Chloride binding behavior of synthesized reaction products in alkali-activated slag

Abstract

The chemical composition and surface properties of reaction products in alkali activated slag (AAS) are different from ordinary Portland cement, thus significantly affect their chloride binding behavior. The objective of this study is to investigate the relationship between physical/chemical characteristics and chloride binding capacity of the reaction products in AAS, i.e. C-S-H gel, C-A-S-H gel and LDHs. In this study, typical reaction products with different chemical compositions in AAS were chemically synthesized and characterized by XRD, NMR, BET and zeta potential. The chloride binding capacity of synthesized reaction products was evaluated in neutral and alkaline NaCl solution. The results indicate that C-S-H and C-A-S-H gels bind chlorides more physically. The increase of Ca/Si ratio and Al/Si ratio results in more positive zeta potential, which is beneficial for chloride binding of C-(A)-S-H gels. Further, the chloride binding capacity of C-A-S-H gels is higher than C-S-H gel, due to more positive zeta potential and larger layer spacing in the presence of Al. LDHs possess higher chloride binding capacity than C-(A)-S-H gels, due to its higher specific surface area (120.3–125.5 m2/g) and positive zeta potential (22.5–28.5 mV); the increase of Mg/Al ratio results in increased layer spacing and subsequently higher chloride binding capacity. The physical binding contributed most chloride binding of LDHs; due to the difficulty of exchange between Cl− and intercalated CO32− in Mg-Al LDH, its chemical binding is weak, which is resulted from the entering of chlorides into the interlayer from the edge of LDHs.