From Phase-separated Precursors to Coordinated Assembly: A New Nucleation Pathway for Calcium-Silicate-Hydrate

Calcium-silicate-hydrate (C-S-H), the primary binding phase of cement, governs the mechanical performance and durability of concrete, yet its nucleation mechanism remains unresolved due to its non-stoichiometric composition and transient early-stage structures. In this study, we propose a new multi-step nucleation pathway for C-S-H governed by phase-separated precursors and sequential calcium-silicate coordination. The early-stage Ca-O lamellar fragments at the periphery of nanodroplets demonstrate that Ca-species and Si-species initially undergo independent condensation rather than forming immediate Ca-Si hydrate complexes. Followed by the chained polymerization of silicates, the progressive coordination through Si-O-Ca linkages drives structural reorganization and ultimately leads to the formation of ordered "dreierketten" motifs and the transition from nanodroplet precursors to foil-like C-S-H. The coordination-driven assembly pathway is thermodynamically favored, with a significant overall energy decrease confirmed by first-principles calculations. This work resolves the long-standing ambiguity in the temporal sequence of calcium and silicate interaction during C-S-H nucleation, and provides a fundamental framework that may help understand non-classical nucleation in cementitious systems. These insights provide a mechanistic basis for controlling hydration kinetics and microstructure evolution, enabling the rational design of high-performance and low-carbon cement-based materials.