Products of the thermolysis of NH4Al-alum were prepared under dried air and characterized by X-ray powder diffraction, thermogravimetry, and for the first time by solid-state 27Al-MAS-NMR spectroscopy. It was found, that high resolution NMR spectroscopy is applicable to follow up the thermal decomposition and to indicate even minor structural changes. 27Al-MAS-NMR spectra show peaks with characteristic chemical shifts for octahedral Al-units up to a high stage of thermolysis. During the decomposition of Al2(SO4)3 to γ-Al2O3 we again observed three-peak spectra with the specific signal at ≈ 35 ppm for penta-coordinated Al ions as recently proved for the thermolysis of AlCl3 · 6 H2O.
A long-term study on the setting reaction of glass ionomer cements by 27Al MAS-NMR spectroscopy