What is Fly Ash?
Fly ash is a by product of coal combustion. Most commonly the material is produced by coal fired electrical generating facilities. Fly ash is a cementitious material, meaning it has certain properties that cause it to harden upon exposure to water. Typically, fly ash does not develop much compressive strength on its own. However, in the presence of Portland Cement, fly ash can develop strength characteristics very similar to cement. The fly ash reacts with a chemical by-product of the cement hydration reaction called calcium hydroxide -CaOH-. CaOH can cause deleterious effects in concrete such as increased porosity and efflorescence -the formation of calcium carbonate crystals on the concrete or mortar surface-. Because fly ash reacts with the calcium hydroxide to form more calcium silicate hydrate -the binder derivative of cement- fly ash actually adds strength to the concrete and helps to remove an agent that may be harmful. Fly ash is also particularly beneficial for use in hot weat
Fly Ash is a pozzolan. A pozzolan is a siliceous and/ or siliceous and aluminous material which in itself is not cementitious but will, in the presence of moisture and temperature > 40°F (> 5C), combine with calcium hydroxide to form cementitious compounds. The major use of fly ash is as a partial replacement for cement in concrete. The amount of cement replaced depends on the fly ash type, the fly ash quality, the application and the atmospheric conditions.
