Fly ash is a byproduct of the production of energy, from the combustion of coal, at coal-fired power plants. Fly ash is often recycled and, because of its pozzolanic qualities, may be incorporated as an additive in green building products or as a supplement or alternative to Portland cement in concrete production. Fly ash bricks may require less energy to produce if hardening occurs without firing and/or without Portland cement.

While fly ash is known to contain residual chemicals and heavy metals at production, the EPA reports that the process of using fly ash in brick and concrete may effectively lock up any hazardous materials, thereby minimizing leeching of any contaminants.