Structural investigations of perlite and expanded perlite using 1H, 27Al and 29Si solid-state NMR

Abstract Industrial grade perlite and expanded perlite systems were investigated by multifield and multinuclear Solid State Nuclear Magnetic Resonance (SSNMR), Fourier Transform Infra-Red (FTIR) spectroscopies and Scanning Electron Microscopy (SEM). The SSNMR studies focussed on the 1 H, 29 Si and 27 Al nuclei using single pulse excitation (SPE) and 1 H, 29 Si cross-polarization Magic-Angle-Spinning (CP-MAS) conditions, with 27 Al Triple Quantum Magic-Angle-Spinning (3QMAS) measurements also being implemented. The resonances at ~ 52 ppm detected in the 27 Al MAS NMR spectra of the industrial grade and expanded systems are tentatively assigned to tetrahedrally coordinated aluminium, while the resonance at ~ 5 ppm, which was only observed in the industrial grade perlite spectrum, is attributed to octahedrally coordinated aluminium. In neither case could the pentahedrally coordinated positions be identified using 27 Al MAS and 3QMAS techniques. 27 Al data suggests that tetrahedrally coordinated aluminium atoms do not have a significant role in the grain expansion process. The broad resonance envelopes found in the 29 Si MAS NMR and FTIR data of both perlites were analyzed by the second-order derivative. The analyses confirmed complex Q n configurations for the perlitic structure suggesting the prevalence of Q 3 and Q 4 species. 1 H- 29 Si CP-MAS spectra support the presence of Q 2 units suggested by FTIR and SPE data. SEM micrographs revealed the honeycomb morphology of expanded domains in perlite after heat treatment.