This work addresses the optimization of alumina‐based castables designed for petrochemical applications by adding an additive able to speed up the samples' densification at lower temperatures. Hot elastic modulus, thermal shock, hot modulus of rupture, and erosion resistance measurements were carried out to evaluate these castables. The E profiles confirmed the transient feature of the generated liquid phase, as it was crystallized throughout subsequent heating cycles. Thermal fatigue effect was also observed for the compositions with higher boron content. Nevertheless, the boron‐containing castables showed outstanding properties, which could be attained only with using suitable amounts of this engineered additive.