Synergistic strengthening of high-volume coal gangue coarse aggregate concrete via composite activation: Mechanical performance and microstructural evolution

The utilization of coal gangue as coarse aggregate in concrete provides a viable pathway for valorizing coal-based solid waste while alleviating the shortage of natural aggregates. However, raw coal gangue is inherently porous, highly absorptive, and weak in crushing resistance, resulting in pronounced mechanical degradation at high replacement ratios. This study proposes a composite activation strategy integrating high-temperature calcination with cementitious wrapping (HTC–CW) to reinforce the aggregate matrix and improve interfacial performance. Concrete mixtures were prepared with coarse aggregate replacement ratios of 0–100% by volume, and mechanical behavior was analyzed together with pore structure evolution and interfacial micromechanics. The HTC–CW treatment markedly improved aggregate physical properties and mitigated strength loss, achieving a 28-day compressive strength of 50.86 MPa at 100% replacement. Low-field nuclear magnetic resonance (NMR) results indicate that composite activation suppresses harmful large pores and shifts the pore system toward finer regimes. Microhardness mapping further reveals increased and more uniform interfacial transition zone (ITZ) hardness, indicating enhanced load-transfer efficiency. Ryshkewitch–Duckworth analysis confirms an exponential porosity–strength relationship for raw and single-activated systems, whereas HTC–CW exhibits a positive deviation at comparable porosity, evidencing additional interfacial strengthening. Moreover, compressive strength shows a strong linear correlation with ITZ microhardness (R² ≈ 0.99), and a three-phase microhardness-based multivariate model captures strength development within the available dataset. These results demonstrate that synergistic “internal reinforcement–external sealing” and ITZ strengthening enable high-volume and full replacement of coal gangue coarse aggregates in structural concrete.