Evolution of elastic properties and microstructural changes versus temperature in bonding phases of alumina and alumina–magnesia refractory castables

The microstructural evolutions of high alumina refractory concretes, based on the systems CaO–Al2O3 and CaO–Al2O3–MgO, have been studied by the way of ultrasonic high temperature measurements. Since such a refractory concrete can be considered as a composite material with two constituents, a continuous matrix (so called bonding phase) and aggregates, investigations of matrices made of mixtures containing cement, reactive alumina and/or magnesia, constitute a preliminary study which is presented in this paper. The elastic behaviour of these matrices has been followed from room temperature to 1550 °C via a specific ultrasonic method. During the first thermal treatment, different changes of slope are observed in the curve E = f(T). Between 200 °C and 400 °C, dehydration mechanisms involve a microstructural reorganisation correlated with a strong decrease of the elastic properties. At high temperature, the Young's modulus evolutions are associated with the expansive formations of CA2b and/or in-situ spinel at 1100 °C and then CA6 (see endnote b) at 1450 °C, which directly depend on the CaO/Al2O3 and MgO/Al2O3 ratios in the mix. The forming of bond linkage between CA6 and in-situ spinel grains in the matrix is believed to enhance the elastic properties at high temperature.