Performance and deterioration mechanism of low-carbon GGBS/FA/FGD gypsum binders in extreme saline environments

Extreme saline environments are characterized by variable multi-ionic conditions and salt concentrations fluctuations, posing major challenges to the stability of low-carbon waste-based binders. Under such conditions, their hydration and deterioration mechanisms remain unclear, limiting their large-scale application. In this study, the performance, hydration, and deterioration mechanisms of GGBS/FA/FGD gypsum binders were investigated across a wide salinity gradient through multi-scale characterization and thermodynamic modeling, enabling the identification of the transition between activation and deterioration. At 1.15% salinity, ions promoted precursor dissolution and early hydration, shortened setting time, and the maximum loss of strength was only 8.3%. Over 5%, especially at 15% and 20%, the binders shifted into a deterioration-dominated regime, with up to 17.1% mass loss and 52.5% compressive strength loss. This transition involved pore coarsening, hydration suppression, and a shift from hydration-product development to salt-dominated accumulation. Sulfate preferentially penetrated Al-bearing and Ca-rich reactive domains, chloride mainly precipitated in pores and participated in Cl-AFm formation, and magnesium accelerated alkalinity depletion and decalcification through brucite precipitation. Meanwhile, increasing salinity from 1.15% to 20% reduced the mean chain length by 25.4%, indicating interruption of silicate chain growth caused by reactive Al depletion and sulfate-rich surface passivation. Thermodynamic analysis further showed that continued salt ingress destabilized the initial AFt buffering assemblage, promoted conversion toward chloride-bearing AFm, and accelerated free-salt precipitation. These results demonstrate that severe saline deterioration is governed by hindered hydration, phase destabilization, and salt crystallization collapse. This work provides a framework for designing salt-resistant solid-waste binders for severe saline environments.