Fly ash is obtained as waste material in modern thermal stations where a mixture of air and powdered coal is burnt in place of lump coal. The ash that results from combustion resembles Portland cement in appearance. This material is used in the building industry as a partial substitute for cement or as an additive to cement and the sintered fly ash is used as lightweight aggregate for structural concrete and masonry units. Fly ash is also used for preparing cellular concrete.

The addition of fly ash to mass concrete work or ready mixed concrete plants imparts the following properties to the concrete: 1. Aggregate Reaction: The addition of fly-ash reduces the cement aggregate reaction.
2. Heat Evolution: There is low heat evolution when fly ash is used for the preparation of the concrete.
3. Permeability: The use of fly-ash greatly improves the watertightness of the concrete.
4. Placing and finishing: Fly-ash permits easier placing of concrete and finishing because of the improvement in plasticity and cohesiveness of the mixture.
5. Strength: The addition of fly-ash improves the strength of concrete. Fly-ash usually replaces 20 to 25% of cement by weight or volume. The strength of such mixture equals or exceeds at a later stage than the strength of the non fly-ash mixture because of the pozzolanic action of the fly-ash.
6. Water Requirement: It is found that the use of fly-ash results either in small reduction or no change in the quantity of mixing water required per cubic meter of concrete for a given consistency or slump.