## 2 Materials and Methods  
Different mineral waste samples were collected from various levels and positions from the magnesite mine area, and the most representatives were selected in terms of their mineral composition, showing different compositions of major constituents for comparison evaluations. The exact location of sampling sites is shown in Figure S1 (Supplementary Material). The mineralogical content (wt.%) and the chemical composition of these samples are presented in Supplementary Material (Tables S1 and S2, respectively) [1]. Table S1 also contains extended information regarding the mineralogical content of the same samples but after the application of thermal treatment at 850 \({}^{\circ}\)C for 30 min, since these particular samples are also used as raw materials (i.e., prior to pelletization) for the hematite addition and the potential production of refractory materials, denoted hereafter as pre-treated samples (PT). The XRD measurement results in the semi-quantification of the crystalline phases (mineralogical content (wt.%)), so the quantified values do not encounter the presence of amorphous SiO\({}_{2}\) (also not initially considered, see the reaction 1) and as mentioned in the relevant published paper of [PERSON] et al., 2020 [1], there is a deviation in the obtained quantification results e.g., for the sample W5 PT of 8.4% (Table S2, Supplementary Material). The modification of raw serpentine mineral by the application of thermal treatment is shown in the following exothermic reaction and the formation of forsterite shown in Figure S2 (Supplementary Material) [1]. \(2\) [\(2\) Mg(OH)\({}_{2}\cdot\)MgO\(\cdot\)xSiO\({}_{2}\)] \(\rightarrow\)\(3\)Mg\({}_{2}\)SiO\({}_{4}\) + \(2\)(x \(-\) 3)SiO\({}_{2}\) + \(2\)H\({}_{2}\)O (reaction 1 at 810\(-\)850 \({}^{\circ}\)C)the respected XRD pattern to verify Fe\({}_{2}\)O\({}_{3}\) formation is shown in Figure S3, Supplementary Material [12]. The initial material was further pulverized to obtain particle sizes \(<\) 73 \(\upmu\)m) and a small quantity (0.2 g) was dissolved with 20 mL 6 N HCl in Teflon vial on a sand bath and diluted to 200 mL with ultra-pure water. The chemical composition of obtained solution was determined by Flame Atomic Absorption Spectroscopy (FAAS), using the Perkin Elmer Atomic Absorption Spectrometer PinAAcle 500. The Loss-On-Ignition (LOI) test was determined at 950 \({}^{\circ}\)C in the electric furnace SNOL 6.7/1300 LSC01. The respective results are presented in Table 1. The LOI 1.1% reported for the additive is attributed to the moisture absorbed during the cooling of the material after heating at 850 \({}^{\circ}\)C.  
The mining waste samples (W5, W6, W7 and W8) were subjected to co-grounding/re-homogenized with the hematite additive in different ratios of 5 and 10 wt.% after proper homogenization, and these percentages were properly selected because the FeO content in the initial mineral samples is already between 6.5-7.5 wt.% (Table S2 Supplementary Material). All these mixtures (solid mineral powders) were homogenized subsequently with the help of a ball mill. The total weight of each blend was 5 g. Therefore, the quantity of hematite (considering the ratios 5, and 10 wt.%) was 0.25 and 0.5 g respectively, while the rest of the mixture was the examined mining sample. Afterwards, the ground/homogenized samples were pressed to form cylindrical pellets of 2 cm diameter. After the proper pelletization, these samples were thermally treated by using the programmable control electric furnace (type SNOL 6.7/1300 LSC01) for the maximum temperature 1300 \({}^{\circ}\)C and by the Thermowatt TGH30L 380V 10 KW furnace, when applying heating temperatures up to 1600 \({}^{\circ}\)C, to induce the formation of forsterite, consisting mainly of the useful olivine mineral phase. Different temperatures and times applied, i.e., 1300 \({}^{\circ}\)C for the duration of 30 min, and 1600 \({}^{\circ}\)C for 120 min.