Deformation of cement-based materials under drying with thermal cycling: Mechanisms governed by C-S-H solid-moisture interactions

Thermal gradients and fluctuations complicate drying, deformation, and cracking in cement-based materials. Under thermal-cycling drying, the deformation mechanisms of hardened cement pastes (HCP) and the role of calcium-silicate-hydrate (C-S-H) solid-moisture interactions remain unclear. This study decoupled environmental factors (5–60 °C cycling, 33% RH) by combining static drying and sealed thermal-cycling conditions. Through thermal-dependent water population distributions and first desorption isotherms, the hydrothermal relationship among saturation degree, relative humidity, and pore-wall stress was quantitatively evaluated, and a conceptual model describing moisture migration pathways was proposed. Under static drying, higher temperatures lead to a lower drying shrinkage response at saturation degrees below 0.7 by weakening the pore-wall stress development. Thermal variations induce water redistribution and structural rearrangement within C-S-H. Due to C-S-H layer relocation and the reshaped hydrothermal relationship, thermal-cycling drying produces smaller shrinkage than static drying at moderate saturation degrees. Moisture-dependent ultrasonic pulse responses further suggest that hydrothermal expansion magnifies the thermal deformation at moderate saturation degrees, causing a bell-shaped saturation degree dependence of the coefficient of thermal expansion. These findings provide a decoupling framework for understanding the HCP deformation characteristics under drying with complex thermal conditions.