Different approaches to master magnesia hydration in refractory castables have been recently proposed. Among them, the use of hydrating agents can change the Mg(OH)2 crystals morphology and its distribution in the resultant microstructure, minimizing the drawbacks related to the reaction expansion. In this work, the hydrating effect of acetic acid in Al2O3–MgO cement-free castable properties during curing, drying, and firing steps was evaluated by elastic modulus, thermogravimetric, apparent porosity, and SEM analyses. Based on the attained results, adding acetic acid resulted in hydroxide crystals with distinct morphology and flexibility leading to a better accommodation of Mg(OH)2 in the designed microstructure, which inhibited the samples' cracking during curing. In addition, the drying behavior of the evaluated compositions was further optimized by incorporating polypropylene fibers. Thus, this study highlights a novel perspective for fine MgO powders application, indicating that brucite morphology engineering may be a key aspect for the development of advanced Al2O3–MgO cement-free refractory castables.