Molecular Dynamics Study on the Structure and Dynamics of NaCl Solution Transport in the Nanometer Channel of CASH Gel

The transport of water molecules and ions in the nanopores of calcium aluminosilicate hydrate (CASH) influences the durability and sustainability of environmentally friendly cement-based materials with industrial waste substitution. In this study, molecular dynamics was utilized to study aqueous NaCl solution capillary transport through the calcium silicate hydrate (CSH) and calcium aluminosilicate hydrate (CASH) gel pore with pore size of 3.2 nm. The invading depth for the NaCl solution advancing frontier with meniscus shape follows a parabolic relation as a function of time, consistent with the classic Lucas–Washburn equation in capillary adsorption theory. As compared with the solution transport in the CSH pore, both water molecules and ions migrate more slowly in the gel pore of CASH, and sodium ions accumulate in the entrance region of the gel pore. The incorporation of Al atoms in the silicate substrate resists the ingress of ions and water. The Al–Si substitution on the CASH interface enhances the charge negativity of solid oxygen atoms, which polarizes the dipole moment of surface water molecules to a larger extent, strengthens the interfacial hydrogen bond, and elongates the residence time of water near the aluminate substrate. In addition, the silicate–aluminate chains in the CASH substrate provide plenty of oxygen sites to associate with the sodium ions by forming a stable Na–OS bond, immobilizing the cations deeply in the vacancy region of the aluminate–silicate channel. The inner sphere adsorption of Na ions on the CASH surface further contributes to the secondary outer sphere adsorption of the Cl ions by forming the Na–Cl ionic pairs. Hopefully, the transport and adsorption mechanism of the ions and water in the CASH gel can help guide the cementitious material substituted by Al-rich industry waste with sustainability and durability.