Physico-mechanical properties and synergistic hydration mechanism of steel slag-GBFS based alkali-activated composites incorporated with silica fume

Low reactivity and expansion limit steel slag (SS) use, while silica fume (SF) aids high-density matrix formation in cementitious materials due to its unique properties. To investigate the feasibility of enhancing SS-based materials with SF, an alkali-activated SS-GBFS-SF composite was prepared. This work examined the impact of SF on the physico-mechanical properties and microscopic characteristics, revealing the synergistic hydration mechanism among the components. The results show that a reasonable amount of SF can effectively improve the fluidity and mechanical properties of SS-GBFS based alkali-activated systems. When the mass ratio of SS to GBFS is 2:3 and SF dosage is 5 %, the fluidity of SS-GBFS-SF mortar is 170 mm, with compressive strength of 71 MPa at 90 days. The influence of SF on the early-age drying shrinkage rate of mortars is significant. At a dosage of 5 %, the 3-day drying shrinkage rate increases by 10.3 % compared to SS-GBFS group. However, this effect gradually decreases in the later stages. Early dissolution of SF produces soluble silicate, which reduces the activation effect of the alkali-activator. During the hydration process, SF consumes hydroxide, resulting in a decreased dissolution of calcium ion in the cementitious system and delaying the hydration of SS. In addition, SF provides a source of silicon, which increases the polymerization of hydration products. In the later stages of hydration, the unreacted SF acts as a physical filler, refining the microstructure and improving the comprehensive performance of the composites. • 5 % SF improves the fluidity and strength of SS-GBFS-SF system to a certain degree. • Moderate SF increases the early-age drying shrinkage and optimizes pore structure. • SF provides a silicon source and enhances gel polymerization and interlacing. • SF delays SS hydration in early stage and acts as a physical filler in later stage.