Effect of fluorine-containing additive on the synthesis and sintering of compositions from natural raw materials in the “cordierite –mullite” system

The influence of the small addition of topaz on the processes of mineral formation in the “mullite–cordierite” system with a variable ratio of cordierite and mullite has been investigated. For this purpose, a series of experiments with different compositions of the initial mixtures, that is, with predominance of cordierite over mullite (KM mixtures) and predominance of mullite over cordierite (MK mixtures), has been performed. The addition of topaz in the form of topaz concentrate has been introduced in the amount of 1% by the weight of the investigated mixtures. The samples have been fired at various temperatures, that is, samples from KM mixtures (at a temperature of 1100–1300?°C) and samples from MK mixtures (at a temperature of 1400–1550?°C. The activating effect of topaz on mineral formation and sintering processes is determined by complex influence of fluoride products of topaz thermal decomposition. These reactively gaseous fluorides partially activate the solid-phase mass transfer processes and then reduce the viscosity of high-temperature melt formed during firing that intensifies the processes of ceramic matrix consolidation.Porous polycrystalline cordierite–mullitе–corundum ceramics from the KM mixtures, which contains 70%–87% cordierite, 3%–12% mullite, and 4%–11% corundum with water absorption of 3.5%–8% and bulk density of 2.10%–2.12?g/cm³ at the firing temperature of 1300?°C, has been developed. Mullite ceramics with notably corundum content (not more than 8%) from the MK mixtures with various density degrees at the firing temperature of 1500–1550?°C, porous ceramics with water absorption of 3%–11%, and dense ceramics with water absorption of less than 1% have been produced.