Thermogelling polysaccharides for aqueous gelcasting—part II: influence of gelling additives on rheological properties and gelcasting of alumina

In the present work the influence of the addition of aqueous solutions of agar, agarose and carrageenan on the rheological properties of alumina slurries is studied. Precursor binder solutions were prepared to concentrations between 1.5 and 4 wt.% in water by heating at 92 °C. Alumina suspensions were prepared at a solid loading of 50 vol.% and heated to 60 °C. At this temperature different amounts of the starting precursor solutions of each gelling binder were added so that the final concentrations of binder were 0.25, 0.5 and 0.75 wt.% referred to dry solids. For concentrations of the precursor solution ⩽2 wt.%, the viscosity of the slurry tends to decrease when the total amount of gel increases, while for concentrations of the precursor solution ⩾3 wt.% viscosity increases with the amount of gel. This defines a yield concentration of precursor solution where the suspension properties change. Gelcasting performance is a function of slurry viscosity, gelling behaviour and time, and body deformation during drying. In general, carrageenan leads to higher shape distortion in the body. Better shape retention is achieved with precursor solutions with ⩾3 wt.% additive and total content of gel of ⩾0.5 wt.%.