Effect of desulfurization gypsum on dissolution kinetics and phase evolution in the alkali-activated steel slag–fly ash system

To address the issue of limited large-scale application due to the unclear action mechanism of desulfurization gypsum (DG) in alkali-activated systems, this study utilizes steel slag (SS)-fly ash (FA) as the precursor to investigate the influence of DG on the dissolution kinetics, macroscopic properties, phase evolution, and reaction pathways of this alkali-activated system. The results demonstrate that DG can rapidly dissolve in an alkaline environment, releasing Ca²⁺ and SO₄²⁻. It not only directly participates in the formation of Ettringite but also alleviates the mismatch in reaction rates between SS and FA by adjusting the Ca/Si and Al/Si ratios of the system. Under composite alkali activation conditions, an appropriate amount of DG can significantly enhance reaction kinetics, elevating the 28-day compressive strength of the system to 22.4 MPa and shortening the initial and final setting times by over 55 minutes. However, excessive DG leads to sluggish late-stage strength development due to excessive Ettringite deposition and residual gypsum. Phase analysis reveals that an appropriate amount of DG can maintain the Ca/Si ratio of C-(A)-S-H gel within the range of 0.8-1.3 and increase the Si/Al ratio to 2.0-3.1, significantly enhancing the polymerization degree and structural density of the gel. In the absence of DG, the reaction pathway of the system is primarily dominated by alkali-activated gelation, while an appropriate amount of DG leads to sulfate-activated ettringite crystallization as the dominant pathway. Furthermore, through hydration kinetics analysis, the exponential function model proposed in this paper can more accurately describe the hydration heat release behavior of the system, particularly in reflecting the characteristic of reaction rate decay over time. This study provides a theoretical basis for the high-value utilization of desulfurization gypsum in solid waste-based cementitious materials.