Ground granulated blast furnace slag (GBFS) has been used to alter the geopolymerisation behaviour of fly ash. The influence of varying amount of GBFS (5-50%) on the reaction kinetics has been studied using isothermal conduction calorimetry. It was observed that the reaction at 27 A degrees C is dominated by the GBFS activation, whereas the reaction at 60 A degrees C is due to combined interaction of fly ash and GBFS. The reaction product of geopolymerisation has been characterised using X-ray diffraction and scanning electron microscopy-X-ray microanalysis. Alumino-silicate-hydrate (A-S-H) and calcium-silicate-hydrate (C-S-H) gels with varying Si/Al and Ca/Si ratio are found to be the main reaction products. Coexistence of A-S-H and C-S-H gel further indicates the interaction of fly ash and GBFS during geopolymerisation. Attempt has been made to relate the microstructure with the properties of the geopolymers.