Faculty of Natural Sciences

Solid-state, magic-angle spinning (MAS) NMR spectroscopy represents a valuable tool for structural investigations on the nanoscale of the most important phases in anhydrous and hydrated Portland cements and of various admixtures. This is primarily due to the fact that the method reflects the first- and second-coordination spheres of the spin nucleus under investigation while it is less sensitive to long-range order. Thus, crystalline as well as amorphous phases can be detected in a quantitative manner by solid-state NMR. In particular the structure of the calcium-silicate-hydrate (C-S-H) phase have been investigated in detail by ²⁹Si and ²⁷Al MAS NMR where the combination of the results for these spin-nuclei provides important information on the degree of Al-incorporation in the C-S-H structure and of the average chain lengths of tetrahedral SiO₄ and AlO₄ units. This presentation will illustrate the versatile applicability of solid-state NMR in studies of Portland cement hydration and of effects of various admixtures such as silica fume, metakaolin, and clay minerals. This includes kinetic studies on the consumption of alite and belite during Portland cement hydration and of structural characteristics for the C-S-H phase formed during hydration. It will be demonstrated that Al³⁺ and flouride guest-ions in the anhydrous and hydrated calcium silicates can be studied in detail by ²⁷Al and ¹⁹F MAS NMR, thereby providing information on the local structure and the mechanisms for incorporation of these ions in the cement phases. The role of flouride ions is of special interest for mineralized Portland cements and it demonstrated that the location of these anions in anhydrous and hydrated Portland cements can be clarified using ¹⁹F MAS or ²⁹Si{¹⁹F} CP/MAS NMR despite these cements contain only about 0.2 wt.% flourine. Finally, the detection and quantification of strätlingite (2CaO·Al₂O₃·SiO₂·8H₂O) in Portland cement mixtures containing metakaolin will be demonstrated.