X-ray computed tomography investigation on the geopolymer matrix formation during the binder jetting additive manufacturing process

This study investigates the geopolymerization mechanism in a binder jetting additive manufacturing system, where a highly alkaline solution is deposited onto a powder bed comprising sand and metakaolin. Two individual alkaline solutions, sodium- and potassium-based, were systematically compared, along with tap water, to interpret processes governing the formation of the geopolymer gel and the subsequent hardening of printed components. Nondestructive volume analysis via X-ray computed tomography was employed to characterize the multiscale structure of the powder bed, while real-time monitoring of alkaline solution–powder bed interactions provided insights into the reaction kinetics as well as material consolidation, from droplet impact to formation of the geopolymer matrix. The results demonstrate successful activation of metakaolin without mechanical mixing, achieved using a large-scale 3D printer with a voxel resolution of 3.0 × 3.0 × 3.0 mm³, facilitating the production of large-volume geopolymer components, with an appropriate compressive strength of ∼20 MPa, suitable for structural applications. Moreover, Nuclear Magnetic Resonance (NMR) spectroscopy proved the change in the coordination states of aluminium ions, shifting from mixed four-, five-, and six-coordination in metakaolin to predominantly tetrahedral coordination in the final geopolymer. These findings provide critical insights into the microstructural evolution and reaction mechanisms in binder jetting-based geopolymerization.