AbstractIn this work, a cement‐free alumina‐spinel (Al2O3–MgAl2O4) castables with a new aluminum–magnesium gel binder were developed to line the Ruhrstahl Heraeus refining ladle, chemical attacks, and degradation mechanisms of the used alumina‐spinel castables after industrial trials were investigated. The results indicated that a reaction product layer of (Mg, Fe)Al2O4 was observed between the slag layer and penetration layer (PL), which was mainly derived from the reaction between MgAl2O4 spinel in the refractory matrix and FeOt from slag or an oxidation of steel, and thereby prevented the further penetration of FeOt. Meanwhile, the in situ spinel could also entrap slight CaO, SiO2, and FeOt from the infiltrated slag to form composite spinel in the PL. Moreover, chemical corrosion/penetration and structural spalling dominated the degradation process of the refractory lining in this case. Cracks formed between the deteriorated layer and original layer because of mechanical and thermal stress, which caused spalling from the refractory's hot face.