Novel chloride binding-based quantitative method for estimating the degree of geopolymerization in metakaolin geopolymer

Abstract

Geopolymer concrete is a potential partial substitute for Portland cement (PC) concrete in civil engineering. Understanding chloride-binding behavior and mechanisms in geopolymers is essential for evaluating durability of geopolymer concrete in chloride-rich environments. This study investigated the chloride-binding behavior of metakaolin (MK) geopolymers by examining the influence of chloride ion concentrations, alkali activator concentrations, and pore solution pH levels on their chloride-binding capacities. The correlation between the bound chloride ion content and the geopolymerization degree of MK was analyzed. The experimental results indicated that the binding of MK geopolymer to chloride ions is physical adsorption under the competition of hydroxide ions. Thus when the pH of pore solution decreases from 11.69 to 10.55, the amount of bound chloride increases by 1.95 times. The chloride-binding capacity of the MK geopolymer was primarily determined by the N-A-S-H gel content and the pH of pore solution. Under the same pH levels and chloride ion concentration, the chloride ion content bound per unit N-A-S-H gel remained constant, independent of N-A-S-H gel content. Based on these findings, a new method was proposed to evaluate the geopolymerization degree of MK by mearsuring the chloride-binding capacity of the N-A-S-H gel. The proposed method shows a small margin of error (<5 %) compared to selective acid dissolution analysis, providing valuable insights for research into geopolymer precursor reactions.