## 4 Results and Discussion  
### Raw Materials  
The MgO content of the magnesite tailings is approximately 40 wt%, while low amounts of silica (5 wt%) and calcium oxides (2.85 wt%) complete the bulk composition (Table 2). The chemistry of this material corresponds to the mineralogical composition of Figures 2 and 3, where magnesite, dolomite quartz, and serpentine were detected. Moreover, the microscopic study using a polarizing and electron microscope assumes the coexistence of magnesite with significant amounts of other crystalline phases. The last reduces the purity in magnesium of the studied materials which is usually demanded for their uses of high economic value (e.g., refractory production). Nevertheless, these wastes are immediately presented clear and free of possible harmful elements. As it is shown in Figure 2a,b, fragments of serpentine are present in the microstructure of magnesite. Moreover, in Figure 2a, the replacement of magnesite by the coarse grain dolomite can also be observed. In general, the studied magnesite tailings presented are suitable to be used in a variety of applications in which an augmented content of nano MgO is needed. From the conducted microscopic observation and analyses of the studied tailings, it was shown that although these tailings constitute a rich in magnesium appearance containing over 45% of magnesite, it seems very difficult to obtain a high-quality product (bipolar magnesium) with over 90% MgO. The reason why this happens is due to the nature of this appearance, and more specifically calcium silicate impurities, as are presented in Table 2 and Figure 2. Thus, the mineralogy and the adhesion of the present minerals to the magnesite are responsible for the characterization of the magnesite tailings as a poor appearance for immediate exploitation, despite its MgO content. Magnesite tailings contain in their matrix 79% magnesite, 3% quartz, 3% dolomite, and 15% serpentine (Figure 3). It has been proven that magnesium refractory materials consist of 90% MgO and calcium/SiO\({}_{2}\) ratio of >2.85. In order to use magnesite tailings as refractories, they should contain similar characteristics to those of physical magnesites, such as low content of impurities as CaO, SiO\({}_{2}\), FeO (tot), and B\({}_{2}\)O\({}_{3}\), and high amounts of MgO. This conclusion is obvious as the tailings contain thin veins of serpentine and dolomite and very fine distributed quartz, not only in the dolomite veins, but also in the mass of magnesite. The coexistence of these minerals makes their release very difficult and, therefore, the appearance should be processed and enriched with high-cost processes by spending significant amounts of energy. This is exactly the main reason why these tailings are not easy to use in other applications, something that the present study fully investigates when filling the existing research gaps regarding the use of tailings deriving from magnesites from Greece (as these tailings which are used for the production of environmentally friendly cements have not yet been tested). Additionally, similar raw materials have been used in mortars and have been tested regarding the influence of their MgO content on their final performance [6], where it was found that certain MgO content seems to positively affect the final behavior of the produced mortars. The chemical composition of the used cement is shown in Table 3 and its mineral composition is shown in Figure 4. The main mineral composition of the used cement is C\({}_{3}\)S, C\({}_{2}\)S, C\({}_{3}\)A, and C\({}_{4}\)AF, with a small amount of gypsum and bassanite (Figure 4). Nano magnesium oxide is the product produced when the magnesite is heated to 1100 \({}^{\circ}\)C; whereupon most of the CO\({}_{2}\) is removed, while 2-7% CO\({}_{2}\) remains, depending on the amount burned [25]. The burned magnesium oxide produced in this study by the thermal decomposition of magnesite at a relatively low temperature, \(\sim\)1000 \({}^{\circ}\)C, is porous, of low bulk density, and chemically active.