IN-SITU SPINALIZATION ROLE PLAYED BY ZnO AND MgO ON Al2O3-BASED CASTABLES

Due to its suitable mechanical, chemical and thermo-mechanical properties, MgO-containing refractories have been extensively investigated during the last decades, however, the likelihood of applying alternative chemical elements to induce in-situ spinalization in refractory castables remains underexplored. In this regard, zincite (ZnO) seems to be a promising alternative as it can react to alumina at intermediate temperatures (~ 800 ºC), giving rise to the spinel ZnAl₂O₄ (also known as gahnite), which is nontoxic, has high refractoriness (> 1900 ºC), low coefficient of thermal expansion, and suitable corrosion resistance. In this work, the addition of ZnO (zincite) in alumina-based castables was assessed and compared with its MgO counterparts with the same volumetric amount of spinel-like phase formed. In-situ Young’s modulus (E) measurements, and assisted sinterability (AS) pointed out that gahnite formation takes place at lower temperatures (~ 1100 ºC) than the spinel MgAl₂O₄ (~ 1300 ºC). On one hand, this lower formation temperature induces anticipated mechanical strengthening, giving rise to the possibility of firing the ZnO-containing compositions at inferior temperatures. On the other, scanning electron microscopy (SEM) highlighted that this faster kinetics of ZnAl₂O₄ formation leads to remarkable changes in the morphology of the pores generated during the firing process, which is assigned to the Kirkendall effect. In contrast to MgO-containing compositions, the pores created by ZnAl₂O₄ formation are preferentially located at the interface of the alumina aggregates, inducing their partial detachment, which impacts some key properties of the castable as thermal shock damage resistance and cold modulus of rupture.