Analysis of calcination activation modified coal gangue and its acid activation mechanism

Acid activation and utilization of coal gangue is of great significance for effective treatment of bulk solid waste in the environment. However, most gangue is quite inert and requires calcination to induce acid activation. In this paper, coal gangue was calcined, and the optimal calcination temperature for activation of the coal gangue acid was explored. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetry/thermal differential analysis (TG/DTG) and Fourier transform infrared spectroscopy (FTIR) were used to analyze the crystal structures of the gangue samples formed at different calcination temperatures, which showed that increases in the amorphous content and structural degradation of the coal gangue samples occurred during calcination. The solubility of silicon and aluminum in acid solution was demonstrated with inductively coupled plasma optical emission spectroscopy (ICP‒OES). XRD and TG/DTG were used to characterize the acid activation products formed at different calcination temperatures. The results show that with the increase of calcination temperature, the compressive strength first increases and then decreases, and reached the maximum value at 650°C; the compressive strengths at 7 d, 28 d and 90 d were 1.32 MPa, 3.22 MPa and 5.14 MPa. At 650°C, the kaolinite in the gangue was converted into metakaolin, the aluminum-oxygen octahedra had the hydroxyl groups removed, the silicon-oxygen tetrahedra were depolymerized, the absorption bands for Si-O, Si-O-Al and Al-O bonds were broadened, the Al2p and Si2p binding energies were reduced, the splitting were increased, and more Al dissolved in the acid solution, so it has the highest acid reactivity.