The corrosion resistance of lightweight and dense mullite–corundum refractories to cement materials was examined. The specimens following static crucible treatment were investigated using phase analysis, microstructure, and thermodynamic simulation. The results reveal that diffusion of the silica‐rich phase enhances alumina saturation solubility in the slag, reducing the corrosion resistance of the mullite–corundum refractories. The large proportion of closed pores and the continuous net‐like structure of anorthite created by the corrosion reaction in the matrix, on the other hand, prevent further slag penetration into the lightweight mullite–corundum refractories. As a result, the presence of the silica‐rich phase assures better slag penetration resistance of lightweight mullite–corundum refractories.