In Situ Gel Formation, Pore Network Evolution and Mechanical Degradation of Red Sandstone Under Chemical–Wet–Dry Cycles

This study investigates in situ hydrogel formation and its regulating effect on multiscale damage evolution in red sandstone subjected to chemical–wet–dry cycles. Uniaxial compression, X-ray diffraction, scanning electron microscopy coupled with energy-dispersive spectroscopy, mercury intrusion porosimetry, and inductively coupled plasma mass spectrometry tests were performed to characterize mechanical degradation, mineral alteration, pore-network evolution, ion migration, and gel micromorphology. By combining multiscale experimental characterization with a segmented statistical damage constitutive model, this study describes the hydrogel-mediated damage evolution of red sandstone under chemical–wet–dry cycles. The mechanical properties of red sandstone show nonlinear degradation, with a deterioration order of acidic > alkaline > neutral, and this effect intensifies with increasing cycle number. After 15 cycles at pH = 3, the compressive strength and elastic modulus decreased by 38.21% and 27.12%, respectively. Both acidic and alkaline environments promoted pore development in red sandstone. After 15 cycles at pH = 3, the porosity increased from 21.51% to 24.51%, and the most probable pore diameter shifted from 21.32 μm to 25.88 μm. The porosity increased by 2.86% at pH = 11, and in situ hydrogels formed under alkaline conditions partially filled pores and inhibited crack propagation. The developed model effectively reproduced the mechanical evolution of red sandstone, with all fitted results showing R2 values no lower than 0.92. These findings provide a basis for evaluating hydrogel-regulated damage in red sandstone and support the application of in situ gel materials in geotechnical engineering.