Deciphering freeze–thaw induced degradation mechanisms in magnesium phosphate cement paste: Insights from three-dimensional quantitative characterization

Magnesium phosphate cement paste (MPC) has garnered significant attention as a promising material for rapid repair applications. Nevertheless, MPC-based composites exhibit suboptimal freeze–thaw (F-T) durability, and comprehensive quantitative insights into the degradation evolution and underlying deterioration mechanisms induced by F-T cycling are still lacking. In this study, we present a novel approach by integrating high-resolution non-destructive X-ray computed tomography (X-CT) with digital volume correlation (DVC) techniques to elucidate the structural degradation pathways and mechanisms of hardened MPC paste from a three-dimensional (3D) quantitative characterization perspective. The experimental findings reveal that F-T cycles alter the failure mode of MPC specimens, transitioning from a tensile–shear mixed mechanism to a tensile-dominated failure. The application of confining pressure notably enhances the fracture toughness of the material and promotes a shift towards shear-driven failure modes. F-T induced degradation exhibits a pronounced spatial gradient, initiating at the specimen periphery and propagating inward. Concurrently, substantial morphological transformations of pores are observed, with degradation regions evolving from a dispersed distribution to localized concentration. Moreover, the chemical dissolution of K-struvite crystals enlarges internal pores and enhances the connectivity of the pore network, thereby promoting the migration and accumulation of capillary water. This phenomenon intensifies freeze-induced expansion due to water–ice phase transitions at low temperatures, significantly accelerating the structural deterioration of MPC.