Compositional and structural controls on Fe(III) trapping in LDH phases

Understanding Fe incorporation in layered double hydroxide (LDH) phases is important for revealing the steel/cement interface interaction in low carbon cement. In this study, synchrotron-based characterization is combined with density functional theory (DFT) calculations to unravel the atomistic mechanisms controlling Fe(III) incorporation in Mg Al LDHs as a function of composition and local structure. XAS measurements reveal that Fe incorporation is strongly suppressed under trivalent-cation-rich compositions, leading to a preferential association of Fe with edge or defect-related environments. The spectra indicate that Fe(III) is incorporated into the LDH lattice in a well-defined octahedral environment under Mg-rich compositions. DFT calculations of substitution energies support these findings by showing that Fe(III) substitution for Al(III) in the main layers is strongly composition-dependent: Fe(III) incorporation is most unfavorable at Al-rich compositions but becomes progressively more favorable in Mg-rich environments. In Al-rich LDHs, Fe shortens neighboring Al O bonds, introducing local strain and lowering mechanical stability, while in Mg-rich LDHs the more flexible Mg O framework accommodates Fe(III) more effectively. Overall, this study defines molecular-level constraints governing Fe(III) incorporation into LDH phases in low-carbon cements. The results offer transferable insights concerning the influence of LDH composition on the interfacial phase assemblages relevant to steel corrosion protection.