Geopolymer belongs to the family of aluminosilicate polymers. The production of geopolymer generally involves the combination of silicon and aluminium atoms in a hydroxide-promoted activation, followed by a condensation reaction to form an aluminosilicate gel which gives the final hardened binder. Reactivity of geopolymer system is influenced by nature of alkaline activator, ratio of silicon to aluminium, curing time, pH and degree of polycondensation. Fly ash and ground granulated blast furnace slag have been utilized for the synthesis of geopolymers at ambient condition which does not require energy. The investigationaims at assessing the reactivity of geopolymer through various approaches such as degree of reactivity (by means of insoluble residue), infra-red spectroscopy and free alkali content involving various activators and admixtures. The results show that potassium based geopolymer with higher level of alkalinity shows morereactivity. Energy required for the production of geopolymer pastes with sodium silicate as activator ranges from 3.65 to 4.37 MJ/kg, with potassium silicate as activator ranges from 2.24 to 4.45 MJ/kg as against 4.79 MJ/kg for ordinary Portland cement paste. For geopolymer pastes using high volume fly ash 4% less carbon foot print is possible compared to ordinary Portland cement paste.