Early-age hydration of fresh concrete monitored by non-contact electrical resistivity measurement

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 (ρ0(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(t)–t curves) was obtained from the ρ(t)–t and ρ0(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(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.