Mechanical—chemical coupling damage and constitutive model of argillaceous siltstone under acidification

Argillaceous siltstone's mechanical–chemical coupling damage mechanism under acidic solutions is complex and crucial for engineering safety. This study selected typical argillaceous siltstone. Through immersion tests, mechanical property tests, and micro- and mesostructure determination tests, along with equipment such as ion concentration meters, scanning electron microscopy (SEM), and X-ray diffraction (XRD), the damage characteristics in different acidic solutions were explored. The results show that acidic solutions trigger complex ion exchange in argillaceous siltstone, significantly altering its mineral composition, mesostructure, and mechanical properties. The H⁺ concentration notably affects cation release, and the increase in Ca concentration indicates the key role of carbonate mineral dissolution in the ion exchange process. Stronger acidity and longer immersion lead to more severe mesostructure damage and more obvious mechanical property degradation. Using the strain - equivalence hypothesis and Weibull distribution, a damage statistical constitutive equation for rock samples under acidic solutions and immersion time was established, clarifying relationships between model parameters and relevant factors. This study reveals the intrinsic relationship among mineral composition changes, micro–mesostructural evolution, and macroscopic mechanical degradation of argillaceous siltstone under acidic conditions, and clarifies its damage evolution mechanism from a multi-scale perspective. The established model can effectively characterize the damage evolution process of argillaceous siltstone.