The current study investigates the performance of concrete incorporating ground granulated blast-furnace (GGBF) slag in the presence of colloidal nano-silica. A control group of concrete mixtures is compared with a group of mixtures with 50% slag replacement, with each group examined at two different ratios of colloidal nano-silica (3% and 6% of the total cementitious material). Subsequently, the relative performance of the two groups is compared with ordinary Portland cement concrete in relation to strength and durability properties. Evaluation included experimental examination of compressive and tensile strength, rapid chloride penetration, and porosimetry using mercury intrusion tests. Furthermore, the microstructure of the cementitious matrix was evaluated using scanning electron microscopy imaging. Results of the tested concrete mixtures indicated that nano-silica particles can improve the properties of concrete containing GGBF slag. Improvement in high early strength as well as reduction in permeability are observed. Furthermore, nano-silica caused a refinement of the pore structure and an improvement to the interfacial transition zone (ITZ) as seen through mercury intrusion porosimetry (MIP) results and scanning electron microscopy imaging, respectively.