Multi-scale deterioration mechanism of shear strength of gypsum-bearing mudstone induced by water-rock reactions

Investigation of water-rock reactions effects on the shear strength of gypsum-bearing mudstone is necessary for underground engineering design and assessment of engineering safety. A comprehensive study of the multi-scale deterioration effects and mechanisms of water-rock reactions on gypsum-bearing mudstone can contribute to developing better engineering applications. The 25 sets of laboratory dissolution tests and 30 sets of conventional rock mechanical compression tests conducted in this study demonstrate the progressive modification of rock properties on multiple scales, including changes in mineralogy, porosity, grain structure, microstructures, and complexity of micropores, with sodium sulfate and gypsum being the most active minerals. Water-rock reactions cause irreversible morphological changes in microstructures. For instance, the number of intracrystalline pores increased, the contact between grains changed from face-to-face contact to line-to-line contact, and the volume of micropores, mesopores and macropores inside the rock increased by a maximum of 80.0%, 59.9% and 71.4% respectively. The maximum increase in the porosity of the samples was 5.33%. The increasing number of microphysical changes leads to defects that develop into weak areas, significantly decreasing the shear strength of gypsum-bearing mudstone. The maximum decrease in the cohesion of the samples was 79.06% throughout the dissolution process. It was found that the decrease in the cohesion of the samples c at the early stage of dissolution is mainly affected by the increase in the micropore volume. In the middle and late stages, the cohesion c of the samples decreased in a straight line, which is mainly caused by the increase in the macropore volume and porosity. The fluctuation in the cumulative pore volume during the whole dissolution process leads to fluctuation in the internal friction angle φ. The multi-scale degradation model of rock samples induced by water-rock reactions should be formulated to accurately evaluate the effect of water-rock reactions on rock shear strength.