Effect of Na+ and Ca2+on chloride binding behavior of LDHs and AFm

The chloride binding of hydrotalcite is significant for delaying steel reinforcement corrosion. This study investigated the chloride binding capacity of hydrotalcite, including CaAl-NO3 layered double hydroxides (LDHs) and monosulfoaluminate (AFm), under NaCl and CaCl2 penetration, respectively. It quantitatively characterized the total bound chloride content, strongly bound chloride content, and not-strongly bound chloride content as the chloride concentration increased. The results indicated that the total bound chloride content by both LDHs and AFm in CaCl2 solution was higher than in NaCl solution. The increase in LDHs was attributed to Ca2+ suppressing LDHs dissolution and reducing competitive adsorption between OH- and chloride ions. For AFm, the increase was due to Ca2+ promoting AFt decomposition and inhibiting AFm dissolution. During chloride desorption process in deionized water, the stability of chloride binding products formed by LDHs was significantly higher than that of AFm. This difference was primarily due to the involvement of SO4 2- in phase transformation, leading to AFt formation and facilitating the decomposition of Kuzel's salt and Friedel's salt formed by AFm. The strongly bound chloride ratio of AFm in CaCl2 solution was generally lower than in NaCl solution due to gypsum dissolution and subsequent release of SO42-.