In situ elastic modulus evaluation of Al2O3–MgO refractory castables

Elastic modulus evaluation is a simple and very accurate technique to investigate refractory castable behavior during curing, drying and firing steps. Therefore, analyzing this property might be useful to assess microstructural changes in Al2O3–MgO systems, where in situ transformations play an important role at high temperature. Considering this aspect, dead-burnt magnesia containing refractory castables with 0 wt%, 2 wt%, 4 wt% or 6 wt% of calcium aluminate cement and 1 wt% of fumed silica, as well as an additional cement-free composition containing a caustic magnesia source (with higher reactivity) were evaluated using hot elastic modulus measurements, thermodynamic calculations and XRD analysis. The cement-free castables attested that the greater the brucite content, the higher the drop in the elastic modulus. On the other hand, MgAl2O4 formation increased the sample′s stiffness, but the presence of caustic magnesia did not significantly affect the elastic behavior at high temperatures. For the cement bonded systems, reactions between CaO, SiO2 and Al2O3 resulted in a remarkable increase in the elastic modulus above 800 °C, whereas at higher temperatures liquid phase generation spoiled the castable′s properties. Thermodynamic calculations indicated that reducing the cement content, a lower amount of glassy phase was generated in the matrix, leading to a higher E value after the cooling stage. The elastic modulus profile for the second thermal cycle also suggested that a glassy phase transition occurred above 1000 °C for the evaluated castables.