Atomic-level structure evolution of calcium silicate hydrate nucleation process, the amorphous-to-crystalline pathway

Calcium silicate hydrate (C-S-H) synthesis follows multistep nucleation processes ranging from amorphous globules to nanocrystals. This type of nonclassical nucleation pathway attracts extensive interest and yet related molecular processes are mostly unexplored. Herein, atomic-level structure evolution during this process is unveiled. From inside to outside of the globule, the atomic arrangement evolves from amorphous configuration to layered stacking of calcium silicate layers with layers spaced by 1.1 nm. Numerous cations surrounding the globules provide extra calcium ions to form nano C-S-H foils at the edge of the globule. The polymerization to form calcium silicate chains has almost been completed before 5 min reaction, and the C-S-H structure formation is basically based on the self-assembly to be calcium silicate layers and then lamellar crystal inside the globule. Silicate chain defects inhibit a- and c-axes crystallization, but promote b-axes one, leading to anisotropic growth of crystalline C-S-H. The defects lead to the crystallization pathways conversion from Si-O-Si bonds formation to Ca2+ electrostatic linking.