Viscometric and sedimentation experiments were performed on fumed silica suspensions at solid volume fractions phi below 20%. The Newtonian viscosity and sediment volume of the suspensions were studied as a function of phi. All the experimental results are shown to be consistent with a dispersion microstructure, where permanent low-density aggregates are the primary structural units. Using a simple phenomenological model for both viscosity and sedimentation predictions, an average elementary permanent aggregate is shown to have a solid volume fraction of about 20%.