Study on the long-term chloride permeability of nano-silica modified cement pastes cured at negative temperature

This paper investigated the 56-day and 120-day chloride permeability of nano-silica (NS) modified cement pastes with different water-cement ratios (0.3, 0.35, and 0.4) cured at a negative temperature of −5 °C. The influences of water-cement ratio, negative temperature, and the improvement effect of nano-silica were highlighted. Experimental results indicated that negative temperature could result in serve and irreversible deterioration of the chloride permeability, and a high water-cement ratio can also aggravate the deterioration. Meanwhile, the incorporation of nano-silica can effectively improve the resistance to chloride permeability of cement pastes, and the improvement of nano-silica was more evident in the environment of −5 °C compared with the environment of +20 °C. Through mercury intrusion porosimetry (MIP) technology, three key parameters (total porosity, tortuosity, and threshold diameter) were determined, which exhibit a strong relationship with the chloride permeability of cement paste through the statistical analysis. The results of MIP and correlation analysis indicated that, unlike the previous studies, the influence of nano-silica on the long-term pore structure of cement pastes under negative temperature is not total porosity but internal pore structure. Nano-silica has little effect on improving the total porosity, but it can effectively increase the tortuosity and decrease the threshold diameter, which will hinder the diffusion of chloride ions in the cement matrix, thereby improving the chloride permeability resistance. Also, the micro-mechanism of nano-silica on the permeability of cement paste is different under different water-cement ratios. In the case of a low water-cement ratio, nano-silica mainly increases the tortuosity of pore structure. In the case of a high water-cement ratio, nano-silica mainly reduces the connectivity of pore structure.