Abstract This study investigated the effect of colloidal nano-silica on concrete incorporating single (ordinary cement) and binary (ordinary cement + Class F fly ash) binders. In addition to the mechanical properties, the experimental program included tests for adiabatic temperature, rapid chloride ion permeability, mercury intrusion porosimetry, thermogravimetry and backscattered scanning electron microscopy in order to link macro- and micro-scale trends. Significant improvement was observed in mixtures incorporating nano-silica in terms of reactivity, strength development, refinement of pore structure and densification of interfacial transition zone. This improvement can be mainly attributed to the large surface area of nano-silica particles, which has pozzolanic and filler effects on the cementitious matrix. Micro-structural and thermal analyses indicated that the contribution of pozzolanic and filler effects to the pore structure refinement depended on the dosage of nano-silica.