This study investigated the freeze–thaw resistance of ordinary and nano-TiO₂-modified concrete (NTC) based on mass loss, ultrasonic velocity, compressive strength, and fracture toughness. The compressive behavior and internal damage evolution were further analyzed using particle flow code in two dimensions (PFC^2D) simulations. The results show that, although neither material exhibited structural collapse after freeze–thaw cycling, visible surface damage was observed, particularly in ordinary concrete. After 100 cycles, NTC showed a 52.17% reduction in mass loss and a 37.31% increase in ultrasonic velocity compared to ordinary concrete. Compressive strength of ordinary concrete decreased by 24.28 MPa (from 41.53 MPa to 17.25 MPa), while that of NTC decreased by only 13.37 MPa, demonstrating that the incorporation of nano-TiO₂ effectively improves the compressive performance of concrete under freeze–thaw conditions. Fracture toughness after 100 cycles decreased by 89.7% in ordinary concrete and 80.9% in NTC, suggesting that while nano-TiO₂ mitigates damage, its effect on maintaining fracture load-carrying capacity remains limited. The PFC^2D simulations were consistent with the experimental results, effectively capturing peak compressive behavior and validating the model’s applicability for freeze–thaw degradation analysis.

This study optimized the mix proportions of ultra-high-toughness fly ash-based geopolymer composites (EGC) and investigated the effects of sub-elevated temperature curing on its tensile properties using dumbbell-shaped tensile tests, microstructural analysis, and Digital Image Correlation (DIC). The results indicate that fiber incorporation significantly mitigates matrix brittleness, enabling EGC to achieve an ultimate tensile strain of up to 4.5%. Under ambient curing, both tensile strength and strain capacity improved with age. Sub-elevated temperature curing markedly enhanced early-age mechanical properties; at later ages, tensile strength further increased while strain capacity slightly declined. Microstructural analysis revealed that sub-elevated temperature curing accelerated geopolymerization, resulting in a denser matrix, with DIC confirming that EGC’s high ductility stems from multiple micro-cracking propagation. By adopting the Menetrey–Willam damage model, numerical simulations successfully captured the stress concentration and damage redistribution process, mechanically confirming the pseudo-plasticity and multiple damage evolution of EGC.