Effect of nano-SiO₂ on negative temperature mechanical properties and microstructure of sulfoaluminate concrete

To ensure the safety and reliability of concrete in a negative temperature environment, mechanical property testing, ultrasonic wave velocity testing, scanning electron microscopy (SEM), and X-ray diffraction (XRD) combined with Rietveld refinement quantitative analysis and other methods were used to study the properties of sulfoaluminate concrete modified with different nanosized SiO₂ (NS) admixtures under constant negative temperature (−10°C) curing conditions. The results show that the strength of sulfoaluminate concrete increases and then decreases with the increase of NS dosage and reaches the maximum strength at 2% NS dosage; When the curing temperature changes from negative temperature to room temperature, the ultrasonic wave velocity first decreases and then increases, and the ultrasonic wave velocity increase rate of rehydration (elevated section) after mixing NS is higher than that of the blank group, in which the ultrasonic wave velocity increase rate of the concrete with 2% NS admixture is the fastest; SEM and XRD found that with the increase of NS dosage, the internal pore structure of concrete becomes denser,and the relative content of calcium alumina (AFt) rises gradually,the 2% NS admixture AFt content is the highest, indicating that NS can promote the hydration of sulfoaluminate concrete under negative temperature environment.