Geopolymers are aluminosilicate inorganic materials synthesized by dissolution and polycondensation, in alkaline medium, of silicon and aluminium coming from a variety of raw materials [1, 2]. The result is a 3-dimensional structure, composed of SiO4 and AlO4 tetrahedra linked together by oxygen atoms. Positive Na+ and K+ ions in the framework cavities allow the tetrahedrical coordination of Al3+ ions with oxygen, analogous to that of Si4+ ions.
A significant advantage of geopolymer technology is its capability to use different types of industrial silicate waste, which would otherwise be landfilled, as raw materials . The use of waste in the synthesis of geopolymers allows them to be employed in value-added applications thus preserving valuable resources in a move towards sustainable development.
Geopolymers are primarily developed for the construction industry as an ecofriendly substitute for Portland cement thanks to their suitable mechanical properties, chemical durability, and the low-footprint emissions associated with their production . Moreover, geopolymers can be used in many applications beyond concrete such as in fire- and acid-resistant coatings, filters, thermal shock refractories, composites, high-tech resin systems, or for immobilization of hazardous waste [5−9].
Most previous work on geopolymers has focused on fabrication and characterization of simple-shape parts, e.g., for use in the building/construction industry. In most cases, little attention was paid to developing components with complex design shapes by new processing technologies or to investigating machining techniques to achieve parts of superior surface finish and shape accuracy which could enable applications in other sectors, such as mechanical engineering.
The aim of this paper is to highlight the positive factors that favor geopolymer technology, including not only low-cost processing and the stabilization of waste materials but also the possibility to process geopolymer components with unconventional (for geopolymers) and innovative technologies.
In this work, direct ink printing and machinability of geopolymers are investigated to expand the range of possible applications for waste-containing geopolymer materials.
In recent publications by Franchin et al.  and Xia et al.  geopolymers were printed with good accuracy, but there are no previous publications on the printing of geopolymers incorporating high amounts of waste materials. Moreover, to the authors' knowledge, there have not been investigations on the machinability of geopolymers to obtain novel 3D components with predetermined (designed) shape and surface finish.
2. Materials and Method
A new geopolymer was considered for this investigation based on a soda-lime waste glass-added fly ash-based composition. The waste glass was used as a substitute for sodium silicate, which is commonly used in geopolymer synthesis. This new geopolymer formulation is interesting since it represents a significant economic advantage as well as an environmentally friendly solution to the alternative landfilling of such residues. Besides achieving the required mechanical performance, replacement of traditional ceramic materials with geopolymers in technical applications requires that standardized and long-term manufacturing capability is proven for the material to ensure reproducibility for widespread usage.
Fly ash class F (ASTM C 618) , provided by Steag Power Minerals (Gladbeck, Germany), with a mean particle size of 20 µm, and soda-lime waste glass derived from municipal waste collection (SASIL S.p.a, Brusnengo, Biella, Italy) in the form of fine powder with a particle size < 30 µm, were used as the raw materials. Table 1 summarizes the chemical composition of the raw materials determined by means of X-ray fluorescence.
The alkaline activator was a sodium hydroxide solution, prepared at 8 M, using sodium hydroxide flakes (Merck 99.5 %) dissolved in distilled water.
2.2 Sample Preparation
A geopolymer slurry (which can be called "ink" in this case as it will be used for 3D printing) was prepared by mixing 64 mass-% of fly ash and 36 mass-% of waste glass with sodium hydroxide solution at 8 molarity. The liquid-solid ratio was fixed at 0.45 to obtain a good workability. The raw materials were kept under mechanical stirring for 4 h in order to obtain a homogeneous slurry. After mixing, the ink was transferred into a plastic syringe for direct ink plotting.
For the machinability testing, the mixture was cast into polyethylene-sealed cylindrical molds and vibrated for 5 min to remove entrained air. To complete the polycondensation reaction, the samples were cured in an oven at 60 °C for 48 h. Cylindrical specimens of 16 mm in height and 14 mm in diameter were produced.
2.3 Direct Ink Printing
3D geopolymer porous structures were fabricated by additive manufacturing using a bioplotting machine (type BioScaffolder 2.1; GeSiM, Großerkmannsdorf, Germany). This bioplotter is equipped with a printing head movable in three room space axes, a pressurized air system, and a static platform. The air system is compatible with the cartridges (used as a reservoir for the plotting material). Micronozzles were purchased from Nordson EFD Switzerland. This system is conventionally used to fabricate scaffolds for biomedical applications  but it was conveniently applied here for geopolymer parts. To define the dimension and the shape of the scaffolds (grid-like structures in the present case), the "Scaffold generator" software available from the manufacturer was used. With this software the height of the different layers in the z-direction, the edge lengths of the scaffold as well as the plotting speed and the number of plotted struts can be defined. The grid-like structures were directly plotted into a plastic substrate (culture plate) (VWR chemicals), which was placed on the static platform. The samples were cured at room temperature.
As a proof-of-concept experiment, a grid-like scaffold was plotted using a plotting speed of 20 mm/s, three layers and five struts per layer. The two chosen needles had inner diameters of 0.84 mm and 1.36 mm. The morphology of the scaffolds was investigated by an optical microscope (Zeiss Stereolupe Stemi 508).
Cylindrical samples fabricated as described in Section 2.2 were conventionally clamped on a lathe and subsequently machined at a speed of 800 rpm using a Weiler Matador vs2 machine (Weiler Werkzeugmaschinen GmbH, Emskirchen, Germany). The samples were machined using a turning tool made of hard metal. For drilling, geopolymer cylinders were perforated using a stainless steel tool, 5 mm in diameter, at a speed of 800 rpm. No lubricant was used during the process.
3. Results and Discussion
3.1 Direct Ink Printing
Direct ink printing is a challenging task for geopolymers because the slurry is subjected to ongoing polycondensation reactions which continuously modify the rheological properties over time . For this reason different key parameters such as particle size, alkaline solution viscosity, printing speed, and printing pressure should be optimized to obtain an extrudable slurry. It is important to point out that in this study the slurry was used after mechanical mixing of the raw materials, without adding any rheology modifier.
The geopolymer material was extruded through two different needle sizes and the resulting 3D-structures are shown in Figure 1.
The extruded ink possesses the behavior of a gel which is typical of geopolymers after dissolution of the raw materials. The ink was able to retain its shape after each filament was deposited over the other, which confirms a rapid viscosity recovery. Despite this positive result, after 28 days the printed scaffolds showed a slight deformation. The small degree of collapse could be caused by weakly crosslinked structures comparable to that occurring in a soft gel or it could be due to nonefficient material dispersion. Figure 1 shows filaments with limited sagging and deformation, moreover final structures did not present cracks or surface defects. In Figure 1 b, some pores are visible in the structure, probably caused by water evaporation, air entrapped during the preparation of the slurry, or through filling of the slurry into the cartridge for the printing process.
Moreover, the side lattice remains open showing a square shape, especially when extruding through the 0.84 mm needle, which confirmed, qualitatively, that the rheological properties of the ink were suitable for printing a porous geometry. Franchin et al.  produced different mixtures to individuate the one with the best rheology to obtain 3D printed scaffolds with competent mechanical properties. In contrast to the previously mentioned study, in this work no additives such as PAA or PEG were introduced to test the possibility of printing a waste-based geopolymer ink without the need of adding (expensive) chemical reagents. From the results we can conclude that not only a geopolymer mixture incorporating relatively high amount of waste material is printable but also that no retardants or plasticizers are needed, representing thus a starting point for developing a green 3D processing technology for geopolymer components.
3.2 Machinability Test
As indicated above, geopolymers present a range of favorable properties such as environmentally friendly production, chemical stability, compressive resistance and low shrinkage, however so far, to the authors' knowledge, no technical information about the machinability of geopolymer parts is available in the open literature. In the present proof-of-concept experiments we investigated the applicability of two standard machining processes, namely machining on the lathe and drilling, on cylindrical geopolymer samples. In the first part of the experiments, the cylindrical samples were processed on the lathe to obtain a given shape, as represented in Figure 2.
Samples showed the capacity to be machined on a lathe and standard metal tooling to obtain the desired final shape. The microscopy image in Figure. 2, on the far right, illustrates the final geopolymer sample after processing. The final shape was highly accurate and well defined, moreover no cracks propagated during the work on the lathe. In the central part of the final geopolymer specimen some bubbles are visible and these are probably air bubbles that were entrapped in the samples during synthesis and therefore not related to the machining process. The surface of the sample was smooth and no protuberances or defects were visible after the process. Furthermore, the samples showed uniform dimensional accuracy.
Cylindrical geopolymer samples were also drilled with a metal tool to realize a hole of 5 mm in diameter in the center of the sample (Figure 3). From the image of a perforated sample (Figure 3, far right), the precision of the shape of the hole can be appreciated, where the inner wall was seen to have a good surface finish. Moreover, the drilling operation did not initiate the formation of cracks in the material. The sample appears to be without defects, just a small bubble is visible on the surface, probably already present before processing and not related to the drilling process. In an early study , it was reported that to achieve good machinability in ceramics, low brittleness is required, which involves achieving a compromise between hardness and fracture toughness. Such a hardness versus toughness relationship appears to be suitable in the case of the present geopolymer samples given their excellent machinability. These results, even if qualitative, indicate increased versatility and numerous possibilities for industrial applications of geopolymer materials as components with complex shape.
This study evaluated the possibility of direct ink printing and machining of waste-based geopolymers. The experiments demonstrated that fly ash-based geopolymer incorporating waste glass could be 3D printed to obtain porous grid-like components. This proof-of-concept test confirmed the suitability of the geopolymer ink with limited collapse and structural defects. Future work should be done to link the processing parameters with the rheological properties of the ink to achieve specific 3D design shapes of high accuracy.
Geopolymers were successfully worked on using a lathe and drilled with standard metal tools to obtain different shapes with a good surface quality and dimensional accuracy. This first experiment of its kind showed the good machinability of the geopolymer material. Thanks to a precise design and the ability to fabricate complex shapes, the range of applications of geopolymers could increase, particularly in the field of brittle materials where machining is a challenging and cost-intensive process. Geopolymers offer the possibility to combine good mechanical properties, environmentally friendly production, and the possibility for applying alternative shaping and working techniques to produce complex shaped or intricate parts. Applications in mechanical engineering or other sectors requiring designed components of certain structural integrity can be considered. |
 Davidovits, J.: Geopolymer chemistry and applications. 4. ed., Davidovits, Joseph, Saint-Quentin France, (2015). ISBN: 9782951482098
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