Abstract Fracture tests have been carried out on rocks, pastes, and paste/rock interfaces, to attempt to study the interaction of these materials and to assist in explaining their influence on properties of concrete. The test is based on ISRM Method I, and can also be used to test interfaces. Results are presented and discussed with reference to the influence of silica fume (S.F.) addition on pastes and interfaces, and of two different rock types on the interaction with pastes. It is shown that S.F. has a minor influence at the mix ratios and ages used. Paste/andesite rock interfaces have substantially higher fracture energies than paste/dolomite rock interfaces. Keywords: Interfaces, Silica Fume, Fracture, Paste-aggregate bond.