Effects of multiple heating-cooling cycles on the permeability and microstructure of a mortar

•Real-time variation in the gas permeability and porosity of a mortar are measured.•Multiple heating-cooling cycles will induce an increase in gas permeability.•Thermal cycles enlarge the existing microcracks and generate new microcracks. Cement-based materials can be used as a sealing layer in compressed air energy storage (CAES) underground storage caverns to ensure airtightness. This experimental study investigated the effects of multiple heating-cooling cycles with variations in the gas permeability, porosity, and microstructure of a mortar sample. Three different temperature ranges, namely, 20–50 °C, 20–60 °C and 20–70 °C, were selected according to different operation conditions of the CAES plants. The results showed that the gas permeability decreases with an increase in temperature in the heating process within each temperature cycle due to a reduction in the pore volume. After the sample was subjected to multiple heating-cooling cycles, the gas permeability has an overall increasing tendency with an increase in cycle number, and the magnitude of the permeability increase is related to the maximum cycling temperature. The microstructure observed by SEM indicates that the temperature cycles will induce some microcracks, which are mainly distributed on the cement matrix surface and in the interfacial transition zone (ITZ) between the cement matrix and sand aggregate. The expansion of the existing cracks and appearance of new cracks caused by a temperature effect led to an increase in the gas permeability.