Multiscale analysis of mechanical strength and failure characteristics in cementitious waste rock–tailings backfill considering aggregate dosage

Driven by the global transition towards greener and more efficient mineral extraction, waste rock–cementitious tailings backfill (WRCTB) has gained prominence as an innovative solution for repurposing mining solid waste. The strength performance of WRCTB is strongly governed by the aggregate proportion of waste rock (R) to tailings (T). This study systematically evaluates the influence of R/T ratios (4:6, 5:5, 6:4, and 7:3) on mechanical behaviour and microstructural evolution using a multi-scale framework. An integrated multi-scale approach combining uniaxial compression, X-ray CT – based 3D reconstruction, and SEM was employed to evaluate mechanical properties, energy dissipation, pore architecture, and hydration characteristics. The results indicate that an equal rock-to-tailings proportion (R/T = 5:5) yields the optimal composite behaviour, maximising strength and stiffness while minimising pore volume and producing a substantially more compact microstructure. Increasing slurry concentration and prolonging age further enhance strength development. CT analysis indicates a uniform pore distribution at lower R/T ratios, whereas higher ratios lead to sedimentation stratification and pore clustering. SEM observations confirm that at R/T = 5:5, abundant C–S–H gel and ettringite (AFt) are formed, producing strong interfacial bonding and a dense microstructure. Overall, a clear ‘mix ratio–structure–property’ relationship is established, clarifying the intrinsic mechanism governing WRCTB performance.