Freezing of water confined between calcium-silicate-hydrate layers: a coarse-grained molecular dynamics study

Freezing of water has a direct impact on the durability of cementitious materials, which include porous binding phases, like calcium-silicate-hydrate (CSH). Molecular dynamics (MD) simulations can capture behaviors of the water freezing in the pores and may give rise to insightful observations. In this work, the monoatomic water (mW) model and all-atom CSH model are combined to study low-temperature behavior of water molecules restricted within CSH layers. The developed model features good accuracy and accelerated computational rate, and is employed to study how ultra-confined water in CSH phases (<= 6 nm) freezes. The threshold width of CSH gap that allows the stable existence of ice is 1.5 +/- 0.1 nm. A non-icing layer with an average thickness of similar to 0.65 nm exists ambient to CSH surface. The main factor affecting the melting point of confined water is the size of the confined space rather than the water filling fraction. The Gibbs-Thomson model is utilized to describe the relationship between the melting temperature and the width of the confined CSH layers. The utilization of this modeling technique presents a novel approach to replicate the freezing process of nanostructures in cementitious composites subjected to low-temperature erosion.