Concept: Effects of Fly Ash on cement concrete: The following are the effects of Fly Ash on the properties of cement concrete: ● Permeability of concrete reduces. ● Improves resistance of concrete to sulphate attack ● Reduces heat of hydration in concrete such that substitution of 30% fly ash results in the reduction of 50-60% of the heat of hydration. ● Reduces the amount of air-entraining agent ● Increases the initial setting time such that 30% substitution can increase setting time upto 2 hours. ● Coarser fly ashes having high carbon content are more liable to increase dry shrinkage than finer fly ashes having low carbon content. ● Modulus of elasticity of altered concrete is lower at early ages but higher at later ages. ● Lesser compressive strength at early ages but equal strength at later ages.