Electric properties of OPC hydrated under compressive load and temperature variation obtained from impedance spectroscopy

Abstract

The impedance spectra of hydrating Portland cement paste showed a small part of a large depressed arc and a single depressed arc at low and high frequency regions, respectively. A new equivalent circuit was proposed for hydrating cement paste, which consists of a serial combination of resistor and CPE with a resistor in parallel for the bulk effects and a parallel combination of resistor and a CPE for the electrode effects. The microstructural evolution in hydrating cement paste was associated with the changes of the resistances, capacitance and the critical relaxation time. The study was undertaken to investigate the electric behavior of the bulk cement paste and to separate the interface effects from the apparent impedance spectrum by using the bare and insulated electrode system. A modified equivalent circuit for insulated electrode system consists of a parallel combination of capacitance and CPE with capacitance in series. The dielectric constant and resistivity of bulk cement paste were obtained by subtracting the interface effects from the total impedance spectra of two electrode system during hydration. The spectra of complex dielectric constant were measured on a fresh cement paste with water/cement ratio of 0.4 sandwiched between insulated electrodes in the frequency range of 10 KHz - 1 MHz at temperature range between $0\,^\circ\!C$ and $-30\,^\circ\!C$. The bulk dielectric constant, 30 - 20, and conductivity, $6.14 \times 10^{-5} \sim 0.65 \times 10^{-5} \Omega^{-1}m^{-1}$, in the temperature range $-10\,^\circ\!C \sim -28\,^\circ\!C$ were much lower than those at room temperature due to the great decrease of ionic mobility by freezing of cement paste. The activation energy of 0.31 eV for the ionic conduction in fresh cement paste was obtained from Arrehenius plot of conductivity at subzero temperature. The electric and mechanical properties of cement paste under various compressive loading were respectively studied by using the impedance spectroscop...