Application of clay minerals in the synthesis of Cretaceous mudstones and microstructural analysis of their deterioration mechanism

The Cretaceous mudstone undergoes significant argillization and disintegration when in contact with water due to its high content of clay minerals, posing a severe challenge to the stability of roadways in coal mines during construction and operation. This research aimed to prepare water-sensitive mudstone–like materials by the method of clay mineral composition similar to that of natural Cretaceous mudstone to reproduce the mechanical and hydraulic properties of natural rocks and meanwhile reveal the deteriorating effect of clay minerals on its microstructure and macroscopic properties. Using binary clay-gypsum mixtures and considering the aggregate-binder ratio, the clay-gypsum ratio, the iron sand content in fine sand, and the powder sand content in aggregate as controlling factors, 16 groups of proportioning schemes were established using the orthogonal test method and a series of physic mechanical experiments were conducted to determine its mechanical and hydraulic properties. Subsequently, sensitivity analysis was applied to characterize the effects of different influencing factors on mudstone-like materials' mechanical and hydraulic properties. The result shows that the distribution ranges of physical, mechanical, and hydraulic parameters of the newly prepared similar material overlap noticeably with those of the natural mudstone, suggesting that this new material can better meet the requirements of modeling natural mudstone. In addition, the range analysis showed that the aggregate-binder ratio was the dominant factor for the material's UCS, E, and cohesion, while the clay-gypsum ratio had a significant effect on its density and disintegration time. Then, qualitative and quantitative microstructural analysis was carried out on the SEM images of the four samples based on kaolinite-gypsum binary mixtures by Avizo software via the dynamic threshold segmentation method. The result indicates that the material microscopic parameters, such as pore size distribution, equivalent diameter, porosity, fractal dimension, etc., are significantly altered with the increase of kaolinite content, resulting in a marked deterioration of the material's micromechanical properties. This mineralogical and microstructural change transforms the cementation type from homogeneous and dense crystalline cementation to anisotropic and loose argillaceous cementation, exhibiting strong water sensitivity and extremely weak macro mechanical properties, which explains the deterioration and disintegration mechanism of natural mudstones from a micro mineralogical point of view. Importantly, the synthesis and microstructural analysis method based on mineralogy proposed in this study may be widely employed in rock mechanics and engineering.