Resource utilization of excavated soil and coal-based solid wastes under resource coupling mode in energy agglomeration areas: A case study on flowable backfill materials

The massive accumulation of excavated soil (ES) and coal-based solid wastes poses a critical sustainability challenge in energy-intensive industrial regions. Under a resource-coupling mode for an energy agglomeration area, this study developed a low-carbon flowable backfill material (FBM) for the high-volume utilization of ES by combining locally available ES and coal-based solid wastes through a ternary binder system comprising calcined coal gangue (CCG), fly ash (FA), and ordinary Portland cement (OPC). A mixture design was used to quantify the effects of binder composition, followed by response surface methodology to investigate the effects of excavated-soil mass fraction (ES m ) and water-to-solid ratio (W/S) on flowability, bleeding, and unconfined compressive strength (UCS). XRD, TG, SEM–EDS, and NMR were employed to elucidate the hydration mechanisms and pore-structure evolution. The results indicated complementary effects of CCG and FA: CCG provided reactive aluminosilicate species and supported strength development and bleeding resistance, whereas FA improved particle packing and workability through its spherical morphology. The selected ternary formulation containing 60% OPC, 25% CCG, and 15% FA achieved a flow spread of 222 mm, a bleeding rate of 3.52%, and 7 d and 28 d UCS values of 1.08 and 3.62 MPa, respectively, at ES m = 80% and W/S = 0.40. Its 28 d UCS was 14.6% and 27.5% higher than those of the corresponding OPC–CCG and OPC–FA binary systems, respectively. Microstructural observations associated this performance with secondary aluminosilicate reactions, hydrate infilling and particle bridging, and a shift toward finer effective pores. An experimentally informed feasible window of ES m = 80%–84% and W/S = 0.39–0.45 was identified for scenario-oriented mixture selection. Under the adopted localized cradle-to-gate boundary, the selected ternary formulation reduced material cost by 34.6% and decreased carbon-emission intensity from 217.2 to 129.6 kg CO 2 e/m 3 , corresponding to a reduction of 40.3% relative to the pure-OPC reference. These findings demonstrate the case-specific feasibility of a regional resource-coupling mode that integrates locally available ES and coal-based solid wastes to produce low-carbon FBM for infrastructure backfilling and remediation applications in energy agglomeration areas.