Valorization of calcined coal gangue as coarse aggregate in concrete

Coal gangue is a by-product generated from coal mining and washing. Using coal gangue as aggregate is an effective approach of valorization this high-volume industrial waste, not only solving the environmental issue of accumulation but also reducing the mining of natural aggregate. However, compared to natural aggregate, the strength of coal gangue aggregate (CGA) is much lower, and its interfacial bonding in concrete is weaker. This paper studies the effect of calcination processes at 450 °C and 750 °C on the mineral and chemical compositions, microstructure and suitability of coal gangue as aggregate in concrete manufacturing. Water absorption, mechanical properties and interface structure of four concretes containing the original, 450 °C-calcined (CGA450), 750 °C-calcined (CGA750) and calcined-selected coal gangue aggregates (CGAs) were comparatively investigated. Experimental results show that the crushing index of the calcined CGAs decreases but water absorption increases. The interface of concrete prepared by CGA450 is porous, and its water absorption is significantly increased. Crushing index of CGA750 is 6.0% lower than that of original CGA, and the water absorption is increased by 3.9%. Because of thermal activation of Si and Al components on the surface, CGA750 can significantly improve the strength and interface structure in concrete. This study provides a theoretical guidance and technical support for the large-scale and high-value utilization of coal gangue coarse aggregates in concrete industry.