This work aims to study the effect of partial substitution of ordinary Portland cement (OPC) by fly ash (FA) on the physico-chemical and mechanical properties of the hardened on the properties of composite cement pastes (CCP). The effect of FA content was studied by the determining chemically combined water, free lime contents, total porosity, gel/space ratio, bulk density and compressive strength on the hydration progressing up to 90 days. The free lime of pozzolanic cement pastes increases up to 7 days, and then decrease up to 90 days, due to pozzolanic reaction of FA. It was found that, the increase of FA content in FA–CCP decreases the required water of standard consistency values and increases the setting times. The partial substitution of OPC with FA leads to higher porosity values, leads to decrease the compressive strength values in comparison with OPC control pastes. The lower values of free lime contents were obtained for SF–CCP with the formation of further additional amounts of C-S-H, C-A-H and C-A-S-H as a result of the pozzolanic reaction. The hydrated products formed mainly fibril C–S–H. FA show a dense microstructure rim of the inner hydrated product at the surface of reacted and unreacted FA particles.