Multi-modal NMR-SEM approach for deciphering salt crystallization-freeze-thaw synergistic damage in limestone

This study proposes a multi-modal Nuclear Magnetic Resonance-Scanning Electron Microscopy (NMR-SEM) approach to evaluate the synergistic damage caused by salt crystallization-freeze-thaw (SCFT) cycles in heritage limestone. Multi-parameter experiments were designed to systematically investigate the coupled effects of salt ions (from H2O, NaCl, Na2SO₄) and cyclic duration (0d-90d) on pore structure evolution. The research elucidated the dynamic evolution mechanism of micro-cracks under SCFT coupling and established a fractal dimension dynamics methodology to reveal nonlinear degradation patterns in the microstructure. The results show that SCFT-drived deterioration is governed by periodic temperature fluctuations, chemical dissolution of saline solutions and crystallization pressure. After 90 days, the porosity evolution rate followed the sequence the NaCl>Na2SO4 > H2O. Fractal dimension (D) increased nonlinearly with SCFT, 29.11% and 22.78% higher for NaCl and Na₂SO₄ than H₂O group. These results establish quantitative indicators for assessing SCFT coupled deterioration in grotto limestone.