Ecological Alkaline Activator Based on Coffee Waste Ash Applied to Geopolymers

This study proposes the application of coffee waste ash (CWA) as an alkaline activator in geopolymers, replacing the hydroxide used. The motivations for this study are related to the need to develop ecological activators that replace commercial ones. CWA has high potassium content, making it suitable for activator application. Geopolymers were produced using metakaolin and a combination of sodium silicate with potassium hydroxide (KOH) as alkaline activators, both commercial. In these composites, commercial KOH was partially replaced by CWA in proportions of 10%, 25%, 50%, and 100% (wt%), using three KOH concentration levels: ≈6.70, ≈7.40, and ≈8.09 M corresponding to 10, 14, and 18 g KOH. Tests were carried out on geopolymers in fresh and hardened state. In the fresh state; mini-slump, viscosity, and calorimetry tests revealed that the potassium from CWA negatively affected the material’s rheology and the kinetic reaction. Specifically, in the 14 g KOH composition, 25% substitution caused a 7.52% decrease on spread while 100% substitution resulted in a 38.62% decrease. Viscosity increased by 38.86% with 25% substitution and by 428.70% with 100%. At hardening state, compressive strength decreased by 14.62% with 10% CWA substitution and by 85.16% with 100%. X-ray diffraction analysis showed that potassium ions were present as potassium carbonate, further influencing the material’s performance. Despite these effects, compositions with up to 25% CWA substitution showed compressive strength similar to the reference group, suggesting that partial replacement of commercial potassium with CWA-derived activators is viable up to 25%, supporting the sustainable use as partial activator for geopolymers.