The Effect of Silica Fume and GGBFS as Cement Substitutes on the Mechanical Properties and Water Resistance of Uncalcined Desulfurization Gypsum-Based Composite Cementitious Materials

This study aims to reduce cement content and enhance mechanical and water resistance properties of uncalcined desulfurization gypsum-based composite cementitious materials (UDGCMs) by incorporating ground granulated blast furnace slag (GGBFS) and silica fume (SF) as cement replacements. The findings indicate that moderate incorporation of either SF or GGBFS optimizes the mechanical properties and water resistance of UDGCMs. Adding SF alone yields the greatest improvement in mechanical properties and markedly reduces the relative embodied energy, carbon emissions, and cost of UDGCMs. Adding GGBFS alone has a more pronounced effect in enhancing water resistance, and the softening coefficient can be increased by 30.98%. Under cement-free conditions, the blend of GGBFS and SF activated by a weakly alkaline environment provided by quicklime yielded a maximum 28-day compressive strength of 25.48 MPa. Microstructural analysis reveals that, in systems with separate additions of SF or GGBFS, strength enhancement is primarily achieved by modulating the formation of ettringite (AFt) and C-(A)-S-H gel. Under Portland cement-free conditions with simultaneous incorporation of SF and GGBFS, although the extent of the pozzolanic reaction is limited, the synergistic accumulation of ettringite and gel products can further promote material strength. This research provides a theoretical and experimental basis for uncalcined desulfurization gypsum (UDG) application and advances low-carbon cementitious materials.