## 3 Results and Discussion
### XRD Phase Analysis of the Obtained Products after Firing under Different Conditions
When heating at temperatures higher than 1200 \({}^{\circ}\)C, free silica and iron oxide reacts with forsterite to form magnessioferrite and polymorphic modifications of magnesium silicate, MgSiO\({}_{3}\) as enstatite protoenstatite and clinoenstatite (2016). Polymorphism for enstatite, protoenstatite, clinoenstatite and high temperature clinoenstatite was studied by [PERSON] et al., 2004, who reported that protoenstatite crystallization starts at temperatures above 1250 \({}^{\circ}\)C and fully develops at 1420 \({}^{\circ}\)C, and that at higher heating temperatures protoenstatite crystallites are unstable and they inverse to clinoenstatite crystallites after cooling (2017). Even very low mechanical stresses may transform protoenstatite into clinoenstatite, a transformation that leads to negative volume changes and mechanical strength (2017; 2018). Thus, the enstatite of the raw materials similar to several studies (2018; 2019; 2020; 2020) is transformed to protoenstatite or to clinoenstatite accompanied by the residual protoenstatite when the applied thermal treatment is at 1300 \({}^{\circ}\)C (2020), and to clinoenstatite (during the cooling) when the applied thermal treatment is at 1600 \({}^{\circ}\)C. Figures S13-S16 in the Supplementary Material show the detailed identification of the magnesium silicate polymorphs. Similar results to the products obtained from the sample W5 are also found for the products of sample W6 (Figure 8), whereas the respected mineral content is presented in Table 4.  
* These products (W6 with hematite addition) were found to consist mainly of olivine (forsterite, forsterite ferroan), pyroxenes (clinoenstatite, protoenstatite) and magnesi-ferrite. * As aforementioned, the addition of hematite favors the formation of magnesioferrite spinel (MgFe\({}_{2}\)O\({}_{4}\)), as the percentage of this constituent increases with the increase of hematite addition, while the percentage of olivine (forsterite) and of pyroxenes (enstatite) decreases. * For the products obtained from the W6 sample, it was found that the final mineralogical content is similar for both applied thermal treatments (i.e., at 1300 \({}^{\circ}\)C for 30 or 120 min). Figure 9 shows the XRD patterns for the products obtained from the thermally treated sample W7, whereas the respective mineral content is presented in Table 5.  
* These products with hematite addition were found to consist mainly of olivine (forsterite, forsterite ferroan), pyroxenes (clinoenstatite, protoenstatite) and magnesi-ferrite. * As shown from the quantification results, the heating time of 30 min is not considered as sufficient for the hematite additive to react and form the maximum quantity of magnesioferrite in the final product, probably due to the different initial mineralogical content of this sample (W7)--see also Table S1, Supplementary Material. * Moreover, the respective quantification shows that a part of olivine (forsterite) and a part of the pyroxenes (enstatite) also react with hematite to form magnesioferrite, resulting in the decrease of olivine and pyroxenes content alongside a simultaneous increase of magnesioferrite formation in the final product obtained from the W7 sample. _Appl. Sci._**2022**, 12, 2094