Experimental Study on Freeze-Thaw Mechanical Performance Deterioration and Microcrack Evolution of Cement Matrix under Different Freezing Temperatures

To explore the influence of freezing temperature on the freeze-thaw (F-T) damage of cement matrix, two F-T cycling regimes were applied: −15 to 5 °C and −20 to 5 °C. The mass loss, surface damage, and mechanical performance evolution of cement specimens following F-T cycles were evaluated at the macro-scale. Additionally, scanning electron microscopy (SEM) and micro-indentation experiments were used to examine the microcrack evolution and micro-mechanical responses at the micro-scale. The results demonstrate that freezing temperature has a crucial regulatory effect on the F-T damage of cement matrix. Low-temperature environment accelerates the initiation and propagation of microcracks, promoting the rapid formation of microcrack networks. This subsequently causes mass loss, surface spalling, and a marked degradation in the mechanical performance of cement-based materials. Moreover, F-T damage exhibits a two-stage characteristic. The initiation and propagation of microcracks in the early stages have limited influence on the macro- and micro-mechanical properties of the material. In the later stages, microcracks rapidly coalesce into a large-scale crack network, intensifying surface spalling and causing a sharp decline in mechanical performance. This study elucidates the mechanisms of F-T degradation of cement matrix under freezing temperatures from both micro- and macro-scales. It provides experimental support for the freeze-resistance design of cement-based materials in cold regions.