Elucidating the Formation and Structure of Calcium Ferrite Silicate Hydrate: Fe‒O Tetrahedra in the Silicate Chains

The growing iron content in modern low‐clinker cements and solid waste‐integrated binders introduces uncertainties to the structural stability and performance reliability of calcium silicate hydrate (C‒S‒H), yet the role of Fe within its fundamental structure has long remained unclear. This study addresses this key knowledge gap by elucidating how Fe participates in and influences the C‒S‒H structural framework. Through the synthesis of calcium ferrite silicate hydrate (C‒F‒S‒H) and multi‐scale structural characterization, including X‐ray absorption fine structure spectroscopy, it is demonstrated that Fe does not merely exist as an external impurity, but directly incorporates into the C‒S‒H structure. Results show polycarboxylate ether (PCE) enables Fe incorporation via dual complexation with Ca/Fe ions, inhibiting rapid C‒S‒H precipitation and the formation of secondary Fe‐rich phases. More importantly, Fe forms tetrahedral Fe‒O units, occupying the bridging sites, and forming stable Fe‒O‒Si linkages that extend the chain connectivity of C‒S‒H. These molecular‐level findings provide a foundation for optimizing Fe‐rich, low‐clinker, and low‐carbon cementitious systems.