Upcycling coal gangue coarse aggregates into 3D printed concrete: Multi-scale mechanisms of fracture behaviour

The advent of 3D printed concrete (3DPC) has transformed construction industrialization, especially in the context of intelligent construction. Nevertheless, conventional cement-based printable materials, mainly composed of extrusion-adapted mortar without coarse aggregates, exhibit low stiffness, high shrinkage cracking potential, and excessive cement dependence, compromising sustainability and increasing carbon footprints. This study introduces the first use of coal gangue as a sustainable coarse aggregate in 3D printed coal gangue concrete (3DP-CC), offering an innovative strategy for upcycling coal mining waste into printable construction materials. We systematically perform uniaxial compression, three-point bending, interlayer bonding tests, and micro X-ray CT to evaluate the multi-scale mechanical behaviour of 3DP-CC with varying coal gangue contents. Key findings include: (1) Pore structure evolves with coal gangue content, with total porosity first decreasing (to 1.8% at 10% content) then increasing (to 3.4% at 40% content), driven by aggregate skeleton and fine aggregate filling; (2) 3DP-CC’s compressive strength anisotropy is reduced compared to printed mortar due to aggregate interlocking, whereas flexural strength anisotropy increases as a result of pore accumulation and weak interlayers; at equal coarse aggregate content, 3DP-CC exhibits lower compressive anisotropy than printed natural aggregate concrete; (3) Compressive and flexural strengths increase initially and peak at 10%–20% coal gangue content, with values in all directions surpassing those of printed concrete with 40% natural aggregate. This work quantifies relationships between coal gangue content, structural anisotropy, and fracture resistance, offering actionable insights for industrial upcycling of coal wastes and addressing key challenges in eco-friendly 3D concrete printing.