Acid-activated geopolymer mortars: effects of the L/S ratio and precursor type on mechanical performance and sustainability

This study investigates the production of acid-activated geopolymer mortars using phosphoric acid as the activator and metakaolin as the precursor, both individually and in combination with iron oxide. The optimum liquid-to-solid (L/S) ratio was determined for each formulation, considering a precursor-to-natural sand ratio of 1:3 by mass. Compressive strength, pH, ultrasonic pulse velocity, and chemical and microstructural characteristics were evaluated. In addition, a cradle-to-gate Life Cycle Assessment (LCA) was performed using SimaPro software and the EcoInvent database. A theoretical optimum L/S ratio of 0.24 was identified for metakaolin-based mortars and 0.32 for mortars containing metakaolin and iron oxide, considering a phosphoric acid solution with a molarity of 10 mol/L. Mortars containing iron oxide exhibited compressive strengths approximately twice those of mortars produced solely with metakaolin, reaching 28.8 MPa after 28 days. However, these mortars experienced a significant loss of strength over 90 days, whereas formulations containing only metakaolin showed a strength increase of up to 288.4% during the same period. Potential applications for this material include the production of non-load-bearing masonry blocks, structural masonry blocks, precast partition panels, and pedestrian pavements. Regarding the environmental impacts, the use of phosphoric acid as the activator was found to significantly increase the environmental footprint of the mortars, contributing 64.5 kg CO₂-eq per tonne of mortar produced with metakaolin alone and 83.3 kg CO₂-eq per tonne of mortar containing iron oxide, making it the primary contributor to the overall environmental impact of the system. Furthermore, the use of commercial iron oxide not only failed to improve the long-term mechanical performance but also further increased the overall environmental impact, indicating that the exclusive use of metakaolin as the precursor represents the more sustainable alternative.