Molecular simulations of premelted films between C-S-H and ice: Implication for cryo-suction in cement-based materials

The premelted films between cement and ice observed at freezing temperatures play a role in the freeze-thaw damage of cement-based materials, as they play a role in cryo-suction. However, their properties are poorly understood. In this work, we unveil the dynamics and local structure of the premelted film using molecular simulations. Combining those results with a thermodynamic analytical derivation, we obtain a characteristic time of the cryo-suction process, which depends on temperature, the transport properties and thickness of the premelted film, and the pore geometry. Our calculations show that, down to about 230 K, cryo-suction occurs relatively fast at the pore scale and the hypothesis of local thermodynamic equilibrium used in most poromechanical models of freezing of cement-based materials is reasonable. A fragile-to-strong transition of water in the premelted films is witnessed at about 230 K. Below this temperature, the significant deceleration of water dynamics makes the cryo-suction significantly slower.