Nanoscale insight on the epoxy-cement interface in salt solution: A molecular dynamics study

The migration of water and ions in the interfacial region of cement hydrate and epoxy determines the durability of FRP reinforced concrete composite. In this study, to better understand the nanoscale mechanism of the composite material subject to chemical attack, molecular dynamics is utilized to investigate capillary adsorption of NaCl and Na2SO4 solution in the nanometer channel of the calcium silicate hydrate (C-S-H) coated with epoxy molecules. The calcium ions immobilize the epoxy stably on the interior surface of C-S-H gel by connecting with silicate tetrahedron in the C-S-H and the oxygen-contained functional group in the epoxy molecule. It explains the good adhesion source between epoxy resin coating and cement-based material from the molecular perspective. The presence of epoxy molecules, narrowing the gel pore entrance and increasing the hydrophobic properties of the interior surface, significantly retards the migration of water in the C-S-H gel channel. Additionally, the silicate chains in the C-S-H gel surface provide non-bridging oxygen sites to attract the Na ions by forming Na-O bond. The SO4 and Cl ions can associate with the Na and Ca ions to form the anion-cation pairs. The interfacial chemical bond and ionic pair formation in the solution further reduce the transport rate of the ions. On the other hand, the ions and water in the gel pore can gradually ingress into the interfacial region between the epoxy molecule and the C-S-H substrate, which results in the Ca ions diffusing away from the substrate, part of epoxy chains dissociating from the silicate chains and finally weakening the interfacial connection. The distorted epoxy resin adsorbed on the C-S-H gel substrate acts like a polydispersity brush, changing the flow rate of water molecules at the interface region of the channel. The water resistance mechanism of epoxy coating and weakening effect from the detrimental ions can provide molecular insights for the durability study on the FRP reinforced concrete composite.