Performance of microwave-activated coal gangue powder as auxiliary cementitious material

Coal gangue is presently the largest stock of industrial solid waste that poses considerable environmental and economic problem. Therefore, the resource utilization of coal gangue must be investigated. In this paper, coal gangue was used as raw material to make an activated coal gangue powder (ACGP) as an auxiliary cementitious material via microwave activation. The effects of ACGP on the mechanical properties of cement–coal gangue power mortar (CCGPM) were evaluated via X-ray diffraction (XRD), scanning electron microscopy (SEM), laser particle size analysis, and mercury intrusion porosimetry (MIP). Results indicated that microwave activation changed the mineral composition of coal gangue powder (CGP), separated its active substances, and made its particles more round and finer that those prior to the activation, thereby forming a CGP with pozzolanic activity. The strongest activity was recorded within the microwave temperature of 600 °C-700 °C. The activated CCGPM had the highest mechanical properties within this temperature range. The mechanism by which ACGP influenced cement hydration was through the decomposition of active substances in cement hydration, changing the hydration products of CCGPM, and filling the pores during hydration. The particle size of nonhydrated coal gangue was smaller than that of hydrated coal gangue, and these small particles acted as microaggregates in CCGPM that hindered the growth of early expansion cracks, thereby enhancing the strength of CCGPM.