Drying-rewetting cycles in ordinary Portland cement mortars investigated by electrical impedance spectroscopy

Abstract

Changes caused in the porous microstructure of ordinary Portland cement (OPC) mortars were studied using electrical impedance spectroscopy (EIS) and equivalent circuit (EqC). Two successive processes, at 20 ºC and 50 °C, consisting of several drying-rewetting cycles, were applied to the mortars. After each cycle, the electrical impedance and the amount of water absorbed were measured. The EIS-EqC methodology allowed to find two distributed impedance relaxations, associated to capillary and gel-C-S-H porosities, respectively. At room temperature any microstructural change was not detected. Nevertheless, at 50 °C two microstructural changes were inferred: 1) the volume of accessible porosity increased (pore coarsening) and 2) the surface of the conductive path through C-S-H gel became more conductive (surface smoothing).

Monitoring the pozzolanic effect of fly ash in blended OPC mortars by electrical impedance spectroscopy

“…Two relaxations are likewise found in our previous papers on electrical impedance for OPC mortars, with fractional exponents of 0.5 and 0.8, respectively [27], [39]. A decrease in exponent 0.8 by 37% is obtained after applying several cycles of drying-rewetting that reflects a change of microstructure in C-S-H gel, nevertheless exponent 0.5 barely changes [39].…”

“…Two relaxations are likewise found in our previous papers on electrical impedance for OPC mortars, with fractional exponents of 0.5 and 0.8, respectively [27], [39]. A decrease in exponent 0.8 by 37% is obtained after applying several cycles of drying-rewetting that reflects a change of microstructure in C-S-H gel, nevertheless exponent 0.5 barely changes [39]. Regarding the physical meaning of fractional exponents, the 0.8 exponent in hardened cement products is related with the fractal dimension of C-S-H gel [31], [47], [48], [49].…”

“…For mortars, a relaxation is represented in EC by a pair of elements in parallel (RQ) [27], [39]. The inverse of resistance R represents the dc-conductivity, and Q is a constant phase element which represents the frequency dependent conductivity.…”

“…The EC is composed of discrete elements R and C, o distributed elements like constant phase element CPE, o Warburg impedance. Different EC have been used with electrical elements in parallel [27], [31], [32], [33], [34], [35], [36] in series [37], [38], [39], or even specific circuits with Warburg impedance such as the Randles circuit [40], [41].…”

“…Each relaxation was represented by a pair of elements in parallel (RQ). R represents the dc-resistance of bulk pore solution and Q is a constant phase element that represents the frequency-dependent conductance related to the surface or volume around the pore solution [27], [35], [38], [39]. The chosen EC was: (RLQL)(RHQH), according to the circuit description code [50], that represents two relaxations, one of low frequency (LF) and another of high frequency (HF).…”