The mechanical activation in vibromills with rings of Portland cement and Portland cement with various additions (blast furnace slag, ash, and tuff) with the initial specific surface of 300 m(2)/kg was studied. To reach a specific surface of 500 m(2)/kg,. the Portland cement with addition of 20% fly ash needs the shortest time of activation-3 min. Comparing testings of physico-chemical and mechanical properties of the pure Portland cement and Portland cement with various additions, it was found that the mechanical activation improved properties in all samples. Effects of mechanical activation were larger when the mix of Portland cement with addition was activated: then only additions were activated. The highest effects of mechanical activation were with the mix of Portland cement with the addition of 20% fly ash: compressive strength after 28 days of hardening was increased to 57.9%, which is 3.3% more than the compressive strength of the pure Portland cement. Due to mechanical activation of cement, changes in the surface microstructure of clinker minerals present in cement take place, which occurs during investigation of diffraction maxima intensities-characteristic of the basic clinker minerals. It is also noticed that there is an interchange of clinker minerals from larger to smaller particle sizes. Thus, the reactivity of the present mineral phases increases with the hydration reaction rates, which occurs in the process of hardening of Portland cement and Portland cement with mineral additions. A somewhat greater presence of clinker minerals in smaller grain size classes (i.e., in the class of -20divided by0 mum) enables more complete hydration of their grains, which ensures the improvement of mechanical properties of tested cements. (C) 2004 Elsevier B.V. All rights reserved.