The evolution of structural changes in ettringite during thermal decomposition

The thermal decomposition of ettringite, Ca6[Al(OH)6]2(SO4)3·∼26H2O, was studied with pulsed neutron time-of-flight diffraction combined with Rietveld structure refinement. Like prior investigations, transition from a crystalline to amorphous state occurred following the loss of ∼20 water molecules. In contrast to earlier investigations, which relied upon indirect measurements of water and hydroxyl occupancies, the present study inferred the occupancies directly from Rietveld crystal structure refinement of the diffraction data. The decomposition pathway was shown to be more complex than previously envisioned, involving the simultaneous loss of hydroxyl and water molecules. Nuclear magnetic resonance (NMR) spectroscopy studies of the rigid lattice lineshapes of fully and partially hydrated ettringite were performed and confirmed our decomposition model.