Pozzolana combines with lime and alkali in cement when water is added and forms compounds which contribute to strength, impermeability and sulphate resistance. It also contributes to workability, reduced bleeding and controls destructive expansion from alkali-aggregate reaction. It reduces heat of hydration thereby controlling temperature differentials, which causes thermal strain and resultant cracking n mass concrete structures like dams. The colour of PPC comes from the colour of the pozzolanic material used. PPC containing fly ash as a pozzolana will invariably be slightly different colour than the OPC.