Conduction, relaxation and complex impedance studies on Portland cement mortars during freezing and thawing

The influence of freezing and thawing on the complex electrical impedance of Portland cement mortars is presented with measurements obtained over the bandwidth 20 Hz-1 MHz. Samples were exposed to a cyclic temperature regime within the range - 70 degrees C to + 20 degrees C. In addition to Nyquist plots, the bulk conductivity and permittivity were de-embedded from the impedance measurements and presented in the frequency domain to study ice formation/thawing, and its influence on conduction and polarisation processes within the capillary and gel pore network. The activation energy for bulk ionic conduction and polarisation processes was also established using an Arrhenius relationship and it was shown that hysteresis was present over a significant portion of the thermal cycle. Where hysteresis effects were present, the conductivity of the sample was lower on thawing portion of the cycle than on the freezing. It was also shown that when the bulk conductivity was presented in an Arrhenius format, four distinct regions were identified on the cooling part of the cycle, whereas on the heating part of the cycle only two regions were present. It was found that the contribution of dissipative conduction processes to the overall conduction increased with decreasing temperature and with increasing frequency. The water/cement ratio is shown to have a significant influence on complex impedance measurements, the depression in freezing and melting point of the pore-water and activation energy.