Mixing cement with seawater will cause the hydration process of cement to be different from that of ordinary cement, which will significantly affect cement’s mechanical properties and durability. This article investigates the effects of chloride ion concentration, curing temperature, and nano-metakaolin content on the evolution process of Friedel’s salts and ettringite (AFt) crystals in cement pastes. The study was conducted using X-ray diffraction (XRD), thermal analysis (TG), scanning electron microscopy (SEM), and mercury-intrusion porosimetry (MIP). The results show that chlorine salt can increase the production of Friedel’s salt and ettringite, and the delayed AFt production increases by up to 27.95% after the addition of chlorine salt, which has an adverse effect on cement-based materials. Increasing the curing temperature and increasing the nano-metakaolin dosage increased the generation of Friedel’s salt and decreased the delayed AFt generation, which resulted in a decrease in the length and diameter of the AFt crystals. After 28 days of high-temperature curing and the addition of nano-metakaolin, Friedel’s salt production increased by 13.40% and 14.34%, respectively, and ettringite production decreased by 9.68% and 7.93%, respectively. Increasing the curing temperature and adding nano-metakaolin can reduce the adverse effect of delayed ettringite increases due to chloride ion binding.

The effect of nano-kaolinite clay (NKC) on the bond behavior between rebar and concrete was studied in a series of experiments. An electrical corrosion method was used to corrode the rebar embedded in concrete specimens. Chloride diffusion and chloride content tests were performed to evaluate the effect of nanoclay on the chloride diffusion depth and chloride ion content in concrete. Pullout tests were performed to assess the bond properties, including the bond-slip curve and ultimate bond strength, between concrete and rebar before and after corrosion. The results revealed that NKC improved the chloride resistivity and bonding in the concrete specimens. The specimens with a 5% nanoclay addition showed the lowest percentage of the rebar corrosion level and bond strength degradation due to corrosion. After 48 h of corrosion, the rebar corrosion ratio decreased by 52%, and the ultimate bond strength increased by 35% compared to the control specimen.

To improve the ability of concrete structures to resist freeze-thaw damage in cold environments, explore the effect and mechanism of nano-metakaolin (NMK) on frost resistance of concrete. And make up for the deficiencies in the mechanical properties and deformation process of na-no-metakaolin concrete in freeze-thaw environments. Rapid freeze-thaw cycle experiment was car-ried out to detect the deterioration law of concrete. Physical and mechanical properties under freeze-thaw environment was measured. The modification mechanism of nano-metakaolin on con-crete frost resistance from micro and meso scales was analyzed. The effect of freeze-thaw damage on nano-metakaolin concrete was characterized. The influence law of stress strain is established, and the meso-statistical damage constitutive model of nano-metakaolin concrete under freeze-thaw action is established. The results show that: Compared with other nano-clays, adding 5% nano-metakaolin can effectively slow down concrete’s freeze-thaw cracking and crack propagation. After 125 freeze-thaw cycles, the surface crack width of concrete mixed with 5% nano-metakaolin is only 0.1mm. Without freeze-thaw cycles, the compressive strength of concrete mixed with 3% nano-metakaolin is the highest, which is 28.75% higher than that of ordinary concrete; after 125 freeze-thaw cycles, the loss rate of compressive strength of concrete mixed with 5% nano-metakaolin was 12.07%. After 125 freeze-thaw cycles, the peak strain is 0.45 times that of concrete without NMK, and the peak stress is 3 times that of concrete without NMK.