The state of Fe3+ in the C–F–A–S–H system with varying Fe/Si and Ca/Si ratios

Abstract

Synthetic CaO–Fe₂O₃–Al₂O₃–SiO₂–H₂O (C–F–A–S–H) gels with Fe/Si and Ca/Si ratios in the ranges 1/8–1/4 and 1.0–2.0, respectively, are investigated to reveal the coordination, location, and doping configuration of Fe³⁺. The bonding between Fe³⁺ and Ca²⁺/Si⁴⁺ species is the most favorable, resulting in the transformation from phyllosilicates (i.e., layered C–A–S–H) to network silicates (hydroandradite and disordered gel with a crosslinked structure). The increasing Ca/Si ratio has a critical effect on the evolution of the C–F–A–S–H system, resulting in layered C–(F)–A–S–H because a sufficient concentration of Ca ions is provided. Using Mössbauer spectroscopy, two kinds of octahedral Fe³⁺ with distinct chemical environments are identified: the distorted one acts as a network modifier, while the highly symmetric one acts as a layered structure former. The former exists in the disordered gel, while the latter mainly exists in the interlayer space of the layered C–(F)–A–S–H. Moreover, Fe³⁺ is unlikely to enter the tetrahedral position of the aluminosilicate chain in layered C–(F)–A–S–H.