A ternary system polypropylene (PP)/calcium carbonate (CaCO3)/high molar mass polysiloxane has been investigated with respect to its processing, crystallization, and dynamic mechanical properties. The filling level of PP varied from 2.5 vol % to 6.5 vol % CaCO3 and 0.6 vol % to 2.5 vol % polysiloxane, and the mixing was done in a single screw extruder. The polysiloxane molecules had different functionalities added (epoxy and methacrylate reactivity, respectively). However, the functionality did not influence processing, crystallinity, or mechanical properties. It was found that the polysiloxane molecules tended to surround the CaCO3 particles, thus forming a core-shell structure. This structure was achieved without surface treatment of the filler. The polysiloxane also provided a lubrication effect in the melt and thus enabled an easier processing of the composite. A nucleating effect of the filler could be detected. The mechanical properties were similar to those found in other core-shell structured samples, where the structure was achieved by surface treatment. It was found that the polysiloxane exhibited a lubricant effect only if it was mixed with PP together with CaCO3. If polysiloxane was added to the PP matrix alone, screw slippage occurred and the polysiloxane molecules agglomerated. The mechanical properties of these composites were similar to those of pure PP. No influence on the matrix crystallisation could be detected in this case.