Colloidal radioactive transuranic wastes are Currently buried in large tanks in the form of dense colloids in high salt, high pH aqueous media. These facilities are beginning to fail, so it is necessary to transport and "package" them for more permanent disposal, processes requiring Understanding of the microstructure that develops under such conditions. Laponite RD clay is believed to be a good simulant for the colloids ill the waste tanks, and file present study addresses their behavior under high salt condition, where previous studies have frequently observed the phenomenon of "restabilization," i.e., the attainment of aggregation stability, at high electrolyte conditions. Specifically, the aggregation kinetics and the resulting cluster Structure (fractal dimension) of Laponite RD clay colloids at high concentrations of BaCl2 (an electrolyte previous, shown to lead to restabilization) are investigated. At low-to-intermediate electrolyte concentrations, the clay is found to behave in accord with DLVO theory, i.e., low salt conditions yield slow aggregation into densely-packed aggregates, whereas intermediate salt concentrations, sufficient to cause double layer collapse, produce rapid aggregation into open aggregates. High salt concentrations, however, show slow rates of aggregation. The aggregate Structure under these conditions is found to mimic that found for very low electrolyte concentrations, i.e., high fractal dimension. Further experiments show that a sudden increase in salt concentration in a system containing Young open aggregates produced under intermediate salt concentrations causes them to reform into more compact Structures. (c) 2005 Elsevier Inc. All rights reserved.