Multiscale Evolution Mechanism of Expansion in Red-Bed Mudstone under Varied Salt Solution Type and Concentration

Red-bed mudstone is a typical soft rock with significant swelling characteristics, which often leads to expansion deformation when used in (ultra)high-speed railway subgrades. Given the widespread presence of soluble salts in natural environments, it is essential to investigate the influence of external ions on the rock expansion behavior. In this study, a multiscale approach integrating macroscopic expansion tests, microstructural evolution analysis, and molecular dynamics simulations was employed to elucidate the mechanism by which salt solutions inhibit mudstone expansion. The results demonstrate that salt solutions significantly suppress mudstone expansion within low concentration ranges, with a reduced expansion potential correlated to increased concentration or decreased cation valence. The critical concentrations for the equilibrium state of mudstone expansion in NaCl and CaCl2 solutions are 1.20 and 1.00 mol/L, respectively. Positive correlation between the concentration of Na2SO4 solution and the expansion rate can be well predicted using a logistic model. Mudstone expansion induced by Na2SO4 includes three stages: a rapid expansion stage, a slow expansion stage, and an expansion growth stage. Three stages correspond to gypsum expansion, saltpeter expansion, and the crack-penetration crystal-expansion cycle. The ability of monovalent cations to neutralize the negative charge on the surface of clay minerals is weak, and the thicker double layer formed implies a wider diffusion layer and a larger water film thickness, while divalent cations significantly compress the thickness of the double layer. Molecular dynamics simulation reveals the nanoscale mechanisms of influence of monovalent and divalent ions on water–rock reactions, where cations reduce the diffusion rate of water molecules and weaken the interaction energy at the water–rock interface. Diffusion coefficient under the influence of Ca2+ is 0.62 × 10–6 cm2/s and 0.17 × 10–6 cm2/s, lower than that under the influence of Na+, indicating that divalent cations have a more significant inhibitory effect on the diffusion behavior of water molecules than monovalent cations. This multiscale study provides theoretical insights into the deformation mechanisms of red-bed mudstone in salt-rich environments, offering valuable guidance for the design and maintenance of (ultra)high-speed railway subgrades.