Mechanism of thermal activation on granular coal gangue and its impact on the performance of cement mortars

Coal gangue (CG) is a kind of solid waste discharged from the process of coal mining and washing without efficient utilization. Motivated by using the CG as supplementary cementitious materials (SCMs) for the partial substitution of cement, the CG was thermally activated via calcining under high temperature and the corresponding mechanism was discussed. The effect of calcination on the microstructure and pozzolanic reactivity of CG as well as the corresponding influence on the performance of Portland cement mortars were investigated via using TG/DSC, SEM, XRD and NMR. Results showed that, CG mainly consist of kaolinite and quartz. Under 550 °C, the kaolinite in CG was transformed into metakaolin, and the metakaolin was further transformed into mullite when the calcination temperature was elevated to 950 °C. Thermally activated CG had a high pozzolanic reactivity, which improved the mechanical properties of cement mortars. Compared with the mortars mixed with un-calcined CG, the 28d flexural and compressive strengths of mortars mixed with 30 wt% CG calcined at 750 °C were increased by 16.1% and 46.4%, respectively. This is attributed to the increased relative contents of Q3 (29Si NMR) and Al[5]+Al[4] (27Al NMR). The higher the relative contents of [SiO4] with Q3(-100.26 ppm) structure and Al[5]+Al[4], the higher the reactivity of calcined CG. This study develops a potential approach to recycle CG and develop greener cement materials.