Thermal expansion kinetics of cementitious materials under different heating regimes: Implications for thermal energy storage durability

The heating rate is an important parameter controlling the thermos mechanical degradation of cementitious materials during high temperature cyclic exposure. This is relevant to thermal energy storage and fire scenarios. This paper studies the influence of heating rate on thermal expansion, thermophysical properties and mechanical stability of standardized EN 196-1 mortar. The peak strain was not very different in direction, ranging from 0.751% to 0.824% indicating nearly isotropic macroscopic behavior. The thermal expansion showed a striking increase with the rise in heating rate from 2 °C/min to 10 °C/min, attaining 0.4607% and 0.8240% at 400 °C and 600 °C, respectively, showing an increase of 52.6% and 55.4%, respectively. The coefficient of thermal expansion increased as much as 43.1% in the temperature range of 100–400 °C. The volumetric heat capacity increased slightly by 1.83–3.32% after thermal cycling at 400 °C, and rapid cycling to 600 °C resulted in a maximum reduction of 3.48%. The thermal conductivity decreased progressively, and this was more pronounced under rapid heating. Thermogravimetric analysis indicated that the total mass loss was about 8.2%. The short term cyclic repeatability of the thermal expansion curves up to 400 °C was demonstrated, while the irreversible physical and mechanical degradation could still be seen after long term cycling. After 100 thermal cycles, the compressive strength was reduced by approximately 66% at 400 °C and up to 72% at 600 °C. This was accompanied by density loss, increased porosity and water absorption, and increased development of apparent crack-like discontinuities. These findings indicate that fast heating promotes dimensional instability and the cumulative degradation of cementitious materials under cyclic loading at high temperature.