Abstract The aim of this study was to clarify the micro-scale processes which control the fresh and hardened properties of hydrating cement pastes containing up to 10 wt% of α-Al 2 O 3 submicron particles and a polycarboxylate-type superplasticizer. The main factor controlling the changes in the microstructure and properties of the fresh pastes with increasing α-Al 2 O 3 cement replacement levels was found to be the increase in the total specific surface area, whereas particle packing played only a minor role. However, in the case of the hardened pastes much more complex and somewhat counteracting effects were observed. Depending on the cement replacement level of the pastes, the sum of these effects was either a hardly noticeable improvement in their mechanical properties, compared to those of the reference paste, or even a reduction in them. These differences decreased with increasing curing time.