The electrical resistivity changes with time of various geopolymeric pastes with 0, 10%, 30%, 50% and 70% fly ash are continuously monitoring by using electrodeless resistivity apparatus. The physicochemical process occurred during geopolymerization process is also analysized. According to the characteristics of the resistivity evolution curves, the four stages were well defined: ion release, ion reorientation, setting and hardening, stabilization. Finally, a descriptive microstructural evolution model is proposed.