Exploring microwave activation as a novel method for activating coal gangue: Focus on microwave activation mechanisms and hydration characteristics of cementitious supplementary materials

To support the decarbonization of the cement industry, coal gangue (CG) has potential as a supplementary cementitious material. The calcination process significantly impacts the phase transformation of clay minerals and the pozzolanic activity of CG. This study involved calcination of CG at 600 ℃ and 800 ℃ using both microwave and thermal activation methods. Employ XRD, XPS, FTIR, TG, and SEM to examine the impact of calcination mechanism on the crystal structure, element binding energy, chemical bonds, microstructure, and mineral composition of CG. The results indicate that the transformation of kaolinite in CG to metakaolin is completed around 600 °C. Microwave activation increased the reactive aluminate content in CG, enhancing pozzolanic activity and promoting secondary hydration with cement. This process generated additional C-(A)-S-H gels and AFm phases, which filled matrix pores, increased system density, and improved mechanical properties. The optimal activation method for CG was identified as microwave activation at 600 °C.