Impedance spectroscopy measurements to study physio-chemical processes in lime-based composites

The conduction and dielectric behaviour of two different grades of natural hydraulic lime is presented over the frequency range 1 Hz-1 MHz, with measurements taken over the initial six months after gauging with water. Samples containing embedded electrodes were exposed to both a natural atmosphere (20C and 65% relative humidity) and a natural atmosphere with a carbon dioxide concentration maintained at 400 ppm which was used to accelerate the carbonation process. A decrease in relative dielectric permittivity and rise in conductivity, with increasing frequency, was observed at all stages over the time-scale presented. When plotted in the complex plane, the impedance featured a bulk response comprising two depressed semicircles and a low frequency spur, the latter being associated with the electrode/sample interface. The complex impedance plot, together with the application of an equivalent circuit model, indicated a dual arc feature with carbonation and hydration contributing to bulk impedance response. This study demonstrates the applicability of electrical property measurements to monitor the combined processes of hydration and carbonation in this group of materials.