## 3 Results and Discussion
### XRD Phase Analysis of the Obtained Products after Firing under Different Conditions  
The XRD patterns of products obtained from the samples W5 and W5 PT and with the addition of 5 or 10 wt.% hematite are shown in Figure 7, whereas the respected quantification of the mineral phases is presented in Table 3.  
* These products (W5 and W5 PT with hematite addition) were found to consist mainly of olivine (forsterite, forsterite ferroan), pyroxenes (clinoenstatite, protoenstatite, enstatite ferroan) and magnesioferrite. * Comparing the products from sample W5 with that of sample W5 PT, the olivine percentage is slightly higher in the product of the raw material, which was thermally pre-treated at 850 \({}^{\circ}\)C for 30 min. * For the products obtained from the W5 and W5 PT samples, the results show that the final mineralogical content is similar for both applied thermal treatments (i.e., at 1300 \({}^{\circ}\)C for 30 and for 120 min). * The results show that the addition of hematite favors the formation of magnesioferrite spinel (MgFe\({}_{2}\)O\({}_{4}\)), since the percentage of this constituent increases with the increase of hematite addition, while the percentage of (desirable) olivine (forsterite) as well  
Figure 5: The effect of hematite addition (wt.%) on the BD (g/cm\({}^{3}\)) parameter for the examined samples after the thermal treatment at 1300 \({}^{\circ}\)C for 30 min. as of (undesirable) pyroxenes (consisting mainly of enstatite) decreases due to the binding of magnesium by hematite in the respective crystalline phases. Figure 6: The mechanical strength values of examined samples (thermally pre-treated or not), with or without the addition of hematite: (**a**) W5; (**b**) W6; (**c**) W7; (**d**) W8. Figure 7: XRD patterns of the thermally treated products obtained from the respective samples: (**a**) W5, and (**b**) W5 PT (with or without the presence of hematite). Since the magnesioferrite spinel has a melting point close to that of olivine (forsterite), and according to the respective phase equilibrium diagram for the system MgFe\({}_{2}\)O\({}_{4}\)--Mg\({}_{2}\)SiO\({}_{4}\) (see the supplementary material, Figure S12) the decrease of the enstatite percentage is expected to upgrade the refractory properties. More specifically, the melting temperature of forsterite is 1890 \({}^{\circ}\)C, whereas the temperature of incongruent melting and liquidus state for the magnesioferrite are 1713 \({}^{\circ}\)C and about 1900 \({}^{\circ}\)C, respectively, and in the spinels' system MgFe\({}_{2}\)O\({}_{4}\)--Mg\({}_{2}\)SiO\({}_{4}\) the initial melting point occurs at 1665 \(\pm\) 10 \({}^{\circ}\)C, although, as already mentioned, this temperature is reduced at values lower than 1600 \({}^{\circ}\)C when the iron oxide is added [19,21]. The results that show the increase of magnesioferrite in the product by the addition of hematite nanoparticles addition are in accordance with the study of [PERSON] et al., 1994 which reported that by applying annealing temperatures up to 700 \({}^{\circ}\)C for the Mg-Fe Olivine mineral, oxidation results in the formation of magnesioferrite (as the major oxide phase among the different ferrous oxide phases formed) [22]. The study by [PERSON], 2020 showed that the ratios of MgO/SiO\({}_{2}\) and MgO/FeO are the main determinant factors for magnesioferrite formation [23]. Moreover, [PERSON] et al., 2013 also reported magnesioferrite formation when Fe\({}^{0}\) reacted with brucite Mg(OH)\({}_{2}\)[24]. The formation of magnesioferrite with the addition of nano-Fe\({}_{2}\)O\({}_{3}\) due to its high solubility and atomic mobility in magnesia substrate was also reported by [PERSON] et al., 2015 [15]. Finally, according to [PERSON] et al.