Nanoscale insights into the interfacial characteristics between calcium silicate hydrate and silica

The interfacial characteristics between cement paste and silica are far from being fully understood, especially from the nanoscale perspective. Herein, molecular models were used to provide comprehensive insights into the interfacial characteristics between calcium silicate hydrate (C-S-H, the main binding phase of cement paste) and silica. Chemically, various types of bonds existed at the interface, including H-bonds and Ca–O bonds, and proton (H⁺) exchange occurred between C-S-H and silica. An increase in the water content of C-S-H could depress the deprotonation of the Si-OH groups on the silica surface. Structurally, an atomic-level interfacial transition zone (ITZ) with a low density was identified, which was attributed to the rich presence of –OH groups at the C-S-H–silica interface. The water molecules and calcium ions in the ITZ diffused faster than those in the bulk C-S-H. Mechanically, the interfacial bond strength was inversely related to the water content of C-S-H, with the higher water content reducing the interfacial interactions. Under loading, the interfacial fracture underwent three stages: crack propagation, atomic chain bridging (responsible for the interfacial residual strength), and complete failure. These atomic-level findings provide hitherto unknown mechanisms of the interfacial interactions between cement paste and silica.