Long before the invention of portland cement, the Romans created impressive concrete structures using lime and a volcanic ash (with properties that were first discovered in Pozzuoli, Italy) that reacted with the lime and hardened the concrete. Coal fly ash, which is the particulate matter collected by pollution-control equipment from the smokestacks of coal-burning power plants, has a similar pozzolanic effect because of its silica and alumina content. Other widely used pozzolans are blast-furnace slag and silica fume.

Portland and similar cements react with water (hydrate) to create a gel that hardens by absorbing carbon dioxide. As it hardens, the cement binds aggregates (typically sand and crushed stone) together, creating concrete. When portland cement cures, it leaves behind some hydrated lime. Adding fly ash or another pozzolan allows that lime to cure as well (as in the Roman walls), making the concrete stronger and less porous.

Fly ash and other pozzolans increase the durability of concrete and can also be used to shrink its environmental footprint by reducing the amount of portland cement in the mix. Nearly a ton of carbon dioxide is emitted to produce each ton of portland cement, while fly ash is a byproduct of energy generation. Mixes in which up to 25% of the cement is replaced by fly ash are quite common, and some designers are specifying over 50% substitution for certain applications. High-volume fly ash mixes must be tested before each application because the chemistry of fly ash is more variable than that of portland cement. They also have to be managed differently as they cure, because they tend to cure and gain strength more slowly than mixes with more cement.

Use of fly ash in concrete in the U.S. is governed largely by ASTM Standard C618. This standard prohibits the use of fly ash with too much residual carbon, which indicates that the coal was not burned thoroughly enough. Residual carbon impedes air entrainment and reduces the concrete’s freeze-thaw resistance.

Fly ash is classified as Class F or Class C. Class F fly ash comes primarily from burning coal found in the Appalachian Mountains and southeastern U.S., and is purely pozzolanic in its effect. Class C fly ash tends to come from younger coals found in the western U.S. It has cementitious, in addition to pozzolanic, properties, so using it instead of some of the cement in a mix will not affect early strength gain as much.

All fly ash consists mostly of the mineral constituents of coal that don’t burn with the hydrocarbons, including a variety of potentially hazardous heavy metals such as mercury, lead, selenium, arsenic, and cadmium. There is little evidence that these metals will leach out of concrete made with fly ash, but questions remain about whether those hazardous ingredients might complicate eventual reuse or disposal of the concrete (see EBN Vol. 17, No. 9).