The effect of the addition of different amounts of fly ash on the properties of ordinary portland cement (OPC) pastes and mortar were studied. Different mixes of OPC and various proportions of fly ash were prepared and cured in tap water up to 90 days. The hydration behaviour was followed by measuring the free lime and porosity. The physico-mechanical properties of each mix were studied, namely water consistency, setting times, bleeding, heat of hydration, and compressive and splitting strengths. Useful conclusions and recommendations concerning the use of fly ash as a pozzolan to replace some of the portland cement in concrete are recorded. 13 refs.