Controlling the stability and theological properties of colloidal slurries has been an important but challenging issue for various industries such as cosmetics, ceramic processing, and nuclear waste treatment. For example, at the US Department of Energy Hanford and Savannah River sites, operation of the waste treatment facilities with increased solids loading increases waste processing rates but negatively impacts the rheological properties. We investigated various rheological modifiers on a Fe2O3/Al(OH)(3)-rich nuclear waste simulant, characterized by high ionic strength and pH, to reduce yield stress of the colloidal slurry. Rheological modifiers change particle interactions in colloidal slurries; they mainly alter the electrostatic and steric interactions, leading to a change in theological properties. Weak acid-type theological modifiers strengthen electrostatic repulsion whereas nonionic/polymer surfactant-type theological modifiers introduce a steric repulsion. Using theological analysis, it was found that citric acid and polyacrylic acid are good theological modifiers for the simulant tested, effectively reducing yield stresses by as much as 70%. Further analysis supports the idea that adding these theological modifiers increases the stability of the slurry. A likely mechanism for the observed effects of citric acid and polyacrylic acid on slurry behavior is identified as both binding cations in bulk solution and adsorption on the surface of the particles. (C) 2010 Elsevier Inc. All rights reserved.