Pozzolanic evaluation of mechanical-thermal composite activation of low-quality coal gangue and its impacts on the hydration properties of ordinary Portland cement

Although low-quality coal gangue (CG) has potential as a sustainable supplementary cementitious material, its low activity limits practical applications. This study employed mechanical-thermal activation to enhance the activity of CG. The Hinckley index (HI) and Liètard indices (R2) quantified the reduction in crystallinity induced by ball milling. In addition to evaluating the activity of composite-activated coal gangue (CACG), we also investigated the impact of CACG on the mechanical properties, hydration products, and microstructure of ordinary Portland cement (OPC) systems. The findings show that ball milling increases the interplanar spacing and reduces the crystallinity of kaolinite in CG. The HI and R2 decrease respectively by 18.7 % and 7 %, and their correlation coefficient reaches 0.9. Calcined at 600 °C, kaolinite in CG begins to transform into metakaolinite, while agglomeration occurs at 700 °C. The 28-day activity of CG reaches 80 % when it is ball-milled for 60 min and calcined at 650 °C. The strong linear correlation (R2 = 0.8) is observed between the strength activity index (SAI) and the assessed pozzolanic activity index (API), indicating that API is an efficient and economical method for evaluating pozzolanic activity. The CACG effectively promotes the hydration process of cement in the later stage, generating more calcium (alumino) silicate hydrate (C-(A)-S-H) and ettringite (AFt). This decreases the porosity of the cement paste and enhances the densification of the matrix structure. Life cycle assessment results indicate that compared to OPC, M3T2-30 exhibits a 7.9 % reduction in carbon emissions and a 5.3 % decrease in cost. These research results provide a practical approach for the industrial application of CG in low-carbon cementitious materials.