## 2 Materials and Methods  
High-purity, reagent grade magnesium oxide powder (MgO \(\geq\) 98% from Acros Organics, Geel, Belgium), commercial reactive alumina powder (d50 \(\leq\) 0.6 \(\mu\)m, Al\({}_{2}\)O\({}_{3}\)\(\cong\) 99.8%, RG 4000 from Alamtis, Leetsdale, PA, USA), citric acid (C\({}_{6}\)H\({}_{8}\)O\({}_{7}\)-H\({}_{2}\)O \(\geq\) 99.5% from POCH, Gliwice, Poland), and tartaric acid (C\({}_{4}\)H\({}_{4}\)O\({}_{6}\)\(\geq\) 99% from POCH) were used as raw materials. Pictures of the raw materials are shown in Figure 1. The magnesium oxide and alumina powder were mixed at a 1:1 molar ratio, and their samples' compositions are presented in Table 1.  
Powders of the raw materials were homogenized by mechanical mixing in a planetary mill using rubber mixing elements, pressed into pellets (diameter 20 mm, under the pressure of 20 MPa), and subjected to a hydrothermal synthesis at the temperature of 240 \({}^{\circ}\)C, under autogenous water vapor pressure for 8 h.  
The samples, obtained after a hydrothermal treatment, have a powder form. The samples were washed several times with cold acetone to remove free water. Samples for the thermal analysis, XRD analysis as well as FTIR analysis were subjected to grinding in a mortar grinder to obtain powder samples under 0.063 mm.  
To identify the samples' phase changes corresponding to the used additives, the X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and thermal analysis, i.e., thermogravimetry (TG), differential scanning calorimetry (DSC), and evolved gas analysis-mass spectrometry (EGA-MS), were simultaneously used. The chemical composition and the microstructure of the samples were characterized by scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS).  
The thermal analysis was carried out usinga Simultaneous Thermo Analyzer (STA) TG-DSC NETZSCH STA 449F5 Jupiter coupled to QMS 403 D Aeolos (Erich NETZSCH GmbH & Co. Holding KG, Selb, Germany). The provided sample mass \(\sim\)30 mg was heated to 1000 \({}^{\circ}\)C in airflow (50 mL/min) at a heating rate of 10 \({}^{\circ}\)C/min, in a corundum crucible.  
The XRD measurement was carried out using an X'PertProPANalytical X-ray diffractometer, in the range of 5-90 20, with Cu K\(\alpha\) radiation, with 0.02\({}^{\circ}\) per step and a time of 3 s  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **A\({}_{2}\)O\({}_{3}\)** & **MgO** & **Citric Acid** & **Tartaric Acid** & **Sum** \\  & **(\% mas.)** & **(\% mas.)** & **(\% mas.)** & **(\% mas.)** & **(\% mas.)** \\ \hline
1 & 71.67 & 28.33 & - & - & 100 \\ \hline
2 & 70.95 & 28.05 & 1 & - & 100 \\ \hline
3 & 70.95 & 28.05 & - & 1 & 100 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Composition of the studied samples.  
Figure 1: Raw materials used in the experiment: (**a**) alumina powder RG 4000; (**b**) magnesium oxide powder; (**c**) citric acid; and (**d**) tartaric acid.  
per step. Data analysis was made by HighScore Plus software (Panalytical) with the PDF-2 database supported by the ICDD (The International Centre for Diffraction Data, Newtown Square, PA, USA), the full list of reference patterns is provided in Table 2.  
The IR bands were recorded using a Bruker Vertex 70v FTIR spectrometer at a spectral range of 400-4000 cm\({}^{-1}\). Measurements were carried out on pelletized ca. 0.5 wt.% powder samples in a KBr matrix.  
The microstructure images were carried out using a Nova Nano SEM 200 FEI (FeiEurope Company, Eindhoven, Netherlands). Measurements were recorded using a low vacuum detector (LVD) using secondary electrons (SE). Chemical analysis was carried out using an EDS analyzer (EDAX corporate, Mahwah, NJ, USA).