Effects of carbonation on chloride ingress, pH and microstructure of alkali activated FA/GGBS binder

This study presents the influence of carbonation on chloride (Cl - ) ingress, Cl - binding, pH and microstructure of alkali (Na 2 SiO 3 and NaOH) activated Fly ash (FA) and ground granulated blast furnace slag (GGBS) based binders (AAB). The cured AAB were exposed to NaCl and accelerated carbonation under three different regimes: 1) exposure to NaCl solution followed by carbonation (G-A), 2) exposure to carbonation followed by immersion in NaCl solution (G-B), 3) NaCl immersion and carbonation cycles (G-C). Following exposure, the samples were characterized to quantify Cl - concentration (Free and bound chloride content) and pH at various depths and microstructure was analyzed using XRD, SEM-EDS and BET-BJH. It was found that carbonation of AAB facilitated Cl - ingress and Cl - desorption. However the specific nature of this effect was influenced by the exposure regime. When Cl - contaminated AAB was carbonated (G-A), the carbonation induced pH dropped caused decalcification and subsequent Cl - ions desorption. When carbonated AAB was exposed to NaCl solution (G-B), the Cl - concentration increased and penetrated deeper into AAB. The AAB’s ability to bind Cl - ions was also reduced mainly due to prior carbonation induced decalcification. G-C showed that Cl - ingress was significant after 6 cycles likely due to formation of microcracks caused by precipitation of CaCO 3 and possibly by repeated wetting (NaCl exposure) and drying (carbonation). Chloride ion desorption was found to result from decalcification initiated by pH drop to below 12, leading to high risk of corrosion.