Coupled mechanical-microstructural damage behavior of phosphoric acid-modified MOC under dry-wet-freeze-thaw cycling

To address the drawbacks of magnesium oxychloride cement (MOC), phosphoric acid was employed to modify MOC cement. MOC cement exhibits poor water resistance, and in cold regions, it is subjected to a complex, multi-stress environment that significantly exacerbates its poor water resistance. Modifying MOC cement can improve its water resistance and thereby enhance its stability in cold regions. This experiment adopted a multi-scale design to simulate temperature variations in cold regions and investigated the effects of dry-wet-freeze-thaw (DWFT) cycle coupling on MOC cement. Mechanical strength, mineral composition, microstructural changes, and pore distribution were analyzed through uniaxial compressive strength (UCS) testing, X-ray diffraction (XRD), scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR). Damage analysis was conducted on the test specimens based on the test results. The findings revealed that the strength loss of unmodified MOC exceeded 50 % after 20 DWFT cycles, whereas the strength of modified MOC increased by approximately 40 %. The addition of phosphoric acid prevented the dissolution of phase 5 crystals in the MOC system, and the porosity of the modified MOC cement decreased by about 50 %. A damage equation for specimens under DWFT cycles was derived and validated, enabling quantitative analysis of the impact of modifiers on MOC cement performance. The research findings hold significant importance for studying the durability and modification of MOC cement.