Mechanical properties and mechanism of carbonated coal gangue concrete after elevated temperatures

To broaden the engineering applications of coal gangue and address the limited research on the fire resistance of carbonated coal gangue concrete (CGC), this study systematically investigated its mechanical performance and underlying mechanisms under combined carbonation and high-temperature exposure. Accelerated carbonation and thermal tests were conducted on CGC with varying replacement ratios. The macro-mechanical properties, microstructure, and phase evolution were examined through compressive and splitting tensile strength tests, X-ray diffraction, and scanning electron microscopy. Predictive models for carbonation depth and compressive strength before and after high-temperature exposure were also developed and validated against experimental data. Results showed that incorporating 50 % coal gangue significantly improved density and compressive strength, with a 14.36 % increase under ambient conditions and further enhancement after carbonation and heating. Under coupled carbonation–fire conditions, compressive strength increased by up to 89.1 %. These improvements were attributed to the thermal activation and carbonation reactivity of coal gangue, which generated dense reaction products that refined the interfacial transition zone. Although elevated temperatures caused some structural damage, the formation of thermally stable phases strengthened post-fire behavior. Overall, this study is the first to systematically couple carbonation and high-temperature exposure in CGC and to establish predictive models, highlighting its potential as a sustainable and fire-resistant material.