This study investigates the effect of the exsolution behavior of alumina-rich spinel on the formation and distribution of CA₆ (CaAl₁₂O₁₉) in corundum castables bonded with calcium aluminate cement. In this study, alumina-rich spinel is substituted for tabular corundum in the same proportions and grain size. The matrices after curing were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The phase composition and microstructure of the matrices containing alumina-rich spinel were analyzed after firing at 1600 °C. These results showed that the addition of alumina-rich spinel significantly improved the mechanical strength of the castables. This improvement was attributed to the alumina produced by spinel exsolution during firing at 1600 °C, which reacted in situ with CA₂ (CaAl₄O₇) to form CA₆. CA₆ connects the different particles and forms an interspersed interlocking structure within the spinel. The CA₆-MA interspersed interlocking structure replaces part of the CA₆-Al₂O₃ structure and significantly improves the mechanical strength of the castables.