Cementitious properties of coal-based metakaolin prepared from coal gangue via Fe3O4-Assisted microwave activation

Abstract

Coal gangue is the largest emission of solid waste in the coal industry, and its large accumulation has posed serious hazards to environment. Low-quality coal gangue can be transformed into an auxiliary cementing material (coal-based metakaolin) with pozzolanic activity after calcination. However, the conventional calcination has slow heating rate, high energy consumption and serious pollution issues. Therefore, the fast and efficient calcination of coal gangue must be explored. In this study, coal series kaolinite (coal gangue with a kaolinite content of more than 80%) from Inner Mongolia was calcined by Fe3O4-assisted microwave calcination craft to prepare coal-based metakaolin (CBM). Results indicated that the efficiency of the microwave calcination of coal series kaolinite powder (CSKP) was considerably increased by adding analytical-grade Fe3O4 to CSKP. After substituting 30% of cement with CBM prepared by Fe3O4-assisted microwave calcination of CSKP to prepare cement–coal based metakaolin mortar (CCBMM), the mortar specimen made of CBM prepared at 675 °C showed the highest compressive strength (61.3 MPa) and activity index (112%). The analytical-grade Fe3O4 particles in CSKP will form spherical hematite microparticles (Fe2O3) with good dispersion after microwave calcination at appropriate temperatures (650 °C and 675 °C), which acted as microaggregates to optimize the pore structure of CCBMM, thereby enhancing the strength of CCBMM. The findings of this study confirmed that analytical-grade Fe3O4 addition not only increased the efficiency of preparing CBM by microwave calcination and reduced energy consumption, but also improved the cementitious performance of CBM.