Effects of Cl- erosion conditions on Cl- binding properties of fly-ash-slag-based geopolymer

Abstract

This study aims to investigate the Cl--binding ability of calcium aluminate silicate hydrate (C-A-S-H)/sodium aluminosilicate hydrate (N-A-S-H) gel and fly-ash-slag based geopolymer mortar exposed to different Cl- concentration under 0–56 days exposure ages. The concentration of Cl- was from 0.1 to 1.0 mol/L. The physical phase and corresponding layer spacing, degree of crystallization, crystal structure and thermal stability were measured and the effect of changes in the C-A-S-H/N-A-S-H gel structure on the Cl- binding capacity of the geopolymer was analyzed. The results showed that the C-A-S-H/N-A-S-H gels changed from flake structure to flocculent structure after Cl- invasion, leading to the transformation of the geopolymer from laminar structure to a laminar-flocculent structure with crystallization and non-crystallization coexisting. With the erosion of Cl-, the crystal structure of C-A-S-H/N-A-S-H gels was disrupted and the crystallinity decreased, leading to an increase in the crystalline surface spacing of the geopolymer. In addition, the erosion of Cl- led to a decrease in the glass transition temperature of C-A-S-H/N-A-S-H gels and a decrease in the thermal stability of the geopolymer. The binding of Cl- by geopolymer mortar was divided into two aspects, C-A-S-H/N-A-S-H gel adsorption and chemical binding, with gel adsorption as the main source. With the increase of Cl- concentration or exposure age, the Cl- binding ability of C-A-S-H/N-A-S-H gels first rose rapidly and then gradually decreased, which was similar to that of geopolymer mortar.