Mechanical Behavior and Mix Design Optimization of Hybrid Mudstone–Sandstone Coal Gangue Aggregate Concrete

Recycling crushed coal gangue as concrete aggregate enables the clean, low-carbon, and sustainable disposal of coal gangue. This paper systematically investigates how gangue lithology and aggregate grading parameters jointly affect the mechanical properties and mixture design of coal gangue concrete. Test results show that higher grading parameters of coal gangue aggregate improve concrete workability. For grading parameters under 0.4, compressive strength fluctuates with water–cement ratio, whereas it decreases monotonically with increasing water–cement ratio when grading parameter exceeds 0.4. The optimal mixture is determined as water–cement ratio 0.6 and grading parameter 0.6. Mudstone gangue drastically modifies the compressive stress–strain behavior of gangue concrete. Compressive strength and elastic modulus drop by 28.3% and 22.4%, respectively, as mudstone content increases from 0% to 75%, with sharp mechanical degradation observed beyond the 75% threshold. As mudstone proportion rises from 25% to 100%, the dominant failure mode shifts from interface debonding and matrix cracking to a composite failure involving interface debonding, aggregate fracture and matrix rupture. For on-site construction, supplementary non-argillaceous gangue is recommended for mixtures with a mudstone content over 75% to maintain acceptable mechanical performance of coal gangue aggregate concrete.