Structural changes during thermal dehydration of ettringite Available to Purchase

Changes in crystal structure of ettringite on dehydration in normal atmosphere have been re-examined by means of thermogravimetry, X-ray diffraction, and ²⁷Al nuclear magnetic resonance spectroscopy. Synthetic ettringite crystals were heated at various temperatures up to 200°C for durations of up to 7 h. The ettringite structure maintains some long-range order until the coordination number of calcium changes to 5 by dehydration of twelve water molecules from channels and columns with heat treatment at 70°C. When the coordination number of calcium decreases to 5, the short-range order of the structure is disrupted and ettringite becomes X-ray-amorophous. Subsequently, the rest of the water molecules in the columns and bridging OH groups in calcium polyhedra are removed, essentially destroying the framework of the columns. This is accompanied by changes in the coordination number of aluminium from 6 to 4.