H2O and CO2 confined in cement based materials: an ab initio molecular dynamics study with van der Waals interactions

Although the cement has been widely used for a long time, very little is known regarding the atomistic mechanism behind its functionality. Particularly, the dynamics of molecular systems at confined nanoporous and water hydration is largely unknown. Here, we study the dynamical and structural properties of H$_2$O and CO$_2$ confined between Tobermorite 9\AA (T9) surfaces with Car-Parrinello molecular dynamics with and without van der Waals (vdW) interactions, at room temperature. For H$_2$O confined, we have observed a broadening in the intra and intermolecular bond angle distribution. A shift from an ice-like to a liquid-like infrared spectrum with the inclusion of vdW interactions was observed. The bond distance for the confined CO$_2$ was increased, followed with the appearance of shorter (larger) intramolecular (intermolecular) angles. These structural modifications result in variations on the CO$_2$ symmetric stretching Raman active vibration modes. The diffusion coefficient obtained for both confined H$_2$O and CO$_2$ were found to be lower than their bulk counterparts. Interestingly, during the water dynamics, a proton exchange between H$_2$O and the T9 surface was observed. However, for confined CO$_2$, no chemical reactions or bond breaking were observed.