The purpose of this study is to examine the effect of solid loading and aggregate size on the rheological behavior of PDMS (polydimethylsiloxane)/CC (calcium carbonate) suspensions. Mixtures containing 10 to 40 vol% of CC were prepared; the effect of shear rate and particle content on aggregate size and rheological properties was studied. Rheological properties including viscosity, loss modulus, storage modulus and yield stress were measured using rotational and oscillatory rheometry. Casson's equation was found to satisfactorily model the samples' stress response as a function of shear rate for different solid content. While solid content did not have any effect on the aggregate size, shear rate did influence the aggregate median diameter. It was observed that suspensions with higher aggregate size had a higher viscosity. In general, for a given volume fraction, a small change in the aggregate size had a significant impact on the viscosity, particularly at low shear rates.