Effect of different starting hemihydrate powders on the performance of gypsum moulds

Abstract

In order to determine the effect of different starting hemihydrates (Plaster of Paris) on the life of gypsum moulds, microstructure of two different Plaster of Paris and gypsum moulds made from these hemihydrates have been characterised using x-ray diffraction (XRD), thermogravinietry and a scanning electron microscope (SEM) attached with an energy dispersive x-ray spectrometer (EDX). XRD results have shown that one of the Plaster of Paris contains anhydrite phase which retains even after moulds are made. Both the gypsum moulds are immersed in a solution containing sodium silicate (Na2SiO3) deflocculant to determine the possible interactions between the moulds and the Na2SiO3 containing slip casting slurry. Experimental results have shown that a layer formed on the mould surface as a result of a reaction between the deflocculant and the gypsum mould. The reaction layer was thicker for the mould containing anhydrite phase.