## 3 Results and Discussion
### Effect of Calcination of MgO Powder  
The obtained X-ray diffraction patterns of MgO powder samples are shown in Figure 2. It was established that the dominant phase in the studied samples is the target phase with the periclase structure, which is identified by the reflexes 2.43, 2.11, and 1.49 A (Figure 2). The average crystallite size of MgO and calcined MgO was 40 and 62 nm, respectively, which corresponds to the requirements [12]. It should be noted that MgO samples prepared by high-temperature processing do not contain an impurity (Figure 2b) of Mg(OH)2 (brucite) and Mg5(CO3)4(OH)2 \(\times\) 4H2O (hydromagnesite), which present in amounts about 21 and about 7 wt%, respectively, in the initial MgO sample (Figure 2a). It was previously noted that the presence of such phases in MgO during the synthesis of the MPP matrix is extremely undesirable, because it leads to an unacceptable increase in the rate of reaction (1) and produces an inhomogeneous compound with low strength.  
According to XRF analysis, the calcined sample of magnesium oxide contains 0.22 wt% impurities (Table 2), that corresponds to the chemical purity of reagent \"analytical grade\" in accordance with Russian standard [25].  
Figure 2: X-ray diffraction patterns of MgO (**a**) and calcined MgO (**b**). 1—MgO (periclase); 2—Mg(OH)2 (brucite), 3—Mg5(CO3)4(OH)2 \(\times\) 4H2O (hydromagnesite).  
When studying the morphology of MgO powder particles, it was found that the initial sample consists of particles of irregular shape with a size of several to tens of \(\upmu\)m (Figure 3a), and the surface structure of this powder presents staggered flake layer, which is typical of Mg(OH)\({}_{2}\) (Figure 3b). The morphology of MgO changed from a flake appearance into cubic crystals (Figure 3c) after its calcination; the discovered effect was previously also observed in [16].  
The obtained data on the granulometric composition of MgO powders are presented in Figure 4 and in Table 3. The distribution of particle agglomerates by size of the initial and calcined powder can be characterized as monomodal with a value of about 55 \(\upmu\)m and a complication in the region of small sizes with values less than 1 \(\upmu\)m (Figure 4a) and less than 3 \(\upmu\)m (Figure 4c), respectively. The effect of ultrasound on MgO samples leads to the destruction of large agglomerates. For example, as a result of the influence of ultrasound on the initial powder, about 56% of the particles acquire a size of less than 8 \(\upmu\)m (Figure 4a,b), and in the case of a calcined powder, about 79%--less than 31 \(\upmu\)m (Figure 4c,d). It is noted  
\begin{table}
\begin{tabular}{c c} \hline \hline  & **Component Content (wt\%)** \\
**MgO** & **Impurities** \\ \hline  & SiO\({}_{2}\)—0.10; CaO—0.08; \\
99.78 & Fe\({}_{2}\)O\({}_{3}\)—0.01; MnO—0.02; P\({}_{2}\)O\({}_{5}\)—0.01 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Chemical composition of calcined MgO.  
Figure 3: Scanning electron microscope (SEM) images of MgO (**a,b**) and calcined MgO (**c**).  
that, in general, calcination leads to partial agglomeration of MgO particles, for example, to an increase of the number of aggregates with a size of about 100 \(\upmu\)m (Figure 4d).  
It was found that the initial MgO powder has a large specific surface area (64.5 m\({}^{2}\)/g), apparently due to the flake layer structure. In this case, as a result of calcining MgO powder at 1300 \({}^{\circ}\)C for 3 h, the specific surface area of magnesium oxide decreases to 5.4 m\({}^{2}\)/g, which corresponds to the previously obtained data [1].