The effect of admixing nanosilica on the fresh and hardened properties of natural pozzolan (NP) based alkali activated concrete (AAC) was examined. The workability, setting times, engineering properties, durability characteristics and pore structure of concrete were evaluated. In addition, the polymerization mechanism was assessed by SEM and XRD analysis. The results indicated that there was insignificant influence of nanosilica on the flow of mortar, however, the setting times of concrete were prolonged with an increase in the nanosilica content. The prominent phases evolved in XRD pattern were philipsite and anorthite which are form of C/N-A-S-H and C-(A)-S-H gel, respectively. A greater absorption of Al and Ca was observed in the mixes prepared with sizeable amount of nanosilica, which enhanced the microstructure and pore structure characterized by fewer voids (>1000 nm) and more gel pores (<10 nm). It is postulated that both the mechanical properties and durability are beneficially enhanced by the synergistic-interaction of NP-nanosilica.