This paper reports the investigations on the early hydration process of fresh concrete using a non-contact electrical resistivity measurement. The bulk resistivity development (ρ(t)-t curves) and the pore solution resistivity development (ρ(t)-t curves) of fresh concrete, with and without fly ash replacement, were obtained for the period from casting to the age of 48 h. The porosity development (φ{symbol}(t)-t curves) was obtained from the ρ(t)-t and ρ(t)-t evolution curves with the calculated initial porosity and the measured porosity at 1 day by mercury intrusion porosimetry (MIP). The degree of hydration (α(t)-t) of the cement-based materials was then obtained based on the porosity change with time, which was caused by the increase of hydration products with a lower density as compared to that of the original cement in the hydration system. The comparison of the degree of hydration predicted from the porosities (φ{symbol}(t)-t) and obtained from the experimental measurement of non-evaporable water showed a good agreement of the two. The setting and hardening behavior of fresh concrete was characterized by the characteristic points on the ρ(t)-t and dρ(t)/dt-t curves. The retarded hydration caused by the fly ash incorporation could be identified by the characteristic points. © 2007 Elsevier Ltd. All rights reserved.