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• The gangue micropores are converted into mesopores and macropores. • The content of various functional groups increased and reached the maximum at S60. • Dry cracking temperature increases, other characteristic temperatures decrease. • The combustion properties all following the chemical reaction mechanism(n = 3). A large amount of coal gangue (CG) produced in coal mining accumulates to form CG hills, and the combustion properties change under the effect of leaching, increasing the risk of spontaneous combustion. The study was mainly carried out by nitrogen adsorption, infrared spectroscopy and thermogravimetry on the physicochemical properties of the CG under different immersion times (30, 60, 90 days). And the activation energy of the immersion CG at different oxidation or combustion states was calculated. The results show that the pyrite contained in gangue is oxidized after immersion, and the heat released by oxidation and the acid solution formed have an obvious effect on the pore expansion of gangue. The mesoporous and macroporous increase by 2.61% and 0.95% respectively, which strengthens the physical and chemical adsorption of oxygen for CG. The immersion process also destroys the structure of the gangue, transforming stable functional groups into active ones, especially the most significant increase in hydroxyl and oxygen-containing functional groups. The characteristic temperature changed under the action of water immersion, the cracking temperature increased, and other characteristic temperatures decreased, among which the ignition temperature decreased by about 20℃. In addition, the water immersion process inhibits the low-temperature oxidation stage of CG and promotes the high-temperature combustion stage, and the risk of spontaneous combustion is the strongest after 60 days of immersion in water. The results can provide guidance for the prevention of spontaneous combustion in drenched CG hills. [ABSTRACT FROM AUTHOR]

• R 3 -CH structure especially the H atom is the most active site in coal self-heating with oxygen. • The free energy barrier of oxygen attacking R 3 -CH site is 8.28 kJ/mol. • The aromatic hydrocarbon radicals and hydroxyl radicals are significant for coal self-heating. • The carbon radical combining with an O plays a vital role in initial coal spontaneous combustion. R 3 -CH structure is a typical active group during coal self-heating. In this paper, Ph-CH(CH 3)–CH 2 -Ph was selected to analyze reaction activity as well as mechanism between R 3 -CH structure and oxygen in coal self-heating by Multiwfn and Gaussian calculation. The results demonstrate that R 3 -CH structure especially the H atom is the most active site in coal self-heating with oxygen. The oxygen attacks R 3 -CH site in the Ph-CH(CH 3)–CH 2 -Ph, which results in the breaking of C–H bond and formation of O–H bond. The free energy barrier of this oxidation reaction is 8.28 kJ/mol and the heat release of the process is 86.89 kJ/mol, which indicates that it can easily occur at room temperature. From the perspective of products, the aromatic hydrocarbon radicals and hydroxyl radicals related to the initial spontaneous combustion of coal are generated, which is of great significance for the further development of coal self-heating. Subsequently, the carbon radical is combined with a single oxygen atom to generate oxygen-containing free radical which plays vital roles in initial coal spontaneous combustion. [ABSTRACT FROM AUTHOR]