Calcium silicate hydrate is the most important hydrate of cement and significantly impacts the properties of cement-based materials, yet the mechanism of its nucleation is still unclear. In this paper, the changes in chemical composition, crystallinity, morphology and multiscale structure of C-S-H during synthesis were observed by TEM, XRD, FT-IR and ²⁹Si NMR. The results confirm that the homogeneous nucleation of C-S-H is a two-step, non-classical process. They further suggest that the crystallization of the final materials is accelerated with Ca²⁺ concentration. Specifically, discrete globules appear as a metastable precursor and then transform into a foil-like C-S-H accompanied by a change of crystallinity and structure with a heat release. Our findings uncover for the first time the structure of the C-S-H precursor and provide new insights into C-S-H nucleation that will benefit its functionalization and more widespread applications.