Effect of nano silica on freeze-thaw resistance of cement-fly ash mortars, cured in corrosive condition at different temperature

In China, a large amount of electric transmission towers has been built across plateau frozen soil, where the foundation concrete serves under freeze-thaw and erosion condition, and consequently, the durability faces the tough challenges. In this study, the mortars were prepared based on cement, fly ash, and nano silica [NS], which were cured in chloride-sulfate solution for 90 days. The compressive strength, freeze-thaw resistance, pore structure, interfacial transition zone, and hydration products was investigated, and the improvement in freeze-thaw resistance by addition of NS was discussed. The results show that addition of NS can improve the freeze-thaw resistance, and increase in curing temperature can also show improvement in freeze-thaw resistance. This effect can be explained by refine the pore structure and densify the microstructure of ITZ with the addition of NS. Furthermore, negative effect on freeze-thaw resistance can be found that being cured under chloride-sulfate condition, the formation of AFt would cause the volume expansion and cracking of concrete. Such results suggest that in the plateau frozen soil, it is better to cast concrete in summer, which would benefit the strength development of concrete and promote the freeze-thaw resistance.