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

ABSTRACT

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.