Some intriguing research might enable far stronger construction 3D printing materials.
Construction 3D printing is quite different from other forms of 3D printing in that the resulting objects are in many cases life-critical. We do not want a 3D printed structure to collapse for any reason, and a large portion of the risk is directly connected with the material.
Any engineering project attempts to use the minimum amount of material to achieve the technical requirements, so a stronger material means less material can be used.
But how is concrete, the typical construction 3D printer material, made stronger? Traditionally this is done by including “hard” additives to the mix, such as rock. However, there’s a problem with this approach when 3D printing concrete: the mixture must be sufficiently liquid to flow through plumbing to the toolhead, and also must not contain any objects large enough to jam the nozzle.
This constrains the types of additives possible in construction 3D printing concrete.
New research might have found one solution to this dilemma, and it uses graphene oxide.
Graphene is a relatively well-known material. It’s a sheet of pure carbon atoms arranged in a hexagonal lattice. It’s an extremely strong material that would be quite desirable in this situation.
However, graphene is also extremely difficult and expensive to produce, making it ill-suited for use in large-scale concrete projects of any kind.
Enter graphene oxide, which is a form of graphene that includes oxygen. It is still quite strong, but not as strong as pure graphene. It is, however, far easier to produce and thus eligible for use in large-scale concrete projects.
The researchers intended on testing 3D prints made from two different concrete mixes with differing ratios of graphene oxide to see what effect it would have on the strength of the resulting prints.
I’m not personally familiar with graphene oxide, so I was surprised at the number of steps required to produce it. The researchers explained their process:
“The first step is the intercalation of graphene (63–90 μm) using H2SO4. The concentrated (98%) acid is cooled to 10 °C, and the raw graphite powder is added and continuously stirred while maintaining the temperature below 20 °C. The second step is the oxidation of intercalated graphene using KMnO4 as the oxidising agent. KMnO4 is added gradually into the graphite-acid mixture with the reaction temperature maintained at 50 °C for 3 h. A mixture of ice and chilled distilled water is then added, and the suspension is stirred for a further hour at 60 °C. The whole process is then followed by a dropwise addition of H2O2 to quench the reaction and terminate any further oxidation. The final suspension is centrifuged, and the supernatant is decanted away. Next, the residual is washed with HCl and deionised water multiple times to remove residual salts and un-exfoliated GO. Finally, GO is obtained in the form of a slurry with a moisture content of 84%. The slurry is then mixed with water to create a 10 g/L aqueous solution of GO. The mixture is shear mixed for 15 mins at 3000 rpm, followed by the ultrasonication for 10 cycles (1-min ultrasonication at 30 Hz followed by 1-min rest).”
Apparently this process is less challenging than making graphene!
The resulting “sheets” of graphene oxide are quite small, basically nanoparticles, but can easily be mixed into a concrete batch.
The researchers chose two ratios: 0.30% and 0.15% of graphene oxide to concrete.
Surprisingly, they found that the smaller 0.15% ratio worked quite a bit better to improve compressive strength, perhaps the most important factor for concrete structures. This ratio provided at least ten percent greater compressive strength, while the higher ratio provided only five percent.
This seems to be the opposite of what you’d expect: adding more hard bits should make the object stronger, but that was not the case. They believe the difference was due to printability with the higher mix: the flow rates were affected. In fact, the higher ratio actually resulted in a slightly shorter object once complete: underextrusion!
They also discovered via microscope larger voids in the higher ratio sample print, which would contribute to the relative weakness of the object.
This research has not been commercialized, but I am certain there is interest from concrete 3D printer manufacturers. Being able to print structures with ten percent less material is a significant gain. We may see graphene oxide appear in the future on commercial construction 3D printing systems.
Via Science Direct
