The electrical conductivity of a range of Portland cement-based mortars was studied over a period of 450 days hydration. The influence of thermal cycling on conductivity was investigated and the activation energy, Ea, established for conduction processes. Ea was found to be system specific and was in the range 16-30 kJ/mol (0.16-0.31 eV/ion); pozzolanic additions had the effect of increasing Ea in comparison with the plain ordinary Portland cement (OPC) mortar. Results also indicate that on the initial heating cycle, microstructural changes occurred. At the end of the test period, permeability and diffusion tests were carried out and data are presented in this respect.