Silica fume is a by-product in the production of silicon or its alloys. It is very fine material and has a surface area = 20 m2 /gm. It is a siliceous material and contains (85-95) % SiO2 in an amorphous state. It reacts with lime in the presence of moisture at normal temperature to form calcium silicate. The aim of the present work is to study the effect of silica fume substitution on the characteristics of ordinary Portland cement. Different mixes of OPC and various proportions of silica fume with and without superplasticizer were prepared in tap water and hydrated up to 90 days. The hydration behavior was followed by measuring the free lime and porosity. Also, the physico-mechanical properties of each mix were measured such as water of consistency, setting times compressive and splitting strengths. The results have shown that, the water of consistency increases with silica fume content which adversely affects the mechanical properties, especially at a high content (above 8.0 Wt., %). In order to improve the physico-mechanical properties of OPC-silica fume pozzolanic cement, superplasticizer must be added. The free lime content of blended cement pastes decreases with curing time and silica fume contents. Useful conclusions and recommendations concerning the use of silica fume with OPC in the production of blended cements were obtained.