Phase-Scale Fracture Mechanisms of Magnesium Phosphate Cement under Wet-Dry Cycling: Insight from the Real- Structure-Based DEM Simulation

To investigate phase-scale crack propagation in magnesium phosphate cement (Magnesium Phosphate Cement, MPC) subjected to wet–dry (W–D) cycling under uniaxial compression, we propose a CT-RSB (Real Structure-Based) modeling approach that integrates high-resolution internal-structure characterization with particle-flow numerical simulation to construct a two-dimensional RSB model based on the real phase morphology. The mesoscopic parameters of the proposed model were calibrated against macroscopic mechanical parameters and failure patterns obtained from laboratory uniaxial compression tests, and the calibrated model was then used to elucidate phase-scale crack evolution in MPC. Results indicate that the CT-RSB approach can accurately capture the hard-brittle nature of MPC and its splitting-type failure mode. Under uniaxial compressive loading, MPC damage evolution can be divided into four stages: an initial stage, a crack initiation stage, a stable crack propagation stage, and an unstable crack propagation stage. During failure, microcracks sequentially develop as interphase tensile cracks, interphase shear cracks, intraphase tensile cracks, and intraphase shear cracks. MPC without W–D cycling exhibits a typical tensile-dominated failure; with increasing W–D cycles, the proportion of shear cracks rises from 1.3% to 22.9%, and the failure mode gradually transitions to a combined tensile–shear pattern. From the perspective of phase composition, microcracks initiate first within the K-struvite (KS) phase and then propagate into the magnesium oxide (MgO) phase and amorphous hydration products. As W–D cycling intensifies, stress concentration within MPC becomes increasingly pronounced, and the driving factor for crack propagation shifts from KS-dominated to pore-dominated.