Ferrosilicate stabilization controls the anomalously high Fe(III) concentration at alkaline pH

Elevated concentrations of dissolved Fe(III) in silicon-rich, alkaline cementitious pore solutions deviate strongly from the solubility limits controlled by the precipitation of ferric (hydr)oxides. This behavior is commonly attributed to the formation of metastable iron-bearing phases, yet growing experimental and modelling evidence challenges this interpretation. In the study presented here, we analyze the aqueous speciation and solid phase evolution in the Fe(III)-Si(IV)-H2O system. We demonstrate that the solubility of dissolved Fe(III) is governed by the stabilization of siliceous ferrihydrite, a hiterto uncharacterized amorphous corrosion product forming a solid solution with pure 2-line ferrihydrite at increasing levels of silicon incorporation. Spectroscopic investigations into the local electronic structure of siliceous ferrihydrite reveal a mixed tetrahedral/octahedral coordination environment, where both the emergence of structural Fe-Si motifs as well as fraction of tetracoordinated Fe(III) are mutually consistent with the phase composition derived from mass balance calculations. We further assign the elevated solubility of iron at pH 12.5 to 13.0 to the stabilization of Fe(OH)3SiO(OH)3- in equilibrium with siliceous ferrihydrite, conflicting its conventional assignment to the formation of gamma-FeOOH(s). Together, these findings corroborate a coherent picture, identifying aqueous ferrosilicate complex formation as the key factor, controlling the anomalously high concentration of \ch{Fe(III)} in alkaline, silicon-bearing environments.