The stability of monocalcium aluminate decahydrate, with the nominal composition CaAl2O4·10H2O (CAH10), has a decisive role for the strength development and durability of cementitious materials based on high alumina cements. This has prompted an investigation of the thermal transformation of crystalline monocalcium aluminate decahydrate in air to an amorphous phase by in-situsynchrotron X-ray powder diffraction in the temperature range from 25 to 500 °C, by DTA/TGA, and 2H, 27Al MAS NMR spectroscopy. The decomposition includes the loss of hydrogen-bonded water molecules in the temperature range up to 175 °C, coupled with a reduction of the unit cell volume from 1928 Å3 at 25 °C, to 1674 Å3 at 185 °C. Furthermore, X-ray diffraction shows that CaAl2O4·10H2O starts to transform to an amorphous phase at ∼65 °C. This phase is fully developed at ∼175 °C and it converts to crystalline CaAl2O4 when heated to 1300 °C. The thermal decomposition in the temperature range from ∼65 to ∼175 °C involves both formation of an amorphous phase including AlO4 tetrahedra and structural changes in the remaining crystalline phase.
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