Cementitious materials are multiscale and multiphase composites whose frost resistance at the macroscale is closely governed by microstructural characteristics. However, the interfacial transition zone (ITZ) between clinker and hydrates, recognized as the weakest solid phase, plays a decisive role in the initiation and propagation of microcracks under freezing conditions. Understanding the frost damage mechanism of ITZ is therefore essential for improving the durability of concrete in cold regions. The motivation of this study lies in revealing how freezing affects the mechanical integrity and microstructure of ITZ in its early ages, which remains insufficiently understood in existing research. To address this, a nanoscratch technique was employed for its ability to quantify local fracture properties and interfacial adhesion at the submicronscale, providing a direct and high-resolution assessment of ITZ behavior under freeze–thaw action. The ITZ thickness and fracture properties were characterized in unfrozen cement paste and in cement paste frozen at 1 and 7 days of age to elucidate the microscale frost damage mechanism. Moreover, the enhancement effect of nano-silica modification on frozen ITZ was investigated through the combined use of nanoscratch and mercury intrusion porosimetry (MIP). The correlations among clinker particle size, ITZ thickness, and ITZ fracture properties were further established using nanoscratch coupled with scanning electron microscopy (SEM). This study provides a novel micromechanical insight into the frost deterioration of ITZ and demonstrates the innovative application of nanoscratch technology in characterizing freeze-induced damage in cementitious materials, offering theoretical guidance for designing durable concrete for cold environments.

Macroscopic fracture testing of nano-modified cement paste requires the labor-intensive fabrication of notched beams. This study establishes a micro-to-macro surrogate modeling framework mapping localized nanomechanical scratch signatures to macroscopic fracture resistance. A database was compiled for pure Portland cement paste (w/c = 0.30, 28-day curing) across seven nano-silica (NS) dosages (0.0–3.0%). Across 21 independent physical beam specimens (N_macro = 21) and 470 validated phase-segmented nanoscratch observations (N_micro = 470), localized descriptors were acquired across unhydrated clinker, hydration products, and the clinker–hydrate interfacial transition zone (micro-ITZ). Within a leak-free pipeline, an XGBoost surrogate model was trained using specimen-grouped nested cross-validation (Nested GroupKFold), achieving cross-validated R² values of 0.878 ± 0.024 (95% CI [0.834, 0.919]) for macro-fracture toughness ($K_{\mathrm{IC}}^{\mathrm{macro}}$) and 0.865 ± 0.028 (95% CI [0.815, 0.912]) for fracture energy (G_f). Leave-one-dosage-out validation demonstrated that when provided with the target mix’s in situ nanoscratch fingerprint, the model successfully tracked the non-monotonic fracture evolution, including the peak at 2.0% NS and the agglomeration degradation at 3.0% NS. Bootstrap Tree-SHAP attribution identified interfacial micro-toughness ($K_{\mathrm{IC},\mathrm{ITZ}}^{\mathrm{micro}}$) as the primary predictive contributor (42.5% [95% CI: 37.8%, 47.1%]), revealing a model-suggested transition region around 0.80 MPa ⋅ m^1/2 above which interfacial crack resistance associates with substantial macroscopic toughening. This framework provides an efficient surrogate approach for evaluating macro-fracture properties from micromechanical indicators.