Microstructural damage and macro-mechanical degradation characteristics of reef limestone under wet-dry cycling conditions

Under prolonged wet–dry cycling, the structural integrity of reef limestone in island and reef coastal zones deteriorates, resulting in significant reductions in rock strength and engineering stability. This study investigates the degradation behavior and mechanisms of reef limestone subjected to repeated wet–dry cycles. Uniaxial compression, splitting tensile, and Split Hopkinson Pressure Bar (SHPB), combined with X-ray Fluorescence Spectrometer (XRF) and Scanning Electron Microscope (SEM) analyses, were conducted to evaluate strength degradation, damage evolution, and microstructural changes under varying cycle numbers (\(N\)). Fractal analysis of fracture debris established a linear relationship between \(N\) and the fractal dimension, quantitatively linking macroscopic mechanical degradation with microstructural evolution. Results show that as \(N\) increases, compressive and tensile strengths decline from 7.45 MPa to 3.13 MPa and 4.20 MPa to 2.08 MPa, respectively, with failure modes shifting from brittle to shear failure. Dynamic compressive strengths decreased to 20.43 MPa, 23.49 MPa, and 25.50 MPa. Increasing fractal dimensions indicate progressive fragmentation, while SEM and XRF reveal mineral dissolution, pore expansion, and microcrack connectivity as key degradation mechanisms. Overall, reef limestone exhibits significant mechanical and structural weakening under wet–dry cycling, highlighting the strong coupling between microstructural evolution and macroscopic damage—of critical importance to island and reef engineering.