Dimeric and oligomeric interactions between calcium silicate aqua monomers before calcium silicate hydrate nucleation

Understanding the formation mechanism of calcium silicate hydrate (C-S-H) is pivotal to control the kinetics of calcium silicates hydration and the evolution of C-S-H microstructure. However, the reactions between calcium- and silicate-based aqueous species before or during the process of C-S-H nucleation at molecular level are still unclear. Here, based on ab initio metadynamics simulations, we find that the dimeric interaction between calcium silicate aqua monomers leads to the formation of a calcium silicate aqua Q0-dimer through an ionic bond between calcium and silicate ions and a reduction of hydration number of the Ca ion, which is prior to the formation of silicate dimers (Q1). Besides, the first coordination shell of the Ca ion in the most stable calcium silicate aqua cluster has the characteristics of both tobermorite and jennite structures. We further propose a potential formation pathway for C-S-H, where the replacement of water ligands of the Ca ion by silicates initiates the clustering and dimerization of silicate monomers and promotes the formation of C-S-H.