•This study explored the influence of added nano-aluminas on CaO-activated GGBFS.•The effect of the nano-alumina on strength depended on the crystallinity of alumina.•The dissolving γ-alumina led to noticeable increase in the formation of C-S-H.•The nano-aluminas caused fast gypsum consumption, resulting in ettringite formation.•Adding γ-alumina with gypsum was most effective for early strength improvement. This study investigated the role of crystallinity of added nanometer-sized Al2O3 (nano-alumina) (i.e., poorly crystalline nano-alumina (γ-alumina) vs. crystalline nano-alumina (α-alumina)) and the interaction of nano-aluminas with added gypsum in the CaO-activated ground granulated blast furnace slag (GGBFS) binder system. In the results, the use of γ-alumina evidently increased the dissolution degree of silicon (Si) from GGBFS regardless of adding gypsum, which was the main ingredient of calcium silicate hydrate (C-S-H), and thus led to a noticeable increase of C-S-H. However, the sole use of α-alumina without added gypsum barely affected the C-S-H formation, although the added α-alumina alongside gypsum also had a similar effect on the dissolution of Si. In addition, due to the amorphous nature of γ-alumina, the dissolution of γ-alumina was faster than that of α-alumina. Thus, the use of γ-alumina produced ettringite at the faster rate, as well achieving the greater and faster strength increase, although the added α-alumina was also slightly beneficial in accelerating the formation of ettringite. Thus, overall, the use of γ-alumina was significantly more advantageous in improving strength by increasing the quantities of reaction products (C-S-H and/or ettringite) than that of α-alumina.