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.