Sintered gahnite–cordierite glass-ceramic based on raw materials with different fluorine sources

Abstract

Glass-ceramic based on Zn-containing cordierite was prepared from kaolin, silica’sand and commercial ZnO. The addition of AlF₃, MgF₂ and CaF₂ was performed as nucleation catalysts. Dark brown glasses were obtained from the glass batches. The transformation and crystallization temperatures were in the range of 739–773 and 972–1007°C, respectively. Gahnite, cordierite and very little enstatite were the development crystalline phases through the heating and sintering process between 1000 and 1340°C. The microstructure of crystallized samples at 1340°C showed the appearance of dominant euhedral octahedral crystals of gahnite and hexagonal cordierite, in the low micro-scale, disseminated in the glassy matrix. The microanalysis of the crystallized samples indicated that Zn and Mg may replace each other in gahnite and cordierite structure. Densities of the crystallized samples were between 2.2517 and 2.5278 g cm⁻³. The thermal expansion of the crystallized samples was ranging from 19.22 to 59.30 × 10⁻⁷°C⁻¹. However, the higher crystallization of both cordierite and gahnite accompany with the higher values of densities and the lower values of coefficient of thermal expansion.