In situ hot elastic modulus evolution of MgO–C refractories containing Al, Si or Al–Mg antioxidants

Metals and alloys (such as Al, Si, Al–Mg and Al–Si) are commonly added to MgO–C refractory bricks as antioxidants due to their effectiveness to prevent carbon oxidation (in the 600–1400 °C range) and their low cost. These additives act at different temperatures and react with refractory components and gases in the environment, inducing significant changes in the resultant microstructure and affecting the overall thermo-mechanical performance of these products. This work addresses the evaluation of physical properties, cold and hot mechanical resistance, as well as in situ hot elastic modulus (E) measurements in the temperature range of 30–1400 °C for MgO–C bricks containing antioxidants (Al, Si or Al–Mg alloy) in a reducing atmosphere. Cured and fired samples of the designed formulations were evaluated throughout 1 or 2 heating-cooling cycles. Despite the improved mechanical behavior (higher cold crushing strength and hot modulus of rupture) of the antioxidant-containing formulations, compared to the additive-free MgO–C sample, the interaction of the selected additives with the refractory components and CO(g) led to a generation of phases (i.e., Al4C3, Al2O3, SiC, SiO2, MgAl2O4) that could not be well accommodated in the microstructure. Consequently, the in situ E drop was observed during cooling (mainly below 600 °C) of the antioxidant-containing sample due to crack and flaw formations. Si and Al–Mg were the most promising antioxidants, whereas the Al-containing composition showed the highest E damage level after two heating/cooling cycles up to 1400 °C for cured samples. Based on the elastic modulus profiles with the temperature, the results also indicated the best working conditions for these ceramic materials.