The influence of alumina reactivity (specific surface area and Na2O content) on the hydration of mono calcium aluminate (CA) is investigated. It was found that additions of alumina influence the hydration behaviour of CA and lead also to differences in the microstructure of the hardened pastes. Besides the specific surface area and Na2O content, the reasons of these influences seem to be connected with the concentration of weak acid sites which obviously causes changes in the nucleation mechanism of the hydrates.