Rheological behaviour and printability optimization of geopolymer ink for direct ink writing

This study focuses on developing a metakaolin-based geopolymer ink tailored for the Direct Ink Writing (DIW) technique, emphasizing optimizing rheological properties to achieve extrusion and shape retention. Three rheological modifiers, Xanthan gum, Triton X 100, and PEG 400, were evaluated to determine the parameters necessary for effective printing. Additionally, H2O2 was incorporated as a foaming agent to enhance the material's porosity. A significant challenge in this process was managing the ongoing polycondensation reactions, which continuously altered the ink's rheological behaviour. The geopolymer slurry was formulated using metakaolin and an alkaline activator with an optimized activator-to-metakaolin ratio of 1.25. Xanthan gum was identified as the most effective additive, creating a viscoelastic network that enabled the required shear-thinning behaviour, where the viscosity decreases during extrusion and rapidly recovers post-extrusion. This behaviour ensured precise printability while maintaining shape integrity. Key parameters were established for optimal printing performance: a nozzle diameter of 0.84 mm, a printing speed of 5 mm/s, and a layer height of 0.6 mm. Maintaining print quality during a holding time of up to 90 min required fine-tuning the ink dispensing rate. These findings underscore the importance of rheological characterization in addressing the challenges and enabling the design of high-performance Geopolymer ink for DIW applications.