Deterioration of performances and structures of cement pastes under the action of thermal cycling fatigue

• Thermal fatigue of cement-based materials in a mild temperature range. • Quantification of microcracks based on image processing and grayscale algorithm. • The double-edged effect of the w/c ratio on the deterioration. • Evolution of macro-properties and micro-structure under thermal cycling fatigue. • The establishment of the relationship between microcracks and Vickers hardness. Concrete structures in the atmosphere are continuously subjected to thermal cycle (TC) induced by varying environmental temperature. However, the influencing mechanism of thermal cycling fatigue on the mechanical properties and microstructure of cement-based materials has not been thoroughly documented. To address this issue, the relationship between the macro-properties and microstructure of cement paste under TC was analyzed using micro-properties measurement and quantitative microcrack evaluation. In this work, ordinary Poland cement pastes with three different water-to-cement (w/c) ratios (0.4, 0.5, and 0.6) were exposed to TC in the range of 5-45 °C. The mechanical properties, pore structures, and micromorphology of cement pastes after different cycle times were measured. The result indicated that as the number of cycles increased, TC negatively affected the macro-mechanical characteristics, pore structure, and micromorphology of cement pastes. Cement pastes with greater w/c ratios were more susceptible to TC-induced deterioration. In addition, thermal cycling fatigue caused the propagation of microcracks including crack width, length, and density. Thermal cycling fatigue also coarsened the pore structure of cement pastes, especially the large pores (i.e., diameter >100 nm). The underlying damage mechanism caused by TC is the coupled actions of fluid flow, thermal deformation, and thermal fatigue. Moreover, a negative linear relationship was established between microcrack density and Vickers hardness of cement pastes.