Carbonation resistance of calcined clay-ground granulated blast furnace slag alkali-activated mortar

Alkali-activated materials are obtained from the alkaline activation of a precursor rich in SiO2 and Al2O3, which can be derived from by-products or industrial residues. Calcined clay is a potential candidate as a precursor for this type of material. Large amounts of suitable clays for alkali-activated materials are available around the world. This paper evaluated the performance of different alkaline-activated mortars using calcined clays (low grade) and ground-granulated blast-furnace slag (GGBFS), including physical and mechanical performance, reaction products phases, microstructure, and resistance to accelerated carbonation. The results indicate that the increase in GGBFS content up to 60% has a significant effect on the properties of the mortars, due to the effect of Ca-rich phases such as C-(A)-S-H/C-(N)-A-S-H, promoting the densification of the microstructure, and an increase in pH from 11.91 to 12.78 for specimens with 100% calcined clay and 60% GGBFS respectively, leading to a better carbonation resistance. On the other hand, adding GGBFS leads to a reduction in concrete initial and final setting time and lowers thermal stability compared to the reference calcined clay material. The increase in the modulus ratio of the activator from 1 to 1.5 also improves the mechanical properties and resistance to carbonation of geopolymer mortars with various binder compositions.