Composite materials of polypropylene and mineral microparticles have been generated by compounding and tested in terms of mechanical stiffness. In a first step silica, boehmite and functionalized clay microparticle powder have been mixed with the polymer in a twin-screw compounder. The elastic modulus was highest for mixtures with a microparticle concentration of 5 to 10%w/w. An increase of 25% of the elastic modulus was achieved by simple melt extrusion. In a second step, a maleic anhydride-grafted polypropylene (PP-g-MA) was used as a matrix. When measured by nanoindentation, the pure PP-g-MA matrix showed an elastic modulus twice as high as pure PP, probably because of a partial reticulation. During extrusion, amino-silane functionalized clay microparticles were added to the PP-g-MA matrix and reacted with it by building covalent amide group bonds. The resulting compound material showed an elastic modulus of more than four times the stiffness of pure PP.