Molecular dynamics study on the chemical bound, physical adsorbed and ultra-confined water molecules in the nano-pore of calcium silicate hydrate

The movement of water molecules confined in the calcium silicate hydrate gel determines the chemical and physical properties of cement-based materials. In this paper, the molecular dynamics was utilized to investigate the structure and dynamic properties of interlayer water, surface adsorbed water and the capillary water molecules. The molecular structure of water molecule ultra-confined in the C-S-H gel or in the vicinity of calcium silicate surface is changed dramatically as compared with capillary water: enlargement of packing intensity, increasing magnitude of dipole moment, enhancement of the order interfacial organization, improvement of orientation preference and the extending of water-water spatial correlation. These structural features are attributed to the hydrophilic nature of the C-S-H surface that can provide non-bridging oxygen atoms in the silicate chains to accept H-bonds from the surface adsorbed water molecules and highly solvated calcium atoms in the interlayer region to associate confined water molecule to form hydration shell. Furthermore, three types of water molecules can be clearly distinguished by the diffusion coefficient derived from the mean square displacement of water molecules: Interlayer water (0.003 × 10−9m2 s−1) ≪ surface water (1.2 × 10−9 m2 s−1) < capillary water (2 × 10−9 m2 s−1). The slow dynamic properties of interlayer water resemble that in the glassy state: water molecules are hard to escape from the “cage” constructed by the H-bonds and ionic bonds network.