Synergistic mechanisms of steel slag, granulated blast furnace slag, and desulfurization gypsum in high-content steel slag-based cementitious backfill materials

• GBFS releases active Si4+ and Al3+ to synergistically activate Ca2+ from steel slag, promoting C-S-H gel and ettringite formation, and optimizing microstructure. • At 30% GBFS content, C-S-H content increases by 40.8%, large pores decrease by 68.7%, and 90-day compressive strength quintuples compared to baseline. • Desulfurization gypsum accelerates silicate hydration, but excessive addition (>16%) causes AFt expansion-induced microcracks and strength loss. • Optimal synergistic system (8% DG + 30% GBFS) yields peak heat release rate of 0.92 mW/g and cumulative heat of 240 J/g, enabling high-content SS backfill materials.