Evolution and fractal characteristics of pore structure in sandy mudstone under alkaline-rich aqueous conditions

An alkaline water-enriched environment reduces the bearing capacity of surrounding rock, inducing uneven deformation, collapse, and roof fall in roadways, thereby threatening construction safety. Sandy mudstone specimens immersed in alkaline water environments were studied. The strength deterioration and pore structure evolution of these rocks after immersion in solutions with different pH values were investigated using uniaxial compression (UC) tests and nuclear magnetic resonance (NMR) tests. A correlation function between the pore fractal dimension (D) and uniaxial compressive strength Rc was established, revealing the mesoscopic pore structure evolution characteristics and macroscopic strength degradation mechanism of sandy mudstone in alkaline water environments. The results show that: (1) As pH increases, micropores (pore size r1≤0.01 μm) and mesopores (0.01 μm r2≤1.00 μm) in sandy mudstone gradually expand and develop into macropores (pore size r3>1.00 μm), leading to a reduction in Rc. (2) The pore structure of sandy mudstone in alkaline water environments exhibits a multifractal characteristic, and its mechanical strength is closely related to the internal pore structure and distribution morphology. (3) With increasing pH, the number of axial fracture surfaces gradually increases, with numerous secondary cracks developing near the main fracture surface and pronounced spalling occurring. (4) The dissolution effect of alkaline water and the water-absorption swelling behavior of sandy mudstone are the fundamental mechanisms responsible for the evolution of mesoscopic pore structure and macroscopic strength degradation. The findings contribute to improving deformation control and disaster prevention for soft rock roadways in alkaline water environments.