This study explores the rheological properties and 3D printability of metakaolin-based geopolymers (GP) designed for fabricating inorganic filters for liquid waste decontamination. Through this study, we investigate the influence of various additives on the rheological properties essential for Direct Ink Writing (DIW) applications. Specifically, we examine the effects of polyethylene glycol (PEG), carbonate minerals, and hexadecyltrimethylammonium bromide (CTAB) on GP formulations to optimize printability and structural integrity. Our findings demonstrate that incorporating PEG as a shear-thinning agent, along with inorganic particles, significantly enhances the printability of the GP pastes. We model the buildability of each formulation by evaluating the evolution of yield stress and the impacts of PEG, CTAB, and carbonate minerals. The optimal formulation, containing 2 wt% PEG and 30 wt% CaCO3, exhibits superior rheological properties, ensuring precise filament extrusion and stable layer stacking. Additionally, a case study validates the printability of selected GP formulations and examines the porosity and topology of the macro- and micro-porous networks within the 3D-printed lattices, which are critical for effective filtration. This research elucidates the structure-processing-properties relationship in 3D-printed GP lattices, offering valuable insights for advancing filtration and decontamination technologies.