Study on the effect of nano-silica on the early-age strength and microstructure of cement paste mixed with calcium nitrite under negative temperature conditions

During construction at low temperatures, fresh concrete is susceptible to early freeze damage, which can permanently damage the concrete structure. Therefore, this study aimed to enhance the early-age strength and durability properties of the cement paste at −5 ℃ by incorporating calcium nitrite (CN) antifreeze and nano-silica (NS). First, the effects of CN and NS on the fresh properties of the cement paste were measured by setting time and bleeding rate tests. Subsequently, the mechanical properties and chloride content after salt solution erosion were tested for each group of specimens, and the change in slurry densification was further observed by ultrasonic pulse velocity and resistivity tests. Finally, the hydration products and microstructure of the specimens were analyzed using XRD and SEM tests. The results showed that the addition of CN and NS could reduce the setting time of cement paste and avoid the bleeding phenomenon of cement paste under negative temperature environment. The decrease in curing temperature and the use of NS both inhibited the promotion of C₃A hydration by nitrite ions, which led to a decrease in nitrite-AFm content. However, NS can compensate for the reduced nitrite-AFm content by promoting the production of hydrated C-S-H through nucleation and pozzolanic effects. Ultimately, the strength and resistance of the slurry to chloride erosion are improved.