Alkalinity-Dependent Dual Role of Sodium Sulfate in Alkali-Activated Slag: From Synergistic Activation to Competitive Inhibition

Sodium sulfate-activated slag cement is considered a highly promising low-carbon cementitious material; however, its application is limited by low early-age activation efficiency and slow strength development. This study aims to systematically elucidate the coupled regulatory mechanism of alkalinity (2% and 4% Na2O equivalent) and sodium sulfate dosage on the performance of alkali-activated slag (AAS). Under standard curing conditions (20 ± 2 °C, relative humidity ≥ 95%), the macroscopic properties of the samples (workability, setting time, and compressive strength) and the evolution of their microstructure (analyzed by XRD, FTIR, and SEM-EDS) were evaluated. The results indicate that the effect of sodium sulfate on alkali-activated slag (AAS) strongly depends on the alkalinity. Under low-alkalinity conditions (2% Na2O), sodium sulfate exhibits a synergistic activation effect by increasing the ionic concentration, promoting slag depolymerization and the nucleation of ettringite (AFt). Specifically, compared with the control, incorporating 6 wt% sodium sulfate (N2S6 mix) increased compressive strength by approximately 82% at 3 days and 21% at 28 days. In contrast, under high-alkalinity conditions (4% Na2O), excessive sodium sulfate (≥2 wt%) shows an inhibitory effect. This is likely because an excess of sodium sulfate interferes with the normal polymerization pathways of the aluminosilicate network, suppressing the formation of the primary C-(A)-S-H gel and thus significantly reducing later-age strength. Microstructural analysis revealed that the hydration products in the composite-activated system mainly consist of C-(A)-S-H gel, ettringite (AFt), monosulfate (AFm), and hydrotalcite. This study investigates the observed kinetic trends of anion-competitive hydration under different alkalinity conditions, providing a theoretical basis for the mix design of low-carbon alkali-activated materials and the valorization of coal chemical industrial salts.