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. We commonly use a small percentage of fly ash as a cement replacement to save money since it is much cheaper in cost. Be aware fly ash reacts with a chemical by-product of the cement hydration reaction called calcium hydroxide. This can cause deleterious effects in concrete such as increased porosity and efflorescence (the formation of calcium carbonate crystals on the concrete). 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.