Kaolin from a Serbian deposit, characterized by high content of impurities, such as mica and quartz, disordered kaolinite and high specific surface area, was subjected to thermal and mechanical activation on a large scale. Activations were carried out with a goal to investigate the effects of applied methods on the chemical, structural and morphological changes that occurred in the kaolin, and their influence on the pozzolanic activity. The changes were monitored using XRD, FTIR, TG/DTA, PSD, BET and SEM analyses. Pozzolanic activity was evaluated by determination of the 7-day compressive strength conducted on the lime mortars. The results showed that mechanical activation by milling the kaolin for 20h in the conventional horizontal ball mill was competitive for obtaining pozzolana as thermal activation. Pozzolanic activity of thermally activated kaolin was mainly influenced by formation of metakaolin. Combined effects (such as significant specific surface area increase, as well as partial kaolinite and complete mica phase amorphization) were dominant in pozzolanic activity of mechanically activated kaolin. •Mechanical activation of kaolin with impurities was performed on large scale.•Low-energy ball mill was used for mechanical activation of kaolin.•Mechanically activated kaolin showed pozzolanic activity competitive to thermally.•Kaolin modifications induced by thermal and mechanical activation were different.•Mica impurity contributes to pozzolanic activity of mechanically activated kaolin.