Thermal decomposition of monocalcium aluminate decahydrate (CaAl2O4·10H2O) investigated by in-situsynchrotron X-ray powder diffraction, thermal analysis and 27Al, 2H MAS NMR spectroscopy

Abstract

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 ų at 25 °C, to 1674 ų 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.