Extrusion and property characterisation of waste-based ceramic formulations

Abstract

This work describes the studies carried out with various industrial wastes and natural sub-products based on the SiO 2–Al 2O 3–CaO system, aimed at extruding all-waste ceramic products of industrial interest. Four waste materials were selected and characterised, namely, (i) Al-rich anodising sludge (A-sludge), (ii) sludge from the filtration/clarification of potable water (W-sludge), (iii) sludge generated in marble sawing processes (M-sludge), and (iv) foundry sand (F-sand). The plastic behaviour of two different all-waste formulations was first characterised by stress–strain curves and then, after prior adjustment of the plasticity level, the effect of the ram speed and extrusion pressure was evaluated using the Benbow–Bridgwater's model for paste extrusion. Using the waste-based formulations with additives and tube-dies of different die-land dimensions, a good agreement was demonstrated between predicted and measured values. The differences in the static friction coefficient give an effective indication of the surface quality of the extrudate. Extruded rods were then fired at several temperatures and characterised in terms of relevant functional properties (shrinkage, density and mechanical strength). Compositional evolution was assessed by X-ray diffraction. Since interesting performances were observed, the potential of the use of wastes in ceramic formulations of industrial interest was confirmed.