Effect of calcination condition on the microstructure and pozzolanic activity of calcined coal gangue

• Pozzolanicity of coal gangue calcined under different conditions was studied.
• Phase transition temperature of kaolinite was measured by anaerobic TG-DSC.
• Crystallinity, structure and morphology of calcined coal gangue were characterized.
• Pozzolanicity of calcined coal gangue mainly depends on its amorphization degree.

In this paper, a systematic study was conducted to investigate the influence of calcination conditions including grinding time of raw material, temperature, holding time and heating rate on the pozzolanic activity of calcined coal gangue (abbreviated as CCG hereinafter). Furthermore, the changes of mineral composition, chemical structure and morphology of coal gangue during calcination were characterized by means of thermogravimetric-differential scanning calorimeter (TG-DSC), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM) analyses. The results show that heating temperature affects directly the mineralogy and crystallinity of CCG. Kaolinite in coal gangue transforms into metakaolin which is an irregular and amorphous phase as calcined at 600 °C–800 °C because of the dehydroxylation of Al–(O,OH) octahedrons together with the depolymerization of Si–O tetrahedrons. The amorphous metakaolin is of high pozzolanic activity; however, it will lose its activity due to the recrystallization into mullite when heating temperature is over 1000 °C. The pozzolanic activity of CCG is mainly dependent on the non-crystallizing degree of kaolinite after calcination.