Fly ash (Coal Fly Ash) is a by-product produced when finely ground coal is burned in a boiler at electrical generation facilities. Physically, fly ash is a very fine and powdery material that is mainly composed of silica. Adding fly ash to a concrete blend contributes to the total cementitious material available in a mix. Under certain conditions, some fly ash may be added to concrete blends to compliment the use of CSA Type 10 (Type GU) or CSA Type 50 (Type HS).