Four different fractions of a semi-hard material, limestone, were comminuted batch-wise by high pressure in a die press under defined conditions that eliminated the wall effects of the confinements. As the influence of the acting walls were negligible, the particular interactions were modified by using the additive caproic acid and by variation of the lateral confinements. The influence of the feed size, additive amount and the confinement on the specific surface growth, the energy absorption and the energy utilization was investigated. It could be proved that decrease of internal friction due to the additive exerts a small effect on the comminution process, whereas the material transport out of the stressing zone is decisive. Using the caproic acid as an additive only improves the energy utilization in case the particle bed is not fully confined.