Phase-Transition Conductometry to Study Pore Structure Evolution in Fly-Ash Mortar

The durability of cement-based materials depends primarily on the environmental exposure conditions and micro-porous nature of the cementitious binder. In this paper, electrical impedance spectroscopy is exploited to study pore structure evolution in fly-ash mortars. To this end, mortar samples were exposed to a cyclic temperature regime over the temperature range 20oC ⇄ -60oC to establish the relation between pore size and the freezing/melting point of pore-water. It is shown that when the bulk resistivity and thermal response of the mortars were presented in an Arrhenius format, hysteresis was evident over a significant portion of the plots. Depression in both the freezing and thawing temperatures was evident and used to estimate the radii of capillary pore necks and cavities. It is also shown that due to on-going hydration and pozzolanic activity, and resulting pore structure refinement, the bulk resistivity above the freezing point increases with curing time, while that below the freezing point exhibits an opposite trend and attributed to the reduction in pore-neck radius which has a governing role on ice formation.