Low viscosity composites, with a flexural modulus of at least 18 GPa and a flexural strength of at least 90 MPa for filling dental root canals for fixing post or as a build-up material on post, are composed of aluminum oxide with a mean particle size of 1 to 50 μm in combination with at least one additional filler with a mean particle size of 0.5 to 5 μm and a monomer mixture. Also disclosed is a method for the preparation of tooth restorations.