It has long been empirically and phenomenologically understood that there exist close relations between cement hydration and its impedance spectroscopy.Much is left to be desired as to the reason why it should be so.In the present work impedance spectra of cement paste and mortar have been shown in complex plane plots along with its equivalent circuits which in its simplest case are a resistance Rs in series with a component composed of a resistance Rp and a capacitance Cp in parallel.Apart from the fact that Rs has known to be related to the porosity of the paste or the mortar, the Rp which can be measured by the diameter of a semicircle in the complex plane, is proportional to the hydration degree, while Cp is the capacitance of the double layer on the surface of the hydrated particles in the cement paste.As the hydration goes further, the circuit parameters Rs, Rp and Cp change in a definite direction.Mineral admixture such as slag and fly ash in mortar will change these circuit parameters that can be elucidated from the mechanism of the hydration process.