The durability of concrete has been a major concern of civil engineering. Chemical attack caused by aggressive waters is one of the factors causing damage to concrete. The effect of nano-silica (NS) on the durability of composite cement pastes and mortars cements containing granulated blast-furnace slag (GBFS) subjected to seawater attack has been studied. Chemically combined water, free lime, total chloride and sulphate contents, as well as bulk density, compressive and flexural strengths have been determined. The durability of cement pastes has been monitored using SEM and XRD techniques. The obtained results indicate that NS improves the compressive and flexural strengths subjected to seawater up to 12 months. NS decreases the accessibility of SO42- and Cl- to penetrate the pore system, hence the total sulfate and total chloride contents decrease. The accumulation of additional hydration products within the pore system enhances the densification of cement paste matrix to form closed and compact structure with narrow pores.