Hydration of refractory oxides in castable bond systems—I: alumina, magnesia, and alumina–magnesia mixtures

Isothermal conduction calorimetry (ICC) of hydratable alumina and reactive magnesia revealed that alumina was partially hydrated to bayerite while magnesia was fully hydrated to brucite after curing for 22 h at 20 and 30 °C. Hydration of Al2O3–MgO mixtures led to formation of a hydrotalcite-type hydrate accompanied by total heat evolution of ≈600 kJ/kg. This level of heat evolution from the mixed oxide was comparable with that from the hydration of magnesia, but much of it was released later in the hydration process. Such a large and delayed heat evolution might explain serious problems sometimes observed when mixed Al2O3–MgO bond systems are used in castable refractories.