Computational Molecular Modeling of the Multi-Scale Dynamics of Water and Ions at Cement Interfaces

Abstract Structural and dynamic behavior of H2O molecules and aqueous at in-terfaces and in nanopores of model C-S-H binding phase (tobermorite) is quanti-fied on the basis of molecular dynamics computer simulations. At the (001) sur-face of tobermorite in contact with 0.25 M KCl aqueous solution, we can effectively distinguish water molecules that spend most of their time within chan-nels between the drierketten chains of silica on the tobermorite surface from the adsorbed molecules residing slightly above the interface. Within the channels, H2O molecules donate H-bonds to both the bridging and non-bridging oxygens of the Si-tetrahedra as well as to other H2O. Some of these molecules form very strong H-bonds persisting over 100 ps and longer, but many others undergo fre-quent librations and occasional diffusional jumps from one surface site to another. The average diffusion coefficients of the surface-associated H2O molecules that spend most of their time in the channels and those that lie above the nominal inter-face differ by about an order of magnitude (DH2O[internal]=5.0×10-11 m2/s and DH2O[external]=6.0×10-10 m2/s, respectively). The average diffusion coefficient for all surface-associated H2O molecules is about 1.0×10-10 m2/s. All of these values are significantly less than the value of 2.3×10-9 m2/s, characteristic of H2O self-diffusion in bulk liquid water, but they are in very good quantitative agreement with experimental data on the dynamics surface-associated water in similar ce-ment materials obtained be 1H NMR [1,2]