Hydration behavior and micro-pore structural of Portland cement composites with crystalline nano-SiO2 at low temperature

Promoting the early hydration of cement and enhancing its strength at low-temperatures can reduce the occurrence of freeze-thaw damage in concrete structures. Amorphous nano-SiO2 particles (NS) have been proven to effectively facilitate cement hydration at low temperatures. However, crystalline nano-SiO2 particles (CS), which share the same chemical composition as NS, have received little attention from researchers. The aim of this study is to investigate the effectiveness of CS in prompting the properties of cement mortar cured at low temperature. Both CS and NS were utilized as modifiers to enhance the properties of cement mortar. Their effects on cement' setting time, hydration heat, micro-pore structure, and mechanical property were compared and analyzed at low temperature (7 degrees C) and room temperature (23 degrees C). Additionally, the mechanism of their influence was discussed based on experimental data. The results indicate that CS can significantly accelerate the hydration of cement at 7 degrees C, whereas this ability is virtually negligible at 23 degrees C. At 7 degrees C, CS was able to increase the degree of cement hydration by 8.7 % at 3d and enhanced the 28d strength of the cement mortar by 11.1 %. NS showed an increase in these two indices by 11.3 % and 13.4 %, respectively. Moreover, CS and NS can result in a 23.8 % reduction in the total porosity of cement samples hydrated for 3d, and refine the overall pore structure of the specimens. Both CS and NS particles can react with CH and generate C-S-H seeds on their surfaces, thereby promoting cement hydration. The findings are of significant importance for enhancing the quality of concrete engineering in cold regions and broadening the channels for the utilization of solid waste.