This study reported on the synthesis of nano silica (NS) by sol-gel technique at different pH values (2, 4, 6, 8 & 9) and studying its application for enhance hardened cement properties. The phase analysis, particle size/morphology and specific surface area (SSA) of the prepared NS were investigated by X-ray diffraction (XRD), transmission electron microscope, atomic force microscope, and physical gas adsorption Brunauer–Emmett–Teller (BET) method. The obtained results showed that the produced NS is in amorphous phase whatever the pH values. The size of NS particles increases from 12 to 80 nm with increasing the pH value. The morphologies of NS particles depending on the P^H value. In addition, SSA decreases with pH value. Blended cement pastes were prepared by partial replacement of ordinary Portland cement with various ratios of NS changing from 0.5% to 2.5%. The compressive strength test was done at 28 days of curing; phases composition/decomposition were studied by XRD and differential thermal analysis (DTA).The results showed that as the proportion of NS increases the compressive strength increases up to 2.0% by ~ 21% compared with plain. The water permeability index of cement blended with 2.0% NS is 3.5 times less than that of plain cement.