Direct Ink Writing of Geopolymers

Geopolymers are materials that were studied for the first time in the '70s by J. Davidovits as an alternative to Portland cement. Some aspects about production and some physical proprieties aren't completely understood yet. Recently attention is focused on the realization of geopolymer matrix composites, thanks to the lower fabrication costs compared to the mostly used ceramics composites, for the availability of the raw materials and for their lower environmental impact. Additive Manufacturing (AM) processes are recent technologies, that starting from early ‘90s begun to gain more and more interest between industrial companies and scientific communities, because they allow fast custom-made production, impossible to obtain with traditional production processes. 3D-printed objects find their application in a wide variety of fields, spacing from military, to aerospace, from automotive, to sports and even to biomedical. Direct Ink Writing (DIW) is part of direct AM technologies and is based on a layer-by-layer continuous deposition of an extruded filament. This technology was first called “robocasting” by Cesarano in his patent (2001). It is mostly used for the production of porous ceramic structures as tissue repairing lattices and catalyst carriers. Not all materials can be used as ink in DIW, the ideal behavior is that of a Bingham fluid, which can be extruded at low pressure and rapidly recover its viscosity to maintain the shape desired. Subject of this thesis is the implementation of a geopolymeric slurry in a DIW process for the production of porous structures. To attain this score, a deep rheological optimization was needed, and thanks to the use of rheology agents and different kind of fillers was possible to obtain geopolymeric inks and geopolymeric composite inks with enhanced printability features. Further to this work, it was possible to understand the evolution of compressive strength through the temperature in order to choose the best ratio between performance gained and time of treatment. Density, microstructural, XRD and chemical analysis were involved to clarify how temperature affects the overall samples characteristics. Finally, the addition of short fibers, especially carbon fibers, gave the possibility to print lattices with enhanced fracture toughness, as observed in SEM micrographs of fracture surfaces, where the strengthening mechanisms (debonding, delamination and pull-out) have been identified.