Dielectric relaxation of space charge polarization in hydrating cement paste

Abstract

Insulated electrodes were used to avoid the effect of electrode polarization on the dielectric measurements for the hydrating cement paste. Theoretical examination for the dielectric spectrum due to the relaxation of space charge polarization was performed at two types of interface; one is insulated electrodes/conductive dielectric interface(type I) and the other is insulated electrode/conductive dielectric/partially insulated electrode interface(type II). The change of frequency dependence of complex dielectric permitivity, $\varepsilon (\omega)=\varepsilon``(\omega)-i\varepsilon"(\omega)$, during the hydration of ordinary portland cement at the two types of interface was investigated for the frequency range of 104-106Hz. The change of $\varepsilon"$ at type. 1 interface during the hydration showed sensitive change at 1MHz and good correlation with hydration stages. The water/cement (W/C) ratio dependence of the change of $\varepsilon"$ at type I interface was investigated. The change of relaxation time of space charge polarization at the type II interface was obtained from the change of complex permitivity and reflected the hydration mechanism. The W/C ratio and environmental temperature dependence of the relaxation time were determined. The electric conductivity and dielectric permitivity of fresh cement paste were determined in the temperature range between $-10\,^\circ\!C$ and $-30\,^\circ\!C$ from the temperature dependence of $\varepsilon"(\omega)$ and$\varepsilon"(\omega)$ at type I interface. Temperature dependence of the conductivity showed that the ionic conduction in fresh cement paste below $-10\,^\circ\!C$ is a activation process. The $\varepsilon``(\omega)$ and $\varepsilon"(\omega)$ for a hardened cement paste showed distribution of relaxation time below $0\,^\circ\!C$ and the relaxation time showed exponential dependence for temperature.