Exploration of silica fume effect on solid waste based backfilling material: Mechanical properties, hydration mechanism and economic benefits

The resource utilization of solid wastes such as silica fume (SF), ground-granulated blast furnace slag (GGBFS), and steel slag (SS) has become a current research hotspot. This study aims to investigate the effect of SF content on the performance and hydration mechanisms of GGBFS-SS-based cementitious filling materials. A low-cost SF-GGBFS-SS-FGD gypsum-based cementitious filling material (SGM) is proposed for the first time, and its compressive strength, fluidity, setting time, shrinkage, electrical conductivity, and leaching safety are evaluated. The results show that when the SF content is 16 %, the SGM exhibits the highest compressive strength at 7 days and 28 days, reaching 1.72 MPa and 3.44 MPa, respectively. The addition of an appropriate amount of SF (10 %) improves the fluidity of SGM (221 cm). Moreover, when the SF content is 16 %, SGM shows the lowest shrinkage rates (3.56 %/4.87 %) and electrical conductivity (6.25 Ω·m/8.78 Ω·m) at 7 days and 28 days. The hydration mechanism is as follows: The incorporation of an appropriate amount of fine-grade SF leads to nucleation crystallization, which provides more active nucleation sites for hydration reactions, increasing the generation and crystallization polymerization degree of hydration products such as C-S-H gel and ettringite. This is reflected in the increased relative number of bridging oxygen (RBO = 22.79 %) in C-S-H gel and the Al[Ⅵ] content in ettringite. The economic analysis results show that, compared to traditional cement binders, the ton production cost of SGM with 16 % SF content is further reduced by 29.59 %. This study provides new insights into the development of low-cost SGM and is of significant importance for the resource utilization of solid wastes such as SF and the application of filling materials.