Insights into the hydration kinetics and microstructural evolution of ultra-high performance cementitious composite at mid-to-low curing temperatures

Strength insufficiency in ultra-high performance cementitious composites (UHPCC) during winter construction has attracted attention, yet the mechanisms underlying the impacts of mid-to-low temperatures on strength development remain unclear. This study systematically investigated the mechanisms underlying the early-age strength insufficiency of UHPCC within the temperature range of 5°C to 20°C. The effects of curing temperatures on hydration reactions, microstructural evolution, and mechanical strength were comprehensively analyzed. The results revealed that during the early hydration stage (0–3d), lower curing temperatures (5°C, 10°C) significantly reduced hydration reaction rates and the formation of hydration products, leading to lower matrix density and early strength compared to 20°C, demonstrating a pronounced thermodynamic response. With prolonged curing time (7–28d), lower temperatures, particularly at 5°C, facilitated the accumulation of hydration products and pore structure refinement, partially mitigating adverse effects of initial delays. Microscopic characterizations (SEM-EDS, TGA, FTIR, XRD) confirmed that low temperatures influenced the formation and transformation of critical hydration products such as C-(A)-S-H gel and Ca(OH)₂, while potentially inducing nano- and microscale structural defects in the material matrix. Hydration kinetics analysis indicated that UHPCC hydration rates were highly sensitive to temperature, with a reduction to 5°C lowering reaction rates by approximately 58.9 %. These complex hydration behavior of UHPCC contribute to the nonlinear decline in the strength development as temperatures decrease, with strength losses reaching as high as 31.8 % at 10°C. The findings provide scientific insights and practical guidance for optimizing UHPCC performance in real-world low-temperature construction environments.