Mechanical properties and multiscale structure evolution of cement mortars under temperature cycling and low-humidity coupled condition

The degradation of concrete induced by wide temperature differences and drying significantly impacts the durability of constructions in plateau regions. This study systematically investigated the mechanical properties, the structure evolution from nanoscale to macroscale, and the hydration product changes under temperature cycling with low relative humidity (5-60 degrees C cycling, 30+5 % RH). Damage gradients were meticulously discussed based on the experimental measured thermal gradient and mechanical failure modes. Pore coarsening and microcracks which increased with w/c mainly occurred between 50 nm and 50 mu m, becoming key factors contributing to the mechanical degradation. Nevertheless, the relatively small damages enable the mortar to maintain much stable mechanical properties than cement paste and concrete. The hydration and the small amount of decomposition had little effect on the structure. Overall, the strength increased until a certain time window followed by a decreasing trend because of two contradictory factors: enhancement led by the increase of capillary pressure and surface energy and degradation driven by the gradient damage.