## 2 Materials and Methods
### Investigation of MgO and MPP Matrix Samples  
The phase composition of MgO and MPP matrix samples was determined by X-ray diffraction (XRD) method using an Ultima-IV X-ray diffractometer (Rigaku, Tokyo, Japan). The XRD data were interpreted using the Jade 6.5 software package (MDI, Livermore, CA, USA) with PDF-2 powder database. The average crystallite size of the studied MgO samples was calculated by the Scherrer [12]. The composition of MgO was determined using the Rietveld method [17], with a PROFEX GUI software package for BGMN [18].  
The microstructure of MgO and MPP matrix samples was investigated by the scanning electron microscopy (SEM) using a JSM-6700F (Jeol, Tokyo, Japan) and Vega 3 (Tescan, Brno, Czech Republic) microscopes; the electron probe microanalysis of the samples was performed by energy-dispersive X-ray spectroscopy (EDS) using an X-ACT analyzer (Oxford Inst., High Wycombe, UK).  
The elemental composition of MgO powder was studied using an Axious Advanced PW 4400/04 X-ray fluorescence (XRF) spectrometer (PANalytical B.V., Almelo, Netherlands).  
Adsorption measurements of MgO powder samples were carried out in an ASAP 2000 automatic sorbtometer (Micromerities, Norcross, GA, USA); the specific surface area was calculated using the Micromerities software package.  
The particle size distribution of MgO samples was determined using a SALD-7500 nano laser diffraction granulometer (Shimadzu, Kyoto, Japan), including the use of 60 W ultrasound for 5 min.  
The compressive strength of MPP matrix samples was determined using a Cybertronic 500/50 kN test machine (Testing Bluhm & Feuerherdt GmbH, Germany). At least three compound samples were used in experiment.  
The hydrolytic stability of MPP matrix samples was determined in accordance with the semi-dynamic standard test GOST R 52126-2003 [19]. Before leaching, monolithic cubic samples were immersed in ethanol for 5-7 s, then the samples were dried in air for 30 min. Next, the samples were placed in a PTFE container, and double-distilled water was poured in as a leaching agent (pH 6.6 \(\pm\) 0.1, volume 100 mL), which was replaced at regular time intervals. The samples were removed from the container at the set time, washed with double-distilled water (volume 100 mL), and combined with the leachate, and the content of the matrix components in the solutions after leaching was determined by ICP-AES (iCAP-6500 Duo, Thermo Scientific, Waltham, MA, USA). The calculations of the differential leaching rate LR\({}_{\text{diff}}\) (g/(cm\({}^{2}\)-day)) of the matrix components from samples were made according to Equation (2).  
\[\text{LR}=\ \frac{\text{c}\cdot\text{V}}{\text{S}\cdot\text{f}\cdot\text{t}}\, \tag{2}\]  
where c--element concentration in solution after leaching, g/L; V--the volume of leaching agent, L; S--the open geometric surface area of the monolithic samples, cm\({}^{2}\); f--element content in matrix, g/g; and t--leaching time, days (for calculating the differential leaching rate t--duration of the n-th leaching period between shifts of contact solution).  
Leaching mechanism of MPP matrix components was assessed according to a [PERSON] and [PERSON] model [20], which can be presented as Equation (3), where values of the coefficient A (slope of the line) correspond to the following mechanisms: \(<\)0.35--surface wash-off (or a depletion if it is found in the middle or at the end of the test); 0.35-0.65--diffusion transport; and \(>\)0.65--surface dissolution [21]. The calculation of B\({}_{\text{i}}\) was carried out according to Equation (4).  
\[\text{log(B_{\text{i}})}=\text{A}\ \text{log(t_{\text{n}})}+\text{const} \tag{3}\]  
\[\text{B}_{\text{i}}=\ \text{A}_{\text{n}}\cdot\frac{\text{V}}{\text{S}}\cdot\frac{ \sqrt{\text{t}_{\text{n}}}}{\left(\sqrt{\text{t}_{\text{n}}}-\sqrt{\text{t}_{ \text{n}-1}}\right)} \tag{4}\]