Key Microscopic Control Mechanisms for Fracture Properties of Cementitious Materials Under Normal and Extreme Environments

Irreversible deterioration of internal microstructure in cementitious materials is induced by extreme service environments, which leads to severe degradation of macroscopic fracture properties. In this study, the fracture mechanics behavior of cementitious materials was systematically investigated under benchmark (N), continuous drying (D), extreme high-temperature (H, 400 °C), and rapid freeze–thaw cycle (F25–F100) conditions. By means of mercury intrusion porosimetry (MIP), nanoscratch technology, and backscattered electron (BSE) image segmentation processing, the evolution laws and synergistic damage mechanisms of the porous continuum matrix, aggregate–paste interfacial transition zone (ITZ), and microcrack network were quantitatively revealed. The results show that the macroscopic fracture energy Gf exhibits significant environmental sensitivity and nonlinear decay characteristics; after drying, F100 freeze–thaw, and 400 °C high-temperature exposure, Gf was decreased by 11.1%, 67.2%, and 83.0%, respectively. Microstructural analysis confirms that total porosity Pt is increased and critical pore diameter dcr is coarsened and right-shifted by environmental deterioration; nanoscratch tests reveal that the ITZ is always the lowest energy consumption channel for crack propagation, and its local fracture toughness KCITZ suffers a catastrophic drop under high-temperature and severe freeze–thaw conditions (down to 0.18 MPa⋅m1/2); the microcrack area ratio C is sharply increased (up to 4.65%), and the distribution morphology is evolved from local dispersion to three-dimensional network bridging. The macroscopic fracture mechanism is thus degraded from three-dimensional matrix shear and aggregate interlocking energy dissipation to two-dimensional debonding and slip along low-resistance interfaces. Based on micro–meso physical mechanisms, a cross-scale synergistic prediction model for fracture energy considering matrix pore volume loss (Pt), microcrack network segmentation (C), and ITZ shear anti-cracking barrier (KCITZ) was established. The prediction desensitization problem of the single porosity index under extreme conditions was accurately eliminated by this model (R2 = 0.984, RMSE = 4.12 N/m), and theoretical support is provided for service life assessment and crack-resistance toughening design of cementitious engineering structures under extreme environments.