Fly ash (FA)-based geopolymer mortar is known to exhibit less strength development at ambient temperature due to slow progress in the polymerisation reaction. In this paper, studies are reported on geopolymer mortar made of FA, ground-granulated blast-furnace slag (GGBS) and nano silica at ambient temperature. The investigations on mechanical and durability properties of FA/GGBS-based geopolymer mortar were carried out by adding nano silica at 1% and 2% weight of FA into the mixture. For all the mixtures, the concentration of sodium hydroxide (NaOH) was maintained as 3M and the sodium silicate (Na2SiO3)/sodium hydroxide weight ratio was 2·06. The alkaline liquid/binder ratio of 0·3 and ambient curing temperature (28°C) was maintained for all the mixtures. Geopolymer mortar with an addition of 2% nano silica showed significant improvement in compressive strength, split tensile strength, flexural strength and fracture properties. The water absorption, rapid chloride penetration test and sorptivity showed better resistance in mixtures with a 2% addition of nano silica. The improved properties could have been due to an enhancement of geopolymeric reaction by the addition of amorphous nano silica, which formed additional sodium aluminosilicate hydrate (Al2H2Na2O7Si) or geopolymer gel while ultrafine particles of nano silica reduced the size of empty space in the geopolymer matrix.