Direct Ink Writing of metakaolin-based geopolymers: Rheology and printability control using PEG and inorganic fillers

This study aims to optimize the rheological properties of metakaolin (MK) based geopolymers for Direct Ink Writing (DIW) applications. The research explores the effects of Polyethylene Glycol (PEG) and calcite (HP) as rheological modifiers, focusing on their impact on yield stress, shear modulus, the time-dependent evolution of viscosity and structural stability. Through rheological analysis, we identify the formulation with the optimal balance between buildability and extrudability. Results demonstrate that PEG significantly decreases geopolymerization kinetics, increasing the printability window, while HP enhances the mechanical stiffness and yield stress of geopolymer paste. A comprehensive rheological evaluation, including stress growth, oscillatory tests, and creep experiments, highlights the importance of following long-term behavior. A case study on column printing demonstrates the importance of rheological controlling by predicting and confirming critical failure heights (sagging and buckling). X-ray tomography confirms the internal porosity and structural integrity of the printed filters. This work establishes the potential for geopolymer formulations in innovative applications like 3D-printed filtration systems by offering an approach to create formulations that combine printability, shape stability, and mechanical integrity.