Thixotropy and antithixotropy were characterized in nuclear waste simulant slurries. For the neutralized current acid nuclear waste (NCAW) simulant slurries, NCAW with glycolic acid (NCAW + GA), and NCAW with nitric acid (NCAW + NA) (pH approximate to 4), a pre-shear flow at constant shear rate destroys the aggregates in the suspension and reduces viscosity. For the NCAW + NA (pH approximate to 7-9), a pre-shear enhances the aggregates in the suspension and increases viscosity. With the addition of silica to the NCAW + GA and NCAW + NA slurries, the pre-shear effect tends to promote aggregation due to the formation of a network in the suspension, and hence, the viscosity increases. The macroscopic rheological property variation due to the microstructural aspects of the suspensions associated with the shear-induced effect are addressed.