Dispersed, spherical particles of nano silica (NS) with controllable size have been synthesized using a metal alkoxide, tetraethoxysilane, as starting material and ammonia as base catalyst using sol-gel method. The size of particles and dispersivity were controlled by varying the solvent and temperature conditions of the reaction mixture. NS was synthesized using four different alcohols as solvent viz., methanol, ethanol, propanol and butanol at different reaction temperatures 25°C, 50°C, 70°C and 90°C. The results showed that the size of NS became larger and more uniform with the increase of the reaction temperature. The particle size of NS gradually increased with increasing carbon chain length of the solvent. Further, the compressive strength and setting time of cement pastes have been examined by addition of NS (-100 nm) of varied contents (0.25 %, 0.5%, 1.0%, 2.5% and 5.0%). The experimental results of these studies showed that the compressive strength of fresh cement paste was increased by increasing the content of NS up to 5% in cement paste. The setting time of fresh cement paste was decreased by increasing the content of NS. Characterization of synthesized NS particles were carried out using scanning electron microscopy (SEM) powder x-ray diffraction (XRD), ²⁹Si MAS NMR, infra-red spectroscopy (IR) and microstructure of cement paste incorporated with NS were analyzed using SEM, XRD and thermogravimetric analysis (TGA) for morphological and mineralogical attributes.