Abstract
	Magnesium oxide is between the most offen occuring oxides of the oxidic slag melts. MgO refractory particles of MgO clinker are dissolved in slag melt at the contact reaction between refractories and slags.
	MgO solubility in basic slag melts is relatively low and depends on temperature and chemical composition of the melt. Presence of magnesium oxide in slag melts increases melting temperature of slag melt. Study of the MgO influence on the physical properties of fayalitic slag was realised in iron crucible, at 6-8 wt. of CaO and pO2 = 21,28kPa and 1300°C. Calciumferritic slags were melted in platinum crucible at pO2 = 21,28 kPa and 1350°C. There were found, that MgO content decreases density and increases surface tension in the FeO-SiO2-CaO and Fe2O3-CaO systems respectively. Theoretical knowledge of structural and physical properties of oxide melts was used for the interpretaion of measured results and discused influences. MgO influence on the fayalitic structure and calciumferritic structure is different according to the differences of the structures of these both melts. Tetaedron SiO4, anion FeO45- and fayalite claster is typical for the structure of fayalitic slags. On the other hand FeO45- and Fe2O54- anions are the representatives of the calciumferritic structure. Magnesium oxide is decomposed to Mg2+ cation and O2- anion in these melts. O2- anions are bonded to anion complexes created by Si4+ and Fe3+ with O2-. Mg2+ cation is probably bonded to particulary free O- anions in these complex anoins.