Multiscale investigation of thermally activated coal gangue aggregate concrete interfacial transition zone evolution and failure mechanisms.

The large‑scale accumulation of coal gangue poses serious environmental and land‑use challenges, yet its direct use as concrete aggregate has been hindered by low reactivity, high porosity, and weak interfacial bonding. Although prior studies have examined thermal activation of coal gangue, the interplay among calcination temperature, pore‑size evolution, and mesoscale failure mechanisms in a unified multi‑scale framework remains underexplored. This study presents a multi‑scale experimental and numerical investigation of thermally activated coal gangue aggregate concrete, focusing on ITZ evolution and failure mechanisms. Coal gangue was calcined at 500, 600, 700, and 800 °C and substituted for natural coarse aggregate in C30 concrete. Uniaxial compression testing, SEM, XRD, NMR, and PFC3D meso‑scale simulations were employed to elucidate the effects of calcination temperature on 28‑day compressive strength, ITZ microstructure, pore‑size distribution, and crack initiation and propagation. Calcination at 700-800 °C maximized the release of active SiO₂/Al₂O₃, resulting in a denser ITZ, reduced porosity, and a 15.6-22.8% increase in compressive strength over uncalcined concrete. Calibrated meso‑scale models demonstrated that enhanced ITZ bond strength delayed crack onset and altered damage patterns. These findings offer theoretical insight and practical guidance for the sustainable valorization of coal gangue in high‑performance concrete applications.