Coal gangue, a type of kaolinite clay in coal measures, is one of the largest categories of solid industrial wastes in China. Alumina extraction from coal gangue could offer an alternative to bauxite as a source of alumina. Mechanically modification by grinding is an effective activation method to improve alumina recovery from coal gangue. The effect of grinding on coal gangue, however, had not been investigated systematically. Present understanding regarding the effects of grinding on coal gangue is still restricted to that on kaolinite. Furthermore, the differences in the inherent mechanisms between grinding and thermal activation on coal gangue have not been investigated so far. This study aimed to clarify the differences among them. Changes in particle size, surface area, and morphology were followed by means of laser particle size analysis, nitrogen adsorption/desorption isotherms, and scanning electron microscopy (SEM). Possible modifications of the chemical structure, mineral phase and thermal behavior were monitored using X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Infrared analysis (IR), and Thermogravimetry (TG). The results showed that the alumina dissolution from coal gangue increased with longer grinding time. Grinding followed by calcination further improved the alumina extraction. After the calcined coal gangue was ground for 20 h, the alumina extraction reached similar to 95%. The results of XRD, XPS, and IR analysis implied that grinding produced defects in the coal gangue kaolinite; however the main kaolinite layer structure (Si-O-Al-VI) were unaffected in the absence of calcination. In this case, the improved activity might be due to reduced particle size, increased surface area, and aluminum enrichment at the surface. The primary structure of kaolinite in coal gangue was destroyed by calcination after grinding, and kaolinite was altered to amorphous metakaolinite with Si-O-Al-VI structure, which facilitated alumina extraction. (C) 2016 Elsevier B.V. All rights reserved.