Mechanical and microstructural enhancement of coal gangue backfill using polypropylene fiber and silica fume.

The brittle failure characteristics of backfill materials have limited their engineering applications. Herein, we propose a novel synergistic enhancement strategy to improve these characteristics. We prepared coal gangue-based backfill material (SGBS-PsFs) using industrial solid waste carbide slag to activate coal gangue powder and slag, to which polypropylene fibers (PP) and silica fume (SF) were introduced for synergistic enhancement. The workability, uniaxial compressive strength (UCS), splitting tensile strength (STS), and peak strain of the SGBS-PsFs were then analyzed using macroscopic tests. Digital image correlation and super depth microscopy were used to investigate the damage mechanism and three-dimensional state of the fracture surfaces. Various microscopic techniques were also used to characterize the material. The results indicate that the incorporation of PP and SF was negatively correlated with the flowability of the SGBS-PsFs. In addition, excessive addition affected the material's workability. The synergistic use of PP and SF significantly enhanced the UCS, STS, and peak strain by 37.94 %, 42.26 %, and 47.17 %, respectively. Notably, PP more effectively improved the STS, whereas SF enhanced the UCS. The synergy between PP and SF promoted the material's transition from tensile failure to a combined tensile-shear failure mode, increased the number of damage cracks, and enhanced the roughness of the fracture surfaces. Microscopic analyses indicate that SF optimized the density and pore structure of the SGBS-PsFs matrix, increased the C-S-H gel content, and improved interfacial bonding and stress transfer between PP and the matrix. The combined use of PP and SF exhibited a coupling effect. The substitution of the material developed herein for cement-based backfill material offers environmental benefits, as its use would reduce costs and carbon emissions. The findings of this study provide technical support and a theoretical basis for utilizing industrial solid waste to prepare novel cementitious materials for engineering applications that incorporate environmental protection.