Fluoride ions as structural probe-ions in 19F MAS NMR studies of cement materials and thermally activated SCMs

Abstract

The potential of 19F magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy in structural studies of fluoride guest ions in cementitious materials is investigated. This includes detection and structural investigations of fluoride ions incorporated in the model compound boehmite (AlOOH) and in four samples of kaolinite (Al2Si2O5(OH)4) from different sources and experiments, which follow the thermal transformation of kaolinite into metakaolin. Furthermore, the structural environments of fluoride ions incorporated into calcium-silicate-hydrate (C-S-H) phases, either synthesised or originating from hydration of fluoride-mineralised Portland clinkers, are investigated. The favourable NMR properties of 19F provide the basis for a range of double-resonance NMR techniques, such as 27Al{19F} and 29Si{19F} cross-polarisation (CP) and 19F{29Si}19F forth and back CP, which probe internuclear connectivities by way of dipolar couplings. The information gained from these experiments facilitates a reliable assignment of the resonances from the fluoride guest ions in the studied model compounds and calcium-silicate-hydrate phases. It is proposed that fluoride ions may deliberately be introduced into a cement system as a structural probe in model studies that follow the reactivity and structure of specific sites that fluoride substitutes for.