Cement paste hardening process studied by impedance spectroscopy

Impedance Spectroscopy measured in the range of 15 MHz to 10 kHz has been used to monitor the hardening process of cement paste with different water over cement (w:c) ratios. The cement used was of commercial grade portland cement containing pozzolans, according to the Spanish standard CEM IV/A 32.5 MR, a Portland cement containing pozzolans. The influence of sample/electrode interfacial phenomena in the impedance spectra has been eliminated by introduction of a non-contact air gap technique. The impedance spectra obtained were interpreted according to the Debye model for dielectric relaxation. The Nyquist plots of the complex permittivity showed symmetrical dispersion (Cole-Cole type) in the first 15 hardening days while asymmetrical dispersion (Cole-Davidson type) was observed later on. The calculated dielectric constant decreases with as w:c ratio increases, which confirms its relation to the variation in solid phase fraction by volume. The evolution of the fitting parameters can be explained in terms of hardening process of cement paste. (C) 1999 Elsevier Science Ltd. All rights reserved.