Exploring solid-phase reaction mechanism to elucidate the formation of effective silica through thermal–chemical coupled activation of siliceous minerals in coal gangue

Coal gangue (CG) provides an abundant source of siliceous minerals that can be converted into effective silica. This study aimed to gain a deeper understanding of the evolutionary mechanism of quartz (Qtz) and kaolinite (Kln), in the conversion process of thermal–chemical coupled activation to effective silica. The structure transformation of two types of siliceous minerals and thermodynamic calculation were used to develop a phase diagram model for the formation of effective silica, and then provide a scientific basis for the thermodynamic study of solid-phase reactions. We showed that K₂CO₃ outperformed CaCO₃ in activation efficiency in the thermal–chemical coupled activation process, and Kln had a higher activation efficiency than Qtz. Notably, when the calcination temperature reaches 850 °C, the activation efficiency of the Kln system with added K₂CO₃ could exceed 50%. Characterization and analysis of the effective silica properties and structure revealed that low bioactivity effective silica existed in the minerals with (K, Ca)_xSi_yO_z, corresponding to the Q³ structure; while high bioactivity effective silica existed in the minerals with (K, Ca)_xAl_wSi_yO_z, corresponding to the Q³(1Al) structure. Finally, we used FactSage thermodynamic calculation software to further predict the phase transformation process of the solid-phase reaction. The simulation results highly matched the experimental data, providing strong support for effective silica synthesis and process optimization. This study provides theoretical basis for understanding the multi-pathway, multi-phase reaction mechanisms of siliceous minerals transforming into effective silica within CG.