This paper investigates the performance of cement mortar with the addition of various percentages of nano-silica (3%, 5% and 10%) at constant water-to-cement ratio. Early age properties of mortar along with the hydration mechanism were determined using flow test, setting time test, micro calorimetry and thermo-gravimetric analysis tests. In addition, hardened and durability-related properties using migration resistance and thermal conductivity tests were evaluated. It was observed that the workability and the setting time decreased largely according to the increase in the percentage of nano-silica as compared to the reference mortar. This finding is confirmed by micro calorimetry results, which indicate a significant increase for the heat evaluation in the presence of nano additives. The thermo-gravimetric analysis indicated the acceleration of the hydration reaction and continuation of the prolonged pozzolanic reaction, which results in the densification of the microstructure confirmed by the increase of compressive strength by 21% at 112 days for 10% addition of nano-silica as compared to the reference mortar. Moreover, the incorporation of nano-silica had also improved the durability of the mortars by reducing the effective diffusion coefficient and thermal conductivity by 46% and 40%, respectively, especially for mortar containing 10% nano-silica.