Structure and microstructure of sintered ceramic obtained by crystallization of fly ash mixtures and glass cullet

Abstract

Glass waste such as fly ash, granulated slags or glass cullet found their application, among others, as fluxes in the sintered ceramics production. The high content of alkali oxide present in wastes causes the formation of a liquid phase during sintering, intensifying this process. However, the role of these materials in the sintering process is not limited to this function. Glassy waste grains have high free energy associated with the lack of long-range ordering, which can additionally affect the intensification of sintering processes in the initial stages, because the degree of diffusion on the grain surfaces and in the glassy material volume is higher than in the crystalline material. Moreover, during heating, the glass undergoes processes of ordering its structure: transition and subsequent possible crystallization (devitrification). This second process is particularly important because the phase devitrification can change the direction of the reaction sintering of the entire ceramic mixture in which the vitreous additive has been introduced. These phases may be nuclei of homo or heterogeneous crystallization or may be substrates for the synthesis of other phases. Therefore, it is quite important to predict the direction of crystallization of vitreous waste, which is difficult due to the influence of many variables parameters. This research focused on identifying and characterizing sinters of fly ash and glass cullet mixture. To characterize the starting materials, X-ray diffraction (XRD), X-ray fluorescence (XRF) techniques were used. The sinters were obtained through thermal treatment at 850 °C as well as 1050 °C. The structural changes occurring during the devitrification of glass waste mixtures were studied by means of 27Al MAS NMR and FTIR spectroscopy. The microstructure of sintered samples etched in hydrofluoric acid was studied by SEM technique. During the crystallization of glass mixtures, the proportions of octahedral versus tetrahedral coordination of Al substantially change.