Ceramics-Silikáty 63, (1) 11 - 20 (2019)

Waste sludge, which is the sediment of washed down particles from ceramic manufacturing processes, constitutes about 2% by weight of the final products. This sludge consists of a water suspension of coarse particles (feldspar, quartz and ground fired tiles), fine particles (clay minerals such as kaolinite) and glaze waste. Common practice is to filter off this sludge and dump it or to use it in wet grinding of raw materials. In this work, it was re-used in the production of wall and floor tiles. Dried and ground sludge waste was added in percentages from 5% up to 50% to standard wall and floor tile mixes, molded and uniaxially pressed at 30 MPa, dried then fired at temperatures reaching 1180 °C for floor tiles and 1160 °C for wall tiles for a soaking time of 15 min. The properties of both green and fired tiles were investigated as function of percent waste added. The vitrification parameters (linear firing shrinkage, water absorption and apparent porosity) as well as mechanical properties were determined and compared with ISO standards. Samples of tiles having the composition of the proposed mixtures were fired in an industrial roller kiln and their properties determined. The microstructure of some fired sample was investigated by scanning electron microscopy (SEM). It was possible to obtain tiles that abided by ISO standards for sludge addition up to 20% for floor tiles and up to 10% for wall tiles.