In the recent years, there is a great interest in replacing nanomaterials (NMs) in concrete structure to produce concrete with novel function and better performance. NMs are used either to replace part of cement, producing ecological profile concrete or as admixtures in cement pastes. A number of NMs such as nano silica (NS) have been explored and extensively used. This work aims to study, the effect of replacing 2, 4 and 6 mass % (NS) on physico-mechanical properties of OPC containing high CaO. The hydration behavior of cement pastes was followed by the determination of (CH) and combined water (Wn) contents with curing time. The water of consistency (W/C), setting times (IST&FST), penetration of chloride and compressive as well as flexural strengths were also estimated. The hydration products were analyzed using XRD technique. The results showed that, NS improves the hydration behavior and the physico-mechanical properties of Pozzolanic cements.  This is due to that, NS not only acts as filler to improve the microstructure, but also as activator to promote pozzolanic reaction with CH, which enhances the formation of excessive hydration products especially C-S-H. The higher beneficial role of NS is mainly due to its   high surface area, filling effect and pozzolanic activity. The blended cement with 2.0 % NS gave the optimum mechanical properties at early ages while at later ages all  cement pastes are similar and better than the blank sample.