Fly ash is an extremely fine powder consisting of spherical particles less than 50 microns in size.  Fly ash is one of the construction industry’s most commonly used pozzolans.  Pozzolans are siliceous or siliceous/alumino materials possessing the ability to form cementitious compounds when mixed with lime [calcium hydroxide (CaOH)] and water.

When portland cement is mixed with water, most of the cement forms insoluble cementitious compounds, CaOH is formed as part of this reaction.  When fly ash is introduced into concrete, it reacts with the CaOH to form additional cementitious compounds.  In a properly proportioned mix, fly ash can improve many of the properties of concrete, including improved workability and consolidation, increased flexural and compressive strengths, improved pumpability, and decreased permeability.