Fly ash is a by-product of the burning of pulverized coal in power plants. It is removed by mechanical collectors or electrostatic precipitators as fine particles from the combustion gases before they are discharged into the atmosphere. Added to concrete, fly ash plays the dual role of fine aggregate and cementitious component. In the earliest stages of curing, it acts as an inert fine aggregate, but in the presence of moisture, the silica and alumina of the fly ash gradually react with the calcium hydroxide released in the hydration of the Portland cement. As the fly ash combines with the calcium hydroxide it slowly converts it to calcium silicate and calcium aluminate binders. In the process the amount of cement increases, enhancing the strength and reducing the permeability of the concrete.

Physically fly ash modifies the plastic properties of fresh concrete because the particles are very small and essentially spherical. Because it has a lower rate of chemical reactivity than the cement it replaces, it reduces the early heat buildup. On the minus side, it usually requires greater attention to the achievement and control of proper air content when needed. Unlike other pozzolans, fly ash does not increase the water requirement of the concrete mix. Fly ash concretes show less segregation and bleeding as well as better finishability and pumpability than plain concretes. These effects make its use particularly valuable in lean mixes or in concretes made with aggregates deficient in fines.

A common reason for using fly ash in concrete is to achieve the needed compressive strength at a lower cement content. Replacement of Portland cement by fly ash on a one-for-one basis, either by volume or weight, results in lower compressive strengths at ages up to about 3 months, but greater strengths develop at 6 months and beyond. Some of the other properties modified by the use of fly ash in concrete include deterioration caused by chemical action, and alkali-aggregate reactions.