Effect of nano-silica on hydration and microstructure of early-age cement pastes subjected to frost exposure

This study investigated the effect of nano-silica on hydration and microstructure of early-age cement pastes subjected to frost exposures. Fresh cement pastes after pre-curing for 24 h were exposed to −15°C for 4 days to simulate frost exposure, and subsequently carried out the standard curing for 1, 3, and 7 days. Experimental tests of thermogravimetric (TG), mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and nanoindentation were carried out. The hydration results showed that incorporating pozzolanic nano-silica enabled the hydration degree of frozen paste to reach approximately 97% of the paste without frost exposures. Moreover, nano-silica increasesd the proportion of high-density C-S-H gel from 29.69% to 40.63%. MIP results revealed that pore structure refinement induced by nano-silica mainly occurred in the period of standard curing. Specifically, nano-silica reduced the pore volume in the 100–1000 nm range while increasing that in the 50–100 nm range, thereby refining the frost-damaged pore structure (resulting in total porosity reductions of 3.2%, 8.7%, and 6.5% after 1, 3, and 7 days, respectively). SEM-EDS results confirm that nano-silica densified the frost-damaged microstructure, promoted the formation of hydration products, and reduced the Ca/Si ratio of the hydration products from 4.16 to 4.03. Overall, nano-silica mitigates frost-induced deterioration in cement pastes through synergistic pozzolanic and filling effects, leading to restored hydration, increased C-S-H density, and a refined pore structure, which underscores its potential to enhance the performance of concrete structures serving in cold environments.