Freezing mechanism of NaCl solution ultra-confined on surface of calcium-silicate-hydrate: A molecular dynamics study

Cement-based materials in cold regions usually suffer deicer-frost deterioration. To better understand the frozen behavior of the calcium silicate hydrate (C-S-H), molecular dynamics is utilized to investigate the freezing processes of gel surface NaCl solution. The presence of C-S-H substrate significantly reduces the freezing temperature of water molecules ultra-confined on the C-S-H surface, which is 17 K lower than that of bulk water. While majority of random distributed water molecules crystalize to hexagonal ice (Ih) structure, molecules within 0.6 nm of C-S-H substrate cannot form ordered ice crystal at 225 K. The non-icing water layer and lower freezing point are mainly due to oxygen atoms on the silicate chains that provide strong hydrogen bonds with neighboring water and restrict the water orientation. Ionic clusters formed in unfrozen solution are important influence factor for water freezing. Hopefully, this work can provide molecular insights of cement-based materials design in cold regions.