Metakaolin-based geopolymer mortars: Influence of mix design on mechanical properties and durability

The production of Ordinary Portland cement (OPC) contributes significantly to global CO₂ emissions, prompting research into sustainable alternatives such as alkali-activated materials (AAMs) and geopolymers. This study aimed to characterise the fresh, mechanical, and durability properties of three metakaolin-based geopolymer mortars with differing mix designs. It included an experimental investigation on how the mix design affects workability, density, drying shrinkage, compressive and flexural strength development, capillary water absorption, and resistance to chloride, sulphate, and freeze-thaw conditions. Among the most important parameters evaluated are the activator-to-binder ratio, sand content and gradation and partial substitution of metakaolin with ferronickel rotary kiln dust (FRKD). Results revealed that the metakaolin-based mortar, achieved 87 % and 84 % of its 28-day flexural and compressive strength, respectively, after 7 days. Moreover, mortars containing FRKD, exhibited slower early strength growth but enhanced long-term strength due to late-age reactions from calcium and iron compounds in FRKD. Drying shrinkage, influenced by water-to-binder ratio, precursor composition, and curing, reached 1.9–3.7 % under direct drying but decreased by up to 83 % with sealed curing. The mortars’ dense microstructure limited chloride and sulphate ingress, with mass changes below ±2 % after one-year exposure. Furthermore, flexural strength was more susceptible than compressive strength to salt formation. These findings demonstrate that geopolymer mortars offer durable and sustainable alternatives to OPC, with mix design and curing conditions critical to optimising performance. • Mix design and curing significantly affect geopolymer mortar performance. • FRKD improves long-term strength but delays early strength development. • Sealed curing reduces drying shrinkage by up to 83 %. • All mortars showed < ±2 % mass change after 1 year of chemical exposure. • Freeze-thaw cycles reduced flexural strength by up to 24 %, showing good durability.