Experimental study on the effect of nano-SiO₂ on long-term properties of negative-temperature cement paste

The current research on the application of nanomaterials in concrete is mainly carried out under positive-temperature curing conditions, while research under negative-temperature curing conditions is very scarce. This paper investigated the effect of nano-SiO₂ on long-term strength and chloride diffusivity of cement pastes cured at negative temperature (−5 °C). Thermogravimetric analysis (TGA) confirms that the low-dosage nano-SiO₂ can still exert the pozzolanic effect within negative-temperature cement paste even after 120 days. The macro results show that low-dosage nano-SiO₂ effectively shortens the curing age required for strength and durability development. It is further proved that compared to the promotion effect of nano-SiO₂ on hydration, the optimization of internal pore structure by nano-SiO₂ plays the dominant role in shortening the curing age required. Furthermore, it is found that the low sensitivity of strength to the microstructure results in the insignificant effect of nano-SiO₂ on the long-term strength. In contrast, nano-SiO₂ can effectively improve the long-term chloride permeability resistance through increasing the tortuosity of pore structure and decreasing the critical pore diameter and threshold pore diameter.