Valorisation of Mineral Wastes in Ceramic Formulations Using Phase Diagrams as a Tool

Abstract

In recent years scientific issues related to environmental preservation have acquired great importance and a major challenge to be met is the recycling of materials discarded by the different productive sectors. Due to the damage caused to the environment by technological development through the disposal of waste, this study sought to evaluate the possibility of using industrial wastes as alternative raw materials in the manufacture of ceramic tiles. Different industrial wastes classified as non-hazardous were selected: sludge from the crushing process of gneiss, sludge from the cutting and polishing process of varvite, sludge from the process of filtration-clarification of potable water and a clay also classified as waste. All wastes, as they were generated, were dried and disaggregated in ball mills, and characterized by X-ray fluorescence, X-ray diffraction, differential thermal and gravimetric analysis, optical dilatometry and particle size distribution. The applicability of these wastes in the ceramic tiles manufacture was guided by the phase diagram of the system S-K-A, and four formulations were established. For initial testing, these formulations were mixed and pressed into pellets, and sintered at 900, 950, 1000, 1050, 1100 and 1150ºC. The formulations plasticity was evaluated by the Casagrande method with good results, and, based on those preliminary results and after processing conditions optimization, the extrusion technique was used for the shaping process. The extruded samples were fired at 1100 and 1150ºC for 40 minutes and characterized by X-ray diffraction, differential thermal and gravimetric analysis, optical dilatometry, linear shrinkage, water absorption and flexural strength. The identified crystalline phases were associated with the sintering conditions (temperature, time, atmosphere) as well the intrinsic characteristics of raw materials such as chemical composition, particle size and homogeneity. The wastes proved to be good alternative raw materials and the corresponding formulations were shown to be viable in the manufacture of ceramic tiles.
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