Durability degradation and pore structure evolution of sandstone from ancient buildings in Southwest China under water–salt cycling

This study investigates the degradation behavior of architectural sandstone from Southwest China under wet-dry cycling in pure water, 3% NaCl, 3% Na 2 SO 4 , and 1.5% NaCl + 1.5% Na 2 SO 4 solutions. A multiscale framework combining mass measurement, uniaxial compression, X-ray CT, SEM, XRD, pore network modeling, and seepage simulation was used to reveal the relationship between pore-structure evolution and mechanical deterioration. The results show that sandstone degradation is controlled mainly by the development of pore connectivity rather than by total porosity alone. After 50 cycles, the connected pore fraction in the Na 2 SO 4 group increased from 26.51% to 51.63%, while the uniaxial compressive strength decreased from 60.15 MPa to 27.60 MPa. Permeability simulation further showed that the Na 2 SO 4 group exhibited the largest increase in permeability, reaching 14.124 μm 2 . Compared with NaCl and mixed-salt environments, Na 2 SO 4 caused the most severe structural damage and strongest mechanical deterioration. These results indicate that pore connectivity is a key structural parameter governing staged sandstone degradation under water-salt cycling.