On the use of temperature to destabilize deflocculated alumina in colloidal silica suspension in zero cement castables

Future advancements in investment casting require the development of improved ceramic castables. An alumina mullite castable was designed for creep resistance by eliminating liquid phase formation. The general solution to control castable coagulation and setting using cement can not be applied and is replaced by coagulation by acid addition. Lowering the pH of the slurry however, has a negative effect on the green strength of the ceramic castable, as has been shown in the current research using a miniature wedge splitting test. To achieve similar coagulation at a higher pH, the temperature during coagulation is lowered from 25 °C to 5 °C. The lowered temperature assists in coagulation by lowering the electric permittivity, decreasing the diffuse layer and lowering the dissociation constant of silanol. By decreasing the temperature, the absolute zeta potential can be decreased by 15 mV, normally achieved by lowering the pH by 2.5. Finally, an optimized coagulation was achieved at higher pH levels in a deflocculated aluminosilicate slurry.