AC impedance is a new method to study the changes of pore structure and the hydration degree during the hydration and hardening process of cement paste by the change of the electrochemical parameters. Employing AC impedance method, we studied the hydration and hardening process of cement paste with fly ash and slag, and analyzed the influence of different hydration age, water-binder ratio and mineral admixture on the impedance parameters. Moreover, we compared the results with those by the conventional porosity testing method and X-ray diffraction method. The results showed that AC impedance could be taken as a new technology in cement and concrete research. AC impedance is a new method to study the changes of pore structure and the hydration degree during the hydration and hardening process of cement paste by the change of the electrochemical parameters. fly, ash and slag, and analyzed the influence of different hydration age, water-binder ratio and mineral admixture on the impedance parameters. Moreover, we compared the results with those by the conventional porosity testing method and X-ray diffraction method. that AC impedance could be taken as a new technology in cement and concrete research.