Rheological and casting properties of two Egyptian clays

Abstract

Slips from Kalabsha and Malkatta clays, widely used in the Egyptian ceramics industry, were prepared with the aid of an appropriate deflocculant at a constant specific gravity of 1·80—which lies within the practical working range for the slip casting technique. The rheological and casting properties, including plasticity of clay pastes, apparent viscosity and thixotropy of the clay slips, as well as casting rate, water retention, porosity and mechanical strength of their casts, were studied. The two clays behave in a quite different manner, reflecting the marked variation in their nature. Kalabsha clay slip has the minimum apparent viscosity with moderate thixotropy in the presence of 0·8% deflocculant addition, while Malkatta clay slip needs twice as much deflocculant to attain maximum fluidity, which is accompanied by a little higher thixotropic structure. Kalabsha clay slip casts faster and leads to cast having lower water retention, shrinkage, modulus of rupture and bulk density and of higher apparent porosity than that of Malkatta slip. In formulating ceramic bodies processed by the slip casting method, a blend of the two clays must be utilized in order to get a mixture having the proper flow properties and the best casting qualities. For thin-walled articles, the content of Kalabsha clay should be kept lower than that of Malkatta clay; the reverse holds for heavy-section casts.