## 3 Results and Discussion
### Study of the Obtained Samples of the MPP Matrix  
For the synthesis of the MPP matrix, we used a precalined MgO powder, the characteristics of which are given in the previous Section. When studying the phase composition of the synthesized samples of the MPP matrix, it was found that their main crystalline  
Figure 4: Size distribution of MgO (**a**,**b**) and calcined MgO (**c**,**d**) (dashed curve—size distribution after the effects of ultrasound on powders).  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Sample** & \begin{tabular}{c} **Mode** \\ **(\(\upmu\)m)** \\ \end{tabular} & \begin{tabular}{c} **Median** \\ **(\(\upmu\)m)** \\ \end{tabular} & \begin{tabular}{c} **\textgreater{}90\%** \\ **(\(\upmu\)m)** \\ \end{tabular} & \begin{tabular}{c} **\textless{}0.1 \(\upmu\)m** \\ \end{tabular} & \begin{tabular}{c} **\textless{}1 \(\upmu\)m** \\ \end{tabular} & \
\begin{tabular}{c} **\textless{}10 \(\upmu\)m** \\ \end{tabular} &
\begin{tabular}{c} **\textless{}100 m** \\ \end{tabular} \\ \hline MgO & 54.92 & 37.09 & 87.51 & 0.99\% & 8.04\% & 18.20\% & 98.12\% \\ Calined & 54.92 & 38.92 & 110.47 & 0.71\% & 3.51\% & 19.77\% & 92.35\% \\ MgO * & 7.58 & 6.81 & 19.26 & no & 3.70\% & 74.13\% & 100\% \\ Calined & 30.68 & 10.58 & 43.51 & 4.73\% & 14.64\% & 50.32\% & 100\% \\ MgO * & \multicolumn{6}{c}{* After exposure by ultrasound on powders.} \\ \hline \hline \end{tabular}  
* After exposure by ultrasound on powders.  
\end{table}
Table 3: The results of granulometric analysis of MgO.  
phase is the target phase MgKPO\({}_{4}\times\) 6H\({}_{2}\)O, which is an analog of K-struvite [6] with main reflections at 5.86; 5.56; 5.40; 4.25; 4.13 A etc.) (Figure 5). The samples also contain phase of MgO (periclase), which is associated with the excess of the used MgO in relation to the stoichiometry of reaction (1).  
The SEM micrograph of the surface of the MPP compound is shown in Figure 5. The elemental composition of the predominant phases in the compound sample includes matrix components of the basic composition MgKPO\({}_{4}\times\) 6H\({}_{2}\)O with insignificant variations in the Mg/K ratio, as we noted earlier in [1]. Open pores with a linear size of about 100 \(\upmu\)m are also observed (Figure 6).  
The compressive strength of the MPP matrix obtained using MgO after calcining at 1300 \({}^{\circ}\)C for 3 h was 6.19 \(\pm\) 0.45 MPa, which meets the regulatory requirements for a cement compound: no less than 4.9 MPa [26].  
The determination results of hydrolytic stability of MPP compound to the leaching of matrix-forming components are shown in Figure 7a,b. Data in Figure 7a shows that the differential leaching rate of magnesium, potassium, and phosphorus from the compound on the 91 st day of contact of the sample with water is 1.6 \(\times\) 10\({}^{-5}\), 4.7 \(\times\) 10\({}^{-4}\)\(\mu\)8.9 \(\times\) 10\({}^{-5}\) g/(cm\({}^{2}\) day),  
Figure 5: X-ray diffraction pattern of the magnesium potassium phosphate (MPP) matrix. 1—MgKPO\({}_{4}\times\) 6H\({}_{2}\)O (K-struvite); 2—MgO (periclase).  
Figure 6: SEM image of the MPP matrix.  
respectively. It was found that the leaching of the matrix-forming elements for 91 days of contact of the sample with water is controlled by various mechanisms. Leaching of potassium and phosphorus from the MPP matrix in the first seven days occurs due to its wash-off of from the surface of the compound, followed by depletion of the surface layer (Figure 7b, coefficients A in Equation (3) are for potassium \(-\)0.43 and 0.17, and for phosphorus \(-\)0.50, 0.04, and \(-\)0.66). Leaching of magnesium in the first 10 days occurs due to its wash-off of from the surface of the compound, and in the next 81 days due to diffusion from its inner layers (Figure 7b, \(-\)0.81 and 0.48). The obtained results on the hydrolytic stability (rate and mechanism of leaching) of the MPP matrix obtained using MgO obtained from serpentinite are consistent with the previously obtained data for the MPP matrix obtained from commercial MgO [1].