravage concrete. In various tests, FA-concrete demonstrated a greater resistance to the effects of chloride than did traditional concrete. In one study, researchers cast cylinders made from traditional and FA-concrete in high-, medium-, and low-strength mixtures. They fully submerged the cylinders in a powerful 19,350 ppm concentration of sodium chloride for 91 days, and then tested the cylinders to determine the amount of free chloride ions (charged chloride atoms) they contained. FA-concrete showed significantly fewer ions. The paper summarized, "Overall results suggest that a judicious use of fly ash in concrete-making can decrease the incidence of chloride-induced corrosion of the reinforcement in concrete structures." Similar studies show that FA-concrete is better able than traditional concrete to withstand attack by sulfates, which react with lime and can cause concrete expansion and cracking.

Beyond chloride and sulfate, concrete faces other threats, such as freezing and thawing. Unlike most compounds that contract and get smaller when they transform from a liquid to a solid state, water expands. Large voids in concrete are problematic because they can fill with liquid water. As it freezes, the ice exerts pressure on the matrix of the concrete, which can cause cracking and spalling (chipping). At the same time, some voids are desirable to allow excess moisture room to expand, so it does not cause microfractures in the concrete. FA-concrete provides an answer. For one thing, FA-concrete requires up to 10% less water in the initial mix than traditional concrete does, yet it provides the same workability. The smooth, spherical, fly ash particles can maneuver around aggregate and fill voids without as much help from water. In addition, when FA-concrete is used in the proper mix proportions, microscopic voids remain. These tiny air pockets serve as safe storage vessels for freezing water in which it expand safely without compromising the concrete's integrity.

Quality Finished Product

Strength and durability are vital to a quality finished product, and so is appearance. In this aspect, FA-concrete again improves upon traditional concrete.

The augmented flowability of FA-concrete helps ensure that the concrete will more completely fill the formwork used to contain it in its plastic state until it sets or hardens. It will spread from top to bottom and to all edges of the formwork with less work, with an even consistency, and with fewer voids. This is particularly evident along the edges of the formwork. For example, wood forms typically serve as guides for the sides of a driveway. Once the concrete is poured and the forms are removed, traditional concrete often reveals noticeable spaces where the concrete did not completely fill the form. Segregation may also be visible. In segregation, larger aggregates migrate to the bottom of the concrete and finer aggregates move to the top. On the other hand, concrete with fly ash flows much easier, filling forms more completely and more consistently, and decreasing voids and segregation. Although a driveway is usually at ground level and its sides are not usually noticeable, a decorative concrete column, arch, or other design element would require as smooth a finish as possible to be pleasing to the eye and for structural integrity. FA-concrete meets those demands.

Flowable concrete is also critical in road construction. Large road projects use slipforms that slide along with the paver (concrete-laying machinery) to provide a temporary form for the concrete. As the paver moves along the road, it pours concrete that fills a section of roadway between the slipforms. Excellent concrete flow is paramount because the concrete must flow quickly and completely between the slipforms before the paver travels on. Again, FA-concrete is an excellent choice. As a road-construction material, research also shows that it maintains a better surface finish longer than traditional concrete.

In addition, the appearance of concrete can be affected by excess lime. As mentioned earlier, Portland cement reacts with water to generate lime. As the lime leaches onto the surface of the concrete and evaporates, it leaves behind a milk-colored, powdery residue. On vertical and sloped concrete structures, streaks may occur. This problem is greatly reduced with FA-concrete. Because fly ash reacts with the lime to make the cementitious paste, less lime is available as leachate, leachate residue declines, and the finished product is more aesthetically pleasing.

Beyond Concrete

Beyond concrete, fly ash is beneficial to other materials. Recent studies review its advantages in mortar, grouts, and bricks, and in less well-known materials. A 1999 study in China, for example, reviewed the benefits of fly ash in that nation's major cement ingredient, blast furnace slag. The results of the study showed that cement containing the correct proportions of fly ash and slag provided greater strength and better pore structure than cement made with slag alone.

Construction contractors have also found fly ash to be an excellent alternative to the standard backfill blend of sand and gravel used to fill in narrow trenches excavated by utility workers, and to fill various other cavities occurring under buildings, roads, or other structures. Standard backfill is relatively inexpensive in itself, but can rapidly become costly. For instance, when filling a deep, narrow trench, workers must spread a bit of backfill, compact it, spread a bit more, compact it again, and so on, until the hole is filled. Then they level it off. Many companies are now switching to controlled low-strength material (CLSM), which has been described as "a fluid material that flows as easily as thick pancake batter and is self-leveling." This material is basically a slurry made of Portland cement, water, and a fine aggregate, which is often fly ash. While CLSM is a bit more expensive than backfill, many contractors find it saves time and money because it eliminates compacting and leveling work. In addition, CLSM normally can carry higher loads than backfill, it will not settle like backfill can, and it is receptive to removal with standard construction equipment, if required.

Pocket and Planet Friendly

Because fly ash is initially a waste byproduct of coal-burning operations, FA-concrete is a cost-effective alternative to concrete made with only Portland cement as the binder. That alone is enough to pique the interest of concrete producers and contractors. Its versatility as a construction material makes it even more attractive. Because it has greater flowability, contractors find it reduces time at the job site. In addition, FA-concrete has shown improved strength and durability, which means that buildings, bridges, roads, and sewers have the potential for longer life.

Fly ash's "green" qualities provide yet another enticement. Cement production requires heat, which uses energy and releases copious amounts of carbon dioxide, a greenhouse gas, into the atmosphere. In contrast, fly ash is a plentiful waste product. The amount of fly ash generated annually is enormous, with the United States alone producing more than 60 million metric tons every year. By using fly ash as a construction material, contractors are recycling a waste product that would otherwise find its way into a landfill, and reducing the demand for cement.

In summary, fly ash has a wide range of attributes. It saves time and money, provides strength and durability, yields improved concrete flow and workability, and helps produce high-quality finished products. It is an excellent building and structural material.

—LESLIE MERTZ

KEY TERMS

ADSORB:

Adhere in a thin layer of molecules (of gases, solutes, or liquids) to the surfaces of solid bodies or liquids to which the substances are in contact.

AGGREGATE:

Mixture of sand, gravel, slag and/or other mineral materials.

ALKALI-AGGREGATE REACTIONS:

Reactions that occur between certain types of aggregates and the alkali in the pore solutions of cement paste in concrete.

CONCRETE:

Mixture normally composed of cement, water, aggregate, and, frequently, additives such as pozzolans.

DELAYED ETTRINGITE FORMATION (DEF):

Formation of the mineral ettringite that is associated with expansion and cracking in mortars and concrete.

LIME:

Calcium hydroxide; forms in concrete when cement mixes with water.

PORTLAND CEMENT:

Most common cement used in construction, mainly composed of lime, silica, and alumina.

POZZOLANS:

Finely divided siliceous (or siliceous and aluminous) material that reacts with calcium hydroxide, alkalis, and moisture to form cement. Natural pozzolans are mainly of volcanic origin. Artificial pozzolans include fly ash, burned clays, and shales.

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