Experimental study on concrete filling material using gangue as an aggregate: a case for along filling and retained roadway

Abstract This study develops an economical and environmentally sustainable filling material for gob-side entry retaining by utilising coal gangue as aggregate, thereby recycling solid waste and reducing reliance on cement. Orthogonal experiments were conducted to optimise the material’s flowability and mechanical properties by examining the effects of solid-cement ratio, slurry concentration, and water-reducing agent content. MATLAB-based linear regression and 3D visualisation were used to analyse how these factors influence uniaxial compressive strength, slump, and bleeding rate. The optimal mix proportion was identified as 82% slurry concentration, a solid-cement ratio of 6:1, and 0.05% water-reducing agent content. Under this mix, the gangue-based concrete achieved stable strength within 7 days, with a uniaxial compressive strength of 4.86 MPa, and exhibited favourable flow performance, including a slump of 224.5 mm and a layering degree of 14 mm. Triaxial tests further demonstrated its strain-hardening behaviour under confining pressure. Industrial application confirmed that this material effectively controls surrounding rock deformation and minimises surface settlement, meeting all engineering requirements for gob-side entry retaining. These findings provide a practical reference for the mix design of gangue-based filling materials under similar mining conditions.