| Summary: | Magnesium oxide is a necessary binding agent for the synthesis of a magnesium potassium phosphate (MPP) matrix based on MgKPO<sub>4</sub> × 6H<sub>2</sub>O, which is promising for the solidification of radioactive waste (RW) on an industrial scale. The performed research is devoted to finding a cost-effective approach to the synthesis of MPP matrix by using MgO with an optimal ratio of the quality of the binding agent and the cost of its production. A method for obtaining MgO from the widely available natural mineral serpentinite was proposed. The phase composition, particle morphology, and granulometric composition of MgO were studied. It was found that the obtained MgO sample, in addition to the target periclase phase, also contains impurities of brucite and hydromagnesite; however, after calcining at 1300 °C for 3 h, MgO transforms into a monophase state with a periclase structure with an average crystallite size of 62 nm. The aggregate size of the calcined MgO powder in an aqueous medium was about 55 μm (about 30 μm after ultrasonic dispersion), and the specific surface area was 5.4 m<sup>2</sup>/g. This powder was used to prepare samples of the MPP matrix, the compressive strength of which was about 6 MPa. The high hydrolytic stability of the MPP matrix was shown: the differential leaching rate of magnesium, potassium, and phosphorus from the sample on the 91st day of its contact with water does not exceed 1.6 × 10<sup>−5</sup>, 4.7 × 10<sup>−4</sup> и 8.9 × 10<sup>−5</sup> g/(cm<sup>2</sup>·day), respectively. Thus, it was confirmed that the obtained MPP matrix possesses the necessary quality indicators for RW immobilization.
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