Physicochemical deterioration mechanism of red-bed mudstone during water–rock interactions: insights from soaking experiments

The softening of red-bed mudstone upon water exposure is a critical factor triggering geological hazards such as landslides. Previous studies have not sufficiently clarified the physicochemical mechanisms underlying the deterioration of red-bed mudstone under water–rock interactions. This paper takes the red-bed mudstone from three landslides in Qinghai. Through soaking experiments and combined with X-ray diffraction, scanning electron microscopy, pore and crack analysis system, ion chromatography, and inductively coupled plasma optical emission spectrometer, the changes in mineral composition, pore structure, and solution ion concentration of the mudstone before and after the soaking were tested and analyzed. Results indicate that the degradation of red-bed mudstone under water–rock interactions is the outcome of the combination of mineral dissolution and secondary formation, clay mineral expansion, water–rock ion exchange, and hydraulic erosion. Hydrolysis of clay minerals, hydrolysis and carbonation of feldspar and calcite, increased porosity by 52%–172.6% and pore quantity by 63.1%–197.9%. The cations released from the hydrolysis of feldspar and calcite participated in clay mineral transformation, increasing total clay minerals content by 3.95%–7.88%. These chemical reactions are key drivers of the deterioration of microstructure. Additionally, the increase in clay minerals and their expansion further degrade the mechanical properties of mudstone.