Deterioration characteristics of water-rock interaction on combustion metamorphic rocks in northern Shaanxi

There are a lot of combustion metamorphic (CM) rocks formed by high temperature baking of coal fires in the border area of Shaanxi, Shanxi and Inner Mongolia in the northeastern margin of Ordos Basin. Compared with the virgin rock, the rock mass subjected to combustion metamorphism is broken in structure, with pores developed, and a large number of dangerous rock masses are formed under the long-term water-rock interaction. Herein, 50 dry-wet cycle experiments were carried out with the CM rocks in Daliuta Area of Shenmu in Shaanxi Province. Meanwhile, the changes in the physical and mechanical properties of CM rocks were tested after different cycles, and the deterioration characteristics of CM rocks with different degrees of combustion metamorphism were discussed. The results show that: the CM rock has the quality, hardness and mechanical properties lowered, the surface glossiness and brightness decreased, and the roughness and red-yellow degree increased. According to the low-field nuclear magnetic resonance (NMR) testing, the pore volume proportion curve of the CM rock shifts to the right and the maximum pore size expands. That is, the micropores change to macropores, with the macropore content and total porosity increased. Besides, the combustion metamorphism causes the development of pores in the rock mass, improves the anti-erosion ability of the CM rock, and reduces the variation of chromaticity, quality, roughness, hardness and mechanical properties. The long-term water-rock interaction could significantly change the physical and mechanical properties, as well as the pore structure, of the CM rocks. Moreover, the degree of combustion metamorphism and the initial pore structure could affect the degree of deterioration. According to the study on the deterioration process of different CM rocks, it is concluded that the erosion and fracture intensity of water-rock interaction could be reduced by strengthening the surface of rock mass with low degree of combustion metamorphism, reinforcing the sedimentary bedding and the original fissures, and plugging the pore of rock mass through high degree of combustion metamorphism.