Effects of nanomaterials on early-age properties and microstructure of calcium sulfoaluminate-Portland cement-based repair mortar under −10 °C curing

To meet the repair needs of concrete structures in high-latitude and high-altitude areas during winter, improving the early-age properties of repair mortar under negative temperatures has significant engineering significance. Due to the dramatically retarded hydration rate of cement in such environment, nanoadditives are employed to accelerate cement hydration. This study investigates the effects of two types of nanomaterials, 0–3 % nano-SiO 2 (NS) and C-S-H seeds (CSHs), on the setting time, mechanical properties, hydration temperature rise, and microstructure evolution of the calcium sulfoaluminate cement (CSA) - Portland cement (PC) binary system (with a CSA: PC mass ratio of 8:2) at −10 °C. The pore structure changes, phase evolution, and microstructure development of hydration products are analyzed using nitrogen adsorption and desorption (NAD), X-ray diffraction (XRD), thermogravimetric analysis (TG-DTG), and scanning electron microscopy (SEM), respectively. The results demonstrate that incorporating nanomaterials accelerates the setting time and enhances the early hydration heat release in the binary cement system, leading to a substantial improvement in early-age compressive strength, yet may adversely affect flexural strength, indicating a potential trade-off between strength types. Specifically, the addition of 3 % NS and CSHs increases the compressive strength by 27.8 % and 15.4 % at 6 h, and by 41.5 % and 20.8 % at 1 day, respectively. Microstructural analyses reveal that nanomaterials promote the consumption of cement clinker, stimulate the formation of ettringite, and contribute to pore structure refinement via filling effects. This results in a higher proportion of micropores and an overall denser cement matrix. Furthermore, the presence of nanomaterials facilitate the formation of Portland cement hydration products, which helps stabilize ettringite, preventing its decomposition and thereby ensuring the stability of strength. In the binary system, NS exhibits superior performance at −10 °C temperature, attributed to its stronger nucleation effect and finer particle size. These insights offer valuable guidance for the design and development of high-performance cement-based repair mortars suitable for cold-weather applications.