Fine particulate, airborne, typically amorphous, siliceous residue from burning coal in industrial burners. The chemistry of the coal and the type/chemistry of the fly ash collection system determines the composition of the fly ash. Pozzolanic (cementitious) fly ash is commonly used as an additive in cement. Non-pozzolanic fly ash is often used as a filler in wood and plastic products, in asphaltic concrete, in roofing tiles, and in other composite manufactured materials. Fly ash commonly contains a variety of heavy metals that were present in trace concentration in the unburned coal.
Cf., bottom ash