²⁷Al and²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy of Al-substituted tobermorites

Abstract

Solid-state²⁷Al and²⁹Si NMR spectroscopy with magic angle spinning (MAS) of samples was used to study several 1.13 nm tobermorites, most of which were deliberately substituted with aluminium.²⁷Al MASNMR clearly showed that aluminium is tetrahedrally co-ordinated in tobermorite structures. In addition two different aluminium environments resonating at ∼ 57 and 64 ppm from [Al(H₂O)₆]³⁺ were detected.²⁹Si MASNMR of pure, anomalous tobermorites showed resonances at −85.7 and −95.7 ppm from tetramethylsilane representing chain middle groups (Q²) and branching sites (Q₃), respectively. Anomalous Al-substituted tobermorites, on the other hand, showed two to four resonances representing different silicon environments. One Al-substituted tobermorite showed two resonances at −84.6 and −91.5 ppm which were assigned to Q²(0 Al) and Q³ (1 Al), respectively. In the above tobermorite aluminium appeared to have substituted into branching sites only. Two other Al-substituted tobermorites, however, showed four distinct resonances at ∼ −82.0, −85.2, −92.0 and −96.0 and these were assigned to Q² (1 Al), Q² (0 Al), Q³ (1 Al) and Q³ (0 Al), respectively. Thus these two tobermorites showed substitution of aluminium in the chain middle groups as well as branching sites. Another Al-substituted tobermorite which showed a normal thermal behaviour exhibited, as expected, only Q²(0 Al) and Q² (1 Al) sites resonating at −84.7 and ∼ −80.2 ppm, respectively. No Q³ sites were detected because few or no branching sites are present in this normal tobermorite. The results reported here clearly demonstrate the usefulness of solid-state²⁷Al and²⁹Si MASNMR spectroscopy for the investigation of short-range order in alumino-silicate materials.