In radioactive waste repositories and tunnel engineering, the near-field groundwater environment is extremely complex, and changes in multiple chemical components are bound to disrupt the long-term stability of soft rock and soil, thus inducing a series of environmental and engineering geological problems. In this study argillaceous slate was selected as the research object and different types of aqueous chemical solutions were prepared. Lateral constraint expansion tests, X-ray diffraction tests, nuclear magnetic resonance tests and scanning electron microscopy tests were then carried out. The expansion characteristics and evolution mechanism of argillaceous slate in a hydrochemical environment are then revealed at the multiscale level, and the internal relationship between the macroscopic expansion of argillaceous slate and the microscopic and mesoscopic structures explored. Finally, based on experimental data and the grey theory, a non-equal step GM(1, N) grey prediction model was constructed. The results indicated that: (1) the water‒rock physicochemical reactions mainly affect the microstructure evolution of the argillaceous slate expansion process through pore changes, mineral dissolution and ion exchange; (2) there is a certain correlation between the microstructure parameters and the expansion rate, and it was found that pore volume and clay mineral content are the main factors affecting the expansion of soft rocks; and (3) the model verification results show that the non-equal step GM(1, N) grey prediction model established in this study can effectively predict the expansion trend of soft rocks in different hydrochemical environments. The research findings can provide a reference for the study of the expansion mechanism of soft rocks in complex hydrochemical environments and the disposal of expansion problems.

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