Impact of Alternate Wetting–Drying Environment on Ionic Strength of Aqueous Solution, Durability, and Microstructural Properties of Geopolymer Concrete

Abstract Durability properties of geopolymer concrete (Gc) produced using fly ash (FA), and FA plus ground granulated blast furnace slag (GGBFS) were investigated by subjecting specimens to cyclic wetting–drying conditions in various environments, i.e., water, NaCl (NC), NaCl + Na 2 SO 4 (NS), and NaCl + MgSO 4 (MS) solutions. The present work primarily focuses on the evaluation of the influence of cyclic exposure conditions on change in weight and compressive strength of Gc specimens, concentration of Na + , Ca 2 + , K + , chloride, and sulfate ions in aqueous solution of Gc, as well as on microstructure evolution of Gc, and inferring the correlation between these variations. The microstructural properties of Gc were investigated by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and field emission scanning electron microscope (FESEM) analyses. Additionally, the corrosion behavior of steel reinforcement was investigated by performing potentiodynamic polarization tests on a steel specimen immersed in aqueous solution obtained from geopolymer concrete specimens subjected to cyclic exposure. Results showed that the weight loss was higher, and the compressive strength of Gc and concentrations of sodium and calcium ions in aqueous solution of Gc were lower in the case of water exposure than when subjected to salt exposure. The Ca 2 + and K + concentrations were higher in aqueous solution of Gc exposed to chloride solution than chloride-sulfate solution, whereas there was opposite variation in Na + concentration. The aqueous solution of Gc had higher pH in the case of exposure to NC solution than NC + NS solution followed by NC + MS solution, whereas an opposite trend in conductivity among exposure solutions. The Gc subjected to NC + NS solution exhibited a higher range of passive region of steel than NC + MS solution followed by NC solution. Microstructural analyses revealed that the incorporation of GGBFS promoted the formation of calcium-rich binding gels, such as albite and C─S─H gel, as observed from XRD analysis, leading to a higher Ca/Si ratio as obtained from EDS analysis. Increasing the NaOH solution molarity enhanced the formation of geopolymer gels, resulting in higher Si/Al and Na/Al ratios. These changes contributed to the formation of a compact microstructure as evidenced from FESEM micrographs, which reduced the ingress of chloride and sulfate ions, and increased the passive region of steel. These findings are supported by corresponding variations in ionic concentrations of Gc aqueous solution.