Microstructural Mechanisms of Concrete Degradation Under Different Coal Gangue Sand Replacement Ratios

Coal gangue manufactured sand (CGS), a sustainable substitute for natural sand, offers both resource and environmental benefits; however, the micro-mechanisms underlying performance deterioration at different replacement levels remain unclear. In this study, cube specimens with 25%, 50%, 75%, and 100% CGS were tested in uniaxial compression, and the results were integrated with PFC2D discrete-element simulations and SEM observations to establish an energy–force-chain–crack coupling framework. Experiments and simulations showed close agreement in peak stress, peak strain, and overall curve shape (errors generally <5%). With increasing replacement, the interfacial transition zone (ITZ) evolves from a dense three-phase ITZ (NS–CGS–CA; natural sand–CGS–coarse aggregate) to a degraded two-phase ITZ (CGS–CA), accompanied by more pores and microcracks; the proportion of Adhesive cracks decreases while Cohesive (intra-particle) cracks increase. Concurrently, continuous force-chain networks deteriorate into localized short-chain clusters; the peak and fraction of strain-energy decrease, whereas frictional/damping dissipation rises—together driving a macroscopic transition from ductile to brittle behavior. At 28 d, SEM images and DEM evolution of cracks/force chains/energy exhibit strong consistency, further confirming that low replacement (25% and 50%) favors stable load-transfer paths and suppresses early cracking, whereas high replacement (75% and 100%)—through ITZ degradation and force-chain instability—induces more concentrated cracking and higher energy dissipation, thereby diminishing mechanical performance.