Fine rejected fraction run of mine (ROM) material, which is rich in magnesite and cannot be further exploited, was mixed with mining wastes by-products of magnesite mine “Grecian Magnesites SA” (Gerakini, Chalkidiki, N. Greece), to produce refractories. The exploitation of the ROM in combination with mining wastes was investigated with the application of different thermal treatments, aiming to find the optimum thermal treatment process for upgrading the refractory properties of these samples. The examined firing process parameters, were the two final temperatures of 1300 °C and 1600 °C at different dwell times. Firing shrinkage, water absorption, apparent porosity, bulk density, and mechanical strength were determined, as the main parameters of importance. Furthermore, optical microscopy micrographs were appropriately received, while the mineralogical characterization was determined by X-Ray Diffraction (XRD) measurements. Concerning the optimum firing process regarding final temperature and dwell time for improving refractoriness of mining samples mixed with ROM, the results indicate that the best results were obtained firing at 1300 °C for 120 min. In that thermal treatment process, were observed the minimum values for firing shrinkage, water absorption, and apparent porosity and maximum values for bulk density and mechanical strength, which is the desired scope regarding these properties for improving refractoriness of the samples. The XRD patterns reveal that firing at 1300 °C with increasing dwell time olivine (forsterite) percentage increased, while pyroxenes (enstatite ferroan) decreased. Preheating of the samples at 850 °C did not significantly affect the physical properties of the samples, since firing shrinkage had a small decreasing trend, while bulk density and mechanical strength also decreased, degrading the properties of the samples. Thermal treatment at 1600 °C, showed a negative effect due to the significant reduction of mechanical strength combined with the major increase in firing shrinkage. Also, quantification of mineral phases at 1600 °C is rather misleading, due to the presence of high glassy/amorphous phases. •Mining waste/by-products combined to produce added-value forsterite refractories.•Forsterite formation is maximized at the thermal treatment at 1300 °C for 120 min.•Optimization of the properties reported when firing at 1300 °C for 120 min.•Preheating of the mining by-products at 850 °C not recommended.•Initial FeO, MgO and SiO2 content affects the final products.