Synergistic Utilization of High-Volume Desulfurization Gypsum with Fly Ash and Slag in Alkali-Activated Binders

This study investigates a sustainable alkali-activated cementitious system incorporating high-volume desulfurization gypsum (DG) with fly ash (FA) and ground granulated blast-furnace slag (GGBFS) as co-precursors. The reaction mechanism, mechanical properties, water resistance, volumetric stability, and environmental safety were systematically evaluated using XRD, FT-IR, SEM-EDS, and TG-DTG. The optimal DG content was approximately 50%, and the DG:FA:GGBFS ratio of 50:20:30 (Mix B5) exhibited the best overall performance: a 28-day compressive strength of 27.2 MPa, initial and final setting times of 90 and 180 min, water absorption of 6.2%, and a softening coefficient of 0.94. The FA/GGBFS ratio also markedly affected performance, with a ratio of 1:1.5 giving the best synergy at 50% DG. Volumetric stability tests showed that B5 underwent moderate early expansion (2.6‰–3.1‰) that stabilized at 0.40‰ at 28 days. Microstructural analysis revealed that the superior performance of B5 originated from a dense matrix of abundant C-(A)-S-H gel and well-distributed ettringite, whereas excessive DG (80%) left large amounts of unreacted gypsum and produced a porous structure with severely degraded performance. Heavy-metal leaching from the two hardened specimens (A8 and B5) remained below the GB 5085.3-2007 and GB 3838-2002 Class IV limits. This study demonstrates the feasibility of using high-volume DG as a principal raw material in alkali-activated binders, offering a promising route to low-carbon cementitious materials from industrial by-products.