By now, you have probably heard of the controversy brewing about the U.S. Environmental Protection Agency’s (EPA) reclassification of fly ash, a coal combustion byproduct, that would make it a hazardous material. So, why does this matter and what are the facts?

First, let’s see how we got here. Currently, about half of the electricity produced in the United States comes from coal (fossil fuel) power plants. These plants produce byproducts like fly ash, silica fume and blast furnace slag. These and other “coal combustion products” were originally treated as waste and disposed of in landfills.

Fly ash is known as a supplementary cementitious material, or SCM. The use of SCMs dates back to the ancient Greeks who incorporated volcanic ash with hydraulic lime to create a cementitious mortar. The Greeks passed this knowledge on to the Romans, who constructed such engineering marvels as the Roman aqueducts and the Coliseum that still stand today.

Early SCMs consisted of natural, readily available materials such as volcanic ash or diatomaceous earth. SCMs can be divided into two categories based on their reaction type: either hydraulic or pozzolanic. Hydraulic materials react directly with water to form cementitious compounds, while pozzolanic materials chemically react with calcium hydroxide Ca (OH)2, a soluble reaction product, in the presence of moisture to form compounds possessing cementing properties.

Fly ash is a pozzolan and is by far the most widely used supplementary cementitious material in the manufactured concrete products industry because of its low cost, wide availability and concrete propertyenhancing characteristics. By partially replacing portland cement, fly ash reuse as an SCM reduces the overall material cost of producing concrete.

Fly ash was first used in large-mass concrete structures such as dams to reduce material cost and minimize the heat of hydration. Additional research revealed property-enhancing benefits of fly ash, including resistance to certain harmful chemicals; resistance to sulfate attack and alkali silica reaction; and increased durability and strength. ASTM C 618 classifies fly ash based on the sum of its primary constituents (SiO2, Al2O3, Fe2O3): when this sum exceeds 50%, it is classified as a Class C fly ash; when the sum equals or exceeds 70%, the Class F designation is used.

Precast producers like fly ash because it reduces material cost and results in stronger concrete. Owners and specifiers want greater concrete durability and the economical and environmental benefits of recycled fly ash. The U.S. Green Building Council recognizes fly ash as a pre-consumer recycled material – a beneficial strategy for CO2 reduction. The use of fly ash in concrete diverts fly ash from landfills (15 million tons [13.5 million metric tons] diverted in 2007).

Sounds like it’s a win-win for everyone – and it was until Dec. 22, 2008, when a containment dike ruptured at Kingston Fossil Plant near Kingston, Tenn., and sent 1.1 billion gallons (4.2 billion L) of coal fly ash slurry over 300 acres (122 hectares) of surrounding land, damaging homes and flowing into nearby rivers. This spill was the largest fly ash release in U.S. history. Cleanup costs are estimated to run anywhere between $525 million and $825 million, not including potential long-term cleanup. Before the Tennessee spill, the EPA categorized fly ash as a “special waste,” meaning it was exempt from federal hazardous waste regulations under Subtitle C of the Resource Conservation and Recovery Act.

Now, due to increasing pressure from environmental groups, the EPA is considering reclassifying fly ash as a hazardous waste. Although an exemption would stay in place for use as an SCM, designation of fly ash as a hazardous waste will likely lead to its removal from project specifications and national codes and standards because of liability concerns. If designated a hazardous waste, fly ash will also become more expensive to handle, making it less desirable for recycling into concrete mixes and other environmentally beneficial uses, thus increasing industrial waste and the need for even more “hazardous material” containment. Without fly ash as a recycled material component for use in concrete, the industry could face higher costs and loss of competitiveness as a green building material. And, of course, stricter regulation of fly ash would adversely affect concrete’s sustainability.

Many states and industry representatives have stepped forward to urge the EPA to consider nonhazardous waste regulation under RCRA Subtitle D, which would protect the beneficial uses of fly ash while still mandating appropriate practices for storage and disposal. As a result of these efforts, in June 2010, the EPA proposed the first nationwide rules for the disposal of ash from coal-fired power plants and opted not to classify the substance as hazardous. A 90-day public comment period will continue through Sept. 20.

NPCA will continue to monitor any new fly ash developments and has joined with the NACA (North American Concrete Alliance) group in addressing this important issue. If you have any questions about this report or the use of fly ash in concrete, please contact Claude Goguen at (317) 571-9500 or at

Claude Goguen, P.E., LEED AP, is NPCA’s director of Technical Services.