Ceramics-Silikáty

Chloride binding is a key factor affecting chloride-induced corrosion in reinforced concrete. Different types of chloride salts may exhibit distinct binding mechanisms. In this work, the influence of chloride salts (NaCl, MgCl₂, and CaCl₂) on the chloride ion binding mechanism in alkali-activated slag (AAS) was investigated. The pH value, chloride ion binding capacity, and phase evolution of AAS were examined through various experimental and analytical methods. To compare the phase compositions of AAS after chemical equilibrium under exposure to different chloride solutions, thermodynamic modelling was employed to predict the phase assemblages in AAS. The study revealed that the type of chloride salt significantly influences the chloride-binding capacity and pore solution pH of AAS. The binding capacity followed the order: CaCl₂ > MgCl₂ > NaCl. Compared to CaCl₂ and MgC₂, exposure to the NaCl solution increased the pore solution pH of AAS, thereby reducing the solubility of Friedel’s salt and MgAl-LDHs (layered double hydroxides) and releasing more bound chloride ions. The X-ray diffraction (XRD) and thermogravimetric analysis (TGA) results demonstrated that AAS exposed to the CaCl₂ solution exhibited the enhanced formation of C-A-S-H, which contributed to the higher physical adsorption of chloride ions. In the MgCl₂ solution, magnesium plays a dual role: it reacts with C-A-S-H phases, leading to a reduction in the pore solution pH; it provides favourable conditions for the precipitation of MgAl-LDHs, thereby enhancing the chloride ion binding. Furthermore, within the water-to-binder ratio range of 0.35-0.45, increasing the ratio enhances the chloride-binding capacity of AAS, but has negligible effects on the pH of the pore solution.