Sustainable geopolymer binders for extrusion-based 3D printing: rheology, microstructure, and printability under ambient and sub-ambient conditions

Additive Manufacturing (AM) is increasingly recognised as a transformative technology for sustainable construction, yet its development is limited by the lack of printable and environmentally responsible binders. This study investigates extrusion-based AM of geopolymers derived from untreated ceramic sludge, with bentonite and citric acid as functional additives. A systematic methodology was applied, combining rheological modelling (Bingham, Herschel–Bulkley, modified laws), X-ray diffraction (XRD), scanning electron microscopy (SEM), and direct printing trials. The influence of processing temperature was addressed by comparing ambient (≈ 25°C) and sub-ambient (0–5°C) conditions, simulating both conventional and energy-constrained fabrication scenarios. Results show that bentonite markedly enhances yield stress, plastic viscosity, and thixotropic recovery, thus balancing extrudability and buildability. Citric acid further promotes structuration through ion-exchange modulation. Optimised formulations (e.g., GEO6) achieved defect-free multilayer deposition at nozzle diameters of 1.55 and 2.00 mm, with effective interlayer cohesion. In contrast, low-temperature processing delayed geopolymerisation, reducing rheological resistance and matrix consolidation, though selected mixes (e.g., GEO6F) retained partial printability. The proposed rheology-based printability map links apparent viscosity and thixotropic recovery to interlayer performance, providing a transferable framework for designing waste-derived geopolymer inks. These findings advance the application of circular aluminosilicate resources in 3D printing for sustainable and cold-region construction.