Applying nano-technology/modifications in construction and building materials has shown inspiring results. This paper investigates the effects of nano-silica incorporation on an eco-friendly alkali activated slag–fly ash blends. The fresh behaviors, reaction kinetics, gel structure, porosity and strength of samples with different nano-silica contents and slag/fly ash ratios are analyzed. The results indicate that as the nano-silica content increases, the slump flow is significantly reduced, and the reaction process is slightly retarded according to the setting time and isothermal calorimetry results. The microstructure analysis carried out by FTIR and TG/DSC shows that the addition of nano-silica slightly increases the chemically bound water content. The main reaction product is a chain structured C-A-S-H type gel, regardless of the slag/fly ash ratio and nano-silica content. Increasing the nano-silica content up to around 2% benefits the compressive strength and contributes to a reduced porosity, but further higher nano-silica contents show a negative effect on the strength and the pore refinement becomes less significant. It is suggested that the nano-silica benefits the microstructure and strength by providing additional reactive silica source and the filler effect. Furthermore, the slag content exhibits a dominant role on setting times, early age reaction, compressive strength and porosity in this blended alkali system.