Mechanisms of water-softening and freeze–thaw damage in remolded slurry: Linking microscopic structure evolution to macroscopic durability

High water content waste slurry generated from large-diameter bored piles exists in a fluid-plastic state, severely limiting its beneficial reuse in geotechnical engineering applications. This study systematically investigates the macroscopic mechanical response, failure mode evolution, microstructural reorganization, and macro–meso coupling mechanisms of remolded waste slurry subjected to the combined effects of water softening and freeze–thaw action. By integrating unconfined compressive strength (UCS) testing, digital image correlation (DIC) monitoring, mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) microstructural analysis, the deterioration pathways and coupling effects of water softening and freeze–thaw damage are elucidated. The results demonstrate that as the water content increases from 15% to 21%, both the UCS and the deformation modulus undergo exponential decay, and the softening coefficient exhibits a pronounced nonlinear correlation with water content. Following 12 freeze–thaw cycles, the specimen volume expands progressively and the freeze–thaw coefficient declines by 52.6%; the accompanying particle reorientation and the development of a macropore-throat-dominated pore structure constitute the microstructural origin of strength degradation. Water softening is governed by clay mineral swelling and pore water pressure, which induce non-uniform localized deformation, whereas freeze–thaw damage originates from frost heave associated with the water–ice phase transition and differential thermal contraction among mineral constituents, driving the propagation of crack networks. Under coupled conditions, elevated water content markedly amplifies freeze–thaw deterioration. A durability evaluation framework based on the softening coefficient and freeze–thaw coefficient is established. The remolded waste slurry maintains a macrostructural integrity retention rate exceeding 90% after 10 freeze–thaw cycles, demonstrating pronounced structural self-compaction and pore optimization effects.