THE ROLE OF Fe₂O₃ IN THE STRUCTURE AND PROPERTIES OF ALKALI-FREE IRON-CONTAINING MAGNESIUM ALUMOSILICATE GLASSES FOR CONTINUOUS HIGH-MODULUS FIBERGLASS, DEVELOPED ON THE BASIS OF FLY ASH

Fly ash formed as waste at thermal power plants and GRES contains up to 90 % of the amount of silicon (SiO2) and aluminum (Al2O3) oxides, which are basic in the silicate industry, and about 10 % is represented by iron oxide (FeO+Fe2O3), calcium, magnesium oxides (CaO, MgO) and finds its application in as additives in cements, paving, mineral fertilizers, etc. The use of fly ash as a raw material in the glass industry, despite solving environmental (waste disposal) and economic problems (cost reduction), may be limited by the increased iron content, which is traditionally considered an undesirable impurity, causing, first of all, staining of glass and reducing its transparency. However, the study presented in the article demonstrates that under certain conditions, iron with variable valence in the form of Fe2O3 can play the role of a glass-forming oxide, similar to Al2O3, without degrading the properties of glass, while the cost of producing such glass will be lower than the traditional similar composition. To test the hypothesis put forward, the calculation of the oxygen balance in the developed alkaline and alkali-free (with alkaline earth oxides) iron-alumosilicate glasses used in the production of high-modulus glass fibers (HMP), the calculation of the degree of connectivity, modulus of elasticity and density was carried out. At the same time, in the developed formulations, one intermediate glass-forming agent Al2O3 is partially replaced by another, Fe2O3. The oxygen balance shows that for most of the studied glass compositions, oxygen introduced by modifier oxides (Na2O, CaO, MgO) is sufficient to compensate for the negative charge of the formed tetrahedra [FeO4]- and [AlO4]-. This does not contradict, in the conditions of the struggle for oxygen, the oxides Fe2O3 and Al2O3 simultaneously participate in the formation of a glass grid with an optimal ratio of all glass components. The calculation is confirmed by sufficiently high values of the elastic modulus (81-86 GPa) and low density (2,47–2,57 g/cm3) of the glasses, indicating a compact tetrahedral structure. In addition, the calculation of the high-temperature viscosity of the developed compositions indicates the possibility of producing continuous glass fibers of the VMP type at temperatures in the range of 1250-1400 ℃. Reaching the production temperature of 1250 ℃ is a record low for this type of glass, which will lead to an improvement in the molding process of elementary fibers.