Pyrogenic silica is widely used as a filler and thixotropic agent. Its behavior is caused by the fractal structure of the secondary aggregates that exist after good dispersion in liquid media. This paper deals with the granulometric properties measured in suspensions of pyrogenic silica using different measurement techniques. Suspensions of various grades of silica with specific surfaces (BET) of 50 m²/g to 400 m²/g with concentrations ranging from 0.1 to 10 v% have been prepared and alkaline stabilized. Measurements using photon correlation spectroscopy in backscattering mode, ultrasonic attenuation spectroscopy and analytical centrifugation with a photocentrifuge have been conducted. It can be seen that photon correlation spectroscopy measures hydrodynamic radii of the fractal aggregates that are in good correlation to the radius of gyration although a certain contribution of rotational diffusion has to be considered. Ultrasonic attenuation spectroscopy results correlate with the Sauter diameter as measured with BET but there is a distinct difference presumably due to attenuations at the sintered junctions of primary particle chains in the aggregates. The measurement of space and time-resolved extinction profiles during centrifugation reveals single agglomerate sedimentation as well as cluster sedimentation, which allows to determine a hydrodynamic radius as well as fractal properties.

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