Influence of the degree of crystallinity of added nano-alumina on strength and reaction products of the CaO-activated GGBFS system

This study investigated the role of crystallinity of added nanometer-sized Al2O3 (nano-alumina) (i.e., poorly crystalline nano-alumina (gamma-alumina) vs. crystalline nano-alumina (alpha-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 gamma-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 alpha-alumina without added gypsum barely affected the C-S-H formation, although the added alpha-alumina alongside gypsum also had a similar effect on the dissolution of Si. In addition, due to the amorphous nature of gamma-alumina, the dissolution of gamma-alumina was faster than that of alpha-alumina. Thus, the use of gamma-alumina produced ettringite at the faster rate, as well achieving the greater and faster strength increase, although the added alpha-alumina was also slightly beneficial in accelerating the formation of ettringite. Thus, overall, the use of gamma-alumina was significantly more advantageous in improving strength by increasing the quantities of reaction products (C-S-H and/or ettringite) than that of alpha-alumina. (C) 2021 Elsevier Ltd. All rights reserved