Effect of chloride salt on the phase evolution in alkali-activated slag cements through thermodynamic modelling

Abstract

In this study thermodynamic modelling was used to investigate the effect of chloride salt on the phase evolution in alkali-activated slag paste. The effects of different sources of chloride salts (NaCl, CaCl2, and MgCl2) and slag chemistry were studied. In thermodynamic modelling, a new solid solution model, i.e. MgAl–Cl-LDH_ss, was proposed to consider the uptake of chloride ions by hydroxylated hydrotalcite. The modelling results show that the valence of cation in chloride salt and slag chemistry both significantly affect the phase assemblage, pore solution composition and chemical chloride binding capacity of alkali-activated slag paste. The hydroxylated hydrotalcite decomposes upon NaCl attack, while upon the attack by CaCl2 and MgCl2 it transforms to chloride intercalated hydrotalcite. Decalcification, dealumination and altering mean chain length of C-(N-)A-S-H gel are predicted upon chloride salt attack. Increasing Al2O3 content in slag thermodynamically favors the formation of Friedel's salt, leading to a higher chemical chloride binding capacity.