Fly ash isn't just filler in concrete. As a very fine-grained pozzolan, it reacts with the calcium hydroxide that is generated by the hydration reaction of cement and water to take on cementitious properties of its own. The reaction makes concrete stronger, less permeable, and reduces the alkalinity, which can reduce the danger of alkali-silica reactivity in the aggregate—and the need in some parts of the country to import nonreactive aggregate from distant sources.
Fly ash concrete is more workable and pumpable. It also hydrates more slowly, which reduces the heat of hydration—critical to reducing cracking in mass concrete placements. All of this makes fly ash concrete more durable than plain portland cement concrete. And concrete with fly ash costs less, because fly ash is typically less expensive than portland cement.
Due to all these advantages, fly ash is used in about half the concrete placed today. NRMCA estimates that 15% of cementitious materials in concrete are replaced by fly ash and other supplementary cementitious materials. Typically specified as a percentage replacement for portland cement, both Class C and Class F ash is used at dosages as high as 50% (although 15% is more common).
But the percentage of fly ash use has been increasing. To achieve a more sustainable concrete, producers and engineers have been increasing replacement rates. One recent study used fly ash at replacement rates up to 30% with no adverse results on a hard-troweled concrete floor (see “Adding Fly Ash to Concrete Mixes for Floor Construction” CONCRETE CONSTRUCTION Special Floors Issue, November 2007).
There is a limit to how much fly ash our industry can use. First, not all fly ash is equal. “Every power plant has its idiosyncrasies” says Morris “Skip” Huffman, former chairman of ACI Committee 232, Fly Ash, and division manager with Headwaters Resources, a marketer of fly ash. “The quality can vary, depending on exactly how the plant is being run. Some fly ashes have higher sulfur trioxide (SO3) contents and are not suitable for use in concrete. Some have problems with higher carbon content or they are too coarse. But the majority of ash, especially in the western U.S., is suitable for use in concrete and can go straight from the plant to the silo.”
And contractors often object to using fly ash concrete. These concerns include set time, air content, and strength gain. Some fly ash can retard hydration and lead to longer set times and slower strength gain. And some can lead to loss of entrained air. An experienced concrete producer can easily overcome these issues. In January 2006 CC, (Using High Volume Fly Ash Concrete), Lattimore Materials' Richard Szecsy notes that even at replacement rates as high as 50%, the set time and strength gain can be controlled using admixtures and proportioning.Why a hazardous waste?
No one argues that containment pond failures are acceptable—they are not. And no one doubts that there should be tighter regulations governing their design, construction, and maintenance. But to many proponents supporting fly ash use in concrete, regulating final disposal methods is a completely separate matter than designating fly ash as a hazardous waste under Subtitle C.
The problem is that the EPA wants more authority than the current law allows. At the September 2009 meeting of the Environmental Council of the States (ECOS), Matt Hale, director of EPA's Office of Resource Conservation & Recovery, said that while he believes that regulating fly ash under Subtitle D would be sufficient to protect public health and the environment, it doesn't provide the EPA with the authority to enforce more stringent disposal requirements.
InsideEPA.com reports that the agecy's current proposal is an unusual “hybrid” approach: designating fly ash as hazardous if it's disposed of in a containment pond or landfill but not hazardous when recycled for “beneficial uses.” This proposal could mean that producers and power plants would have to transport, store, and treat fly ash as a hazardous waste in its powder form, but not after it's incorporated into concrete.
However well-intended, a hybrid approach to fly ash classification would cause our nation significant problems.