Synergistic effects of fly ash and slag in belite–sulfoaluminate cement composites: Hydration, strength, and sustainable performance

Although high-belite sulfoaluminate cement (HBSC) exhibits excellent environmental benefits, high-early strength, and durability properties, it is limited by high early hydration heat and cost. Blending it with ordinary Portland cement (OPC) and supplementary cementitious materials (SCMs) such as fly ash (FA) and ground granulated blast-furnace slag (GGBS) is an effective strategy to enhance its performance and cost-effectiveness. Nevertheless, the high-aluminate phase and complex ionic environment within HBSC-based binders alter the reactivity of FA and GGBS compared to conventional OPC systems, introducing uncertainties in performance. This study investigated the effects of FA and GGBS on the hydration mechanism of an optimized HBSC–OPC–gypsum ternary system and analyzed their long-term performance and economic indicators. The results show that FA accelerates early ettringite (AFt) crystallization by acting as a physical filler and providing nucleation sites. Conversely, the hydraulic activity of GGBS elevates early-stage alkalinity, which hastens the conversion of AFt to monosulfoaluminate (AFm) but later facilitates AFt regeneration. However, this regenerative effect is reversed in composite systems with high SCM dosages, where an antagonistic interaction leaves the system rich in AFm. Consequently, FA enhances workability, whereas GGBS boosts later-age strength. Both materials improve sulfate resistance through distinct mechanisms: FA by refining the pore structure through its pozzolanic reaction and GGBS by rapidly densifying the matrix, thus hindering sulfate attack. Nonetheless, their combined use can lead to excessive expansion. Crucially, incorporating 10% GGBS reduces the production cost and CO2 emissions per MPa of compressive strength by 2.0% and 4.9%, respectively.