Study on the Binding Behavior of Chloride Ion and Ettringite in Nano-Metakaolin Cement by Seawater Mixing and Curing Temperatures

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.