Changes in Structure of CaAl2O14H20 during Heat Treatments: X-ray Absorption Spectroscopy and 27Al NMR Studies

Abstract

Depending on temperature, various calcium aluminate phases are formed by hydration of high-alumina cements (HACs). At low temperature, CAH10 is formed by hydration of CA, the main constituent of HACs. HACs are commonly used for refractory applications; heat treatments then lead to the thermal decomposition of hydrates, which results in amorphisation and loss of mechanical properties. CAH10 first dehydrates to CAHx (x < 10), and finally to CA. In CAH10, aluminium atoms are octahedrally coordinated (AlVI), whereas they are tetrahedrally coordinated in CA (AlIV). Changes in calcium and aluminium environment during CAH10 dehydration was investigated by X-ray absorption spectroscopy (EXAFS and XANES) and 27Al MAS and CP-MAS NMR. Ca K-edge EXAFS showed a decrease in Ca-O distance and a reduction of the number of oxygen neighbours in the first coordination shell. Al K-edge XANES indicated an important distortion of the aluminium sites when amorphisation occurred. The relative contents of AlIV and AlVI were determined by 27Al MAS NMR. The CP MAS spectra only showed the contribution of AlVI, and indicated a progressive decrease in the AlVI-H dipolar coupling, associated with the progressive loss of water.