Alumina-magnesia refractory castables present suitable performance during application in steel ladles due to the in situ spinel (MgAl2O4) formation. For the steel industry, an early generation of the spinel phase would be beneficial for the castable performance. Taking this aspect into account, alternatives to replace silica fume, which is the most traditional liquid former inducer in this system, were evaluated in this study, by adding boron-containing compounds. Faster spinel and calcium hexaluminate (CA6) formation was attained through a transient liquid, leading to remarkable creep resistance and hot modulus of rupture of the refractory castables. In this context, a novel route to design in situ spinel-forming castable is presented, with great potential to increase the performance of this class of refractory.