From waste to value: Engineering properties and failure mechanisms of green backfills utilizing molybdenum tailings, fly ash and coal gangue

The sustainable reuse of industrial by-products, such as molybdenum tailings (MoTs), fly ash (FA), and coal gangue (CG), is critical for mitigating environmental risks and advancing low-carbon development. This study develops a novel green backfill material (CGMFB) using CG as aggregates and MoTs and FA as supplementary cementitious materials (SCMs), replacing 0 %–90 % of cement. The engineering properties, failure mechanisms, microstructure, and sustainability of CGMFB were systematically investigated through workability tests, quasi-static uniaxial compression tests combined with digital image correlation (DIC), advanced microstructural analyses (XRD, SEM–EDS, FTIR, TG-DTG, and MIP), along with carbon emission evaluations. The results show that SCMs replacement significantly influences CGMFB performance. With increasing SCMs replacement, flowability rises from 24 cm to 27.3 cm at 30 % before dropping to 24.5 cm at 90 %, while settling ratios increase from 5.98 % to 9.73 %. The 3-, 7-, and 28-day compressive strengths decrease linearly with increasing SCMs replacement, dropping from 22.97 to 3.85 MPa at 28 days. With higher SCMs replacement, CGMFB failure shifts from brittle to non-brittle, cracks become more complex, and uniaxial compression energy decreases. Microstructural analyses reveal that major hydration products, including C-S-H and C-A-S-H gels, are formed, while higher SCMs replacement leads to increased porosity. Notably, CGMFB carbon emissions decrease with increasing SCMs replacement. CGMFB-75, meeting engineering requirements, emits 66.33 % less CO₂ than CGMFB-0 without SCMs. This study demonstrates a sustainable pathway to transform hazardous industrial wastes into value-added green backfill materials, presenting a promising approach for low-carbon mining technologies and supporting circular economy initiatives.