Development of Phase-Transition/Foaming Dual-Expansion Material from Industrial Solid Wastes for Double-Layered Goaf Sealing Walls

For connecting roadways in dual-roadway coal mine excavations, traditional brick seals suffer from low construction efficiency and are susceptible to water-induced collapse. To overcome these drawbacks, this research proposes a self-supporting rapid-sealing technology that integrates multisource inorganic solid waste through a designed dual-expansion mechanism. A systematic performance assessment—covering fluidity, initial setting time, unconfined compressive strength, and CT-based 3D pore characterization—led to the following optimized material design: (1) lower layer (W2): 4% air-entraining agent (AEA) for phase-change-induced expansion; (2) upper layer (H16): 2% in-house developed chemical expansion agent (CEA), processed at 15°C with a water-cement ratio of 0.6. The resulting material exhibits a final expansion rate > 25%, an effective balance between setting behavior and expansion performance, and a compressive strength of 1.7 MPa. Phase and microstructural analyses (XRD, TG, SEM) identified ettringite (AFt) and C-S-H gel as the dominant hydration products. By combining CT imaging with theoretical modeling, the expansion mechanism of the dual-layer composite was clarified, offering a new technical route toward efficient and stable goaf sealing.