Properties and Colloidal Behavior of Casting Slip Containing Ratchaburi Red Clay

Abstract

Red clay from Ratchaburi Province in Thailand has been used in place of ball clay in casting slip which normally contains 25:25:20:30 weight ratio of ball clay, kaolin, silica and Na-feldspar. The red clay has small particle size and high specific surface area. Slip viscosity increased as the amount of red clay increased. Na2SiO3 deflocculant was used to adjust the slip properties and its effects on colloidal behavior in terms of thixotropy, particle size and zeta potential were studied. The addition of 1.00-1.05 wt% Na2SiO3 was needed for the slip containing 25.00 wt% red clay (R25). The R25 slip with density of 1.67 g/cm3 had minimum viscosity of 390.5 cP and thixotropy at 5 min of 29.4 cP. It provided a 5 mm thick green body after 30 min casting. The pH was 8.62 and zeta potential was -40.8 mV at the optimum point. The red clay slip tends to have point of zero charge at pH<2. It has possibility to be a better stable colloid at pH around 6 or at pH >9.