Antifreeze Protein for Freeze–Thaw Durability Enhancement of Cement Mortar: Effects and Action Analysis

Enhancing the freeze–thaw resistance of cement-based materials in a green and efficient manner is crucial for hydraulic structures in cold regions. This study investigated the effects of soybean antifreeze protein (AFP) on the freeze–thaw durability of cement mortar through mechanical testing, low-temperature microscopy, NMR analysis, and frost-heaving stress monitoring. The results show that AFP improves freeze–thaw durability, with 0.5% dosage outperforming 1.0%. Relative to the control, the relative ice content at −20 °C decreased from 62.81% to 40.01%, and frost-heaving stress declined from 321.15 kPa to 123.04 kPa. Microscopy and pore structure analyses revealed that AFP transforms ice crystals from needle-like to fine granular forms, inhibiting ordered growth and retarding pore coarsening. A frost-heaving stress model based on the Gibbs–Thomson effect and ice-crystal fractal characteristics indicated that AFP suppresses stress development by reducing effective ice formation, weakening stress transfer, and increasing ice-crystal boundary complexity. This study offers insights for developing green antifreeze admixtures for cement-based materials in cold regions.