Year by year, the total demand for cement production has been growing. To be more competitive, cement producers have implemented new operating practices based on new eco–friendly raw materials. Lately, magnesia-dolomite refractories have been considered chrome-free refractories that might substitute magnesia–spinel and magnesia–chromite lining bricks for the cement industry with benefits to human health and the environment. However, their use is limited due to their low hydration resistance. In the present work, the hercynite's effect on the magnesia-dolomite properties (crystallographic, microstructural, physical, and mechanical) was studied. Refractory specimens were formed by uniaxial pressure, followed by a drying and fyring process, reaching a soaking temperature of 1600 °C. According to the XRD and SEM analysis, hercynite promotes hydration resistance and mechanical strengthening in the refractory body. These improvements are mainly due to brownmillerite formation. Brownmillerite diffuses through the grain boundary and triple points, forming a necklace-like microstructure (solidified liquid network) surrounding the magnesia and free-lime particles. Furthermore, it was observed that this structure avoids the progress of the expansive hydration process in the portlandite particles. Hercynite-containing refractory specimens exhibit superior physical and mechanical properties than conventional magnesia-dolomite refractories. By hercynite addition, an improvement of ∼35% in mechanical strength and ∼50% in hardness was found. Display omitted •Brownmillerite forms a necklace-like microstructure, activating a coating effect.•The necklace-like structure surrounds the magnesia and free-lime particles.•The hydration resistance is almost directly proportional to the hercynite addition.•Hercynite addition resulted in an improvement of ∼35% in mechanical strength.•The magnesia-dolomite hardness was improved by ∼50% with hercynite addition.