Site preferences of F, Al3+ and Fe3+ guest ions in the calcium silicate phases of Portland cement from solid-state NMR spectroscopy

Fluoride mineralizers such as CaF2 are increasingly used in modern Portland cement (Pc) production. Although the bulk fluoride content in the kiln feed is typically only about 0.2%w, it has an important impact on the formation as well as the hydration properties of the calcium silicates in Pc. The present structural investigation of F–, Al3+ and Fe3+ guest ions in the calcium silicate phases reveals that the fluoride ions are incorporated only in the interstitial oxygen sites of alite. The F– ions take part in a coupled substitution of Si4+ + (Ointerstitial)2- → Al3+ + F–, where the Al3+ guest ions are located in the tetrahedral sites in close proximity to the F– ions, thus preserving local charge balance. Fe3+ ions on the other hand do not seem to be involved in the fluoride mineralization, and are more likely to be incorporated in alite by substitution for Ca2+ ions on octahedral sites, i.e., a Ca2+ → Fe3+ substitution. This work also demonstrates the unique potential of solid-state NMR in obtaining structural information about guest ions in the calcium silicate phases of industrial Portland clinkers, which can have important implications for thermodynamic stability of the host phases.
Author: T.T. Tran, D. Herfort, H.J. Jakobsen, J. Skibsted

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