Associate Professor Holger Koss and PhD student Julian Christ, DTU Civil Engineering, were not exactly looking for a new concrete recipe when Julian, as a student at DTU, began optimizing the construction of a shelter for Greenland five years ago. Nor were they really looking to work with 3D concrete printing and invent a completely new solution in print technology that made it possible to print in more directions than merely vertical.
Nevertheless, Holger Koss and Julian Christ are on the cusp of multidirectional 3D concrete printing. They can print arches and larger overhangs, and they are in the process of patenting a new recipe for concrete. A cement-free concrete.
“We’ve developed and tested a type of concrete that completely does away with cement. We’ve replaced cement with biopolymers, i.e. organic substances that are biodegradable,” says Holger Koss.
As a lay person, the most important thing to know is that cement, which consists of clay and lime, can make up to 20 per cent of concrete, and that it is the cement that is the main CO2 culprit in concrete. According to the International Energy Agency (IEA), global cement production is estimated to be responsible for seven per cent of total man-made CO2 emissions. So a cement-free concrete is a very interesting option at a time when we need to find new ways to reduce CO2 emissions.
Optimized shelter design
But how did the two researchers get here? Holger Koss explains:
“I conduct research into the wind’s impact on building structures and therefore became involved in Julian’s MSc project to develop the optimum structure for a shelter for Greenland—one that used as few materials as possible and was strong enough to withstand the harsh wind conditions there. At the same time, we wanted to utilize or recycle materials that exist locally.”
The shelter was a dome-shaped structure similar to the tents festival-goers use. In a wind tunnel, the researchers were able to measure the way in which strong winds affect the shelter, and based on this knowledge Julian Christ began optimizing the structure. The method he used was stochastic topology optimization, which in simple terms means working out where to place the material in a construction based on the desire to use as little material as possible without compromising the strength of the construction. In the stochastic topology optimization, Julian Christ took into account the varying loads that the wind imposes on the shelter structure.
Following the topology optimization process, the researchers were left with a dome-shaped shelter where the load-bearing structures branched around the hemisphere without any straight lines or right angles.
“We ended up with a very organic and biomorphic structure from the optimization. It looks like something that Mother Nature herself would have made,” says Julian Christ.
Can it be 3D printed?
As concrete consists mostly of stone and sand—both of which are found in abundant quantities in Greenland—it would seem to make sense to build the shelter in concrete. But being the structural engineers Holger and Julian are, they immediately realized that this biomorphic structure would be extremely challenging to mould in concrete. Simply producing the moulds for the casting would be far too expensive. Casting the entire dome without moulds and removing all the unnecessary concrete afterwards would result in a disproportionate waste of material.
“So we asked ourselves: can we 3D print it?” explains Holger Koss.
Obvious solution – big challenge. To do so requires a concrete and a printing technology that offers greater spacial freedom than conventional concrete printing technology. The concrete material is too liquid and hardens too slowly to print in any direction but vertical. Julian and Holger need to be able to print in multiple directions and with arches and overhangs as well.
In 2019, Julian Christ was hired as a PhD researcher at DTU Civil Engineering with support from the Villum Experiment programme, which supports radical research ideas. This meant that he could continue the work of finding a solution. His PhD project must find and test new ingredients that can be extracted locally in Greenland to replace the cement—partly to give the concrete the properties that make it possible to print the shelter—and partly to find a more sustainable solution without the use of cement.
“My starting point is still my case from the master project, namely building an optimized concrete shelter in Greenland. The most sustainable solution is to be able to print with a concrete-free cement to avoid the long transport of cement bags that also adds to the carbon footprint. Therefore, the alternative material for cement must be a resource that is present and easily accessible in Greenland,” says Julian Christ, who has limited his research to a list of 14 different biopolymers—organic substances derived from either proteins or carbohydrates.
Organic substances replace cement
The most promising ingredient identified and tested by Julian Christ is extracted from side flows in the food industry which are then recycled to extract biopolymers. Side flows are waste products in a production that are typically considered as waste.
“We’ve done pressure tests on concrete where cement has been replaced with biopolymers, and we’ve had pretty impressive results where the biopolymer concrete is stronger than conventional concrete,” says Julian Christ, who expanded the testing of the new concrete type by 3D printing with it. The result was surprising.
“Our biopolymer concrete allows us to print in multiple directions,” says Julian Christ, who had to invent and implement technical solutions for the 3D printer itself to make this possible. One of the solutions involves heating the material before printing.
“We need to heat up the concrete before printing to make it flow well. Conventional printers don’t allow us to heat the material in the way we need, so we had to come up with a solution ourselves. When the material is hot, it is a liquid, and as soon as it comes out, it cools off and hardens quite rapidly. This is the main reason why we can now print in multiple directions,” explains the PhD researcher, who cannot elaborate on their technological heating solution due to possible future patenting requirements.
Global relevance
The researchers are working to patent the recipe for the concrete where cement is replaced with biopolymers. Although the two researchers have made great strides in a short time, they still see a long road ahead of them before horizontal 3D concrete prints become commonplace.
“We’ve only just begun. There are still many things we need to clarify—for example, the extraction of biopolymers—whether it’s best to harvest them from the plant or animal world, or whether they should be produced synthetically. Also, which biopolymers have the best flotation properties and are suitable for printing and which ones add the most strength in the printed constructions,” says Julian Christ.
Holger Koss brings up another important point:
“Biopolymers are organic materials and therefore easier to break down, and we need to further explore how to protect this material from degradation. It is comparable to wood, which is also an organic material, but with wood we’ve learned how to prevent premature decomposition.”
Although the cement-free 3D concrete print has been developed with the aim of building a shelter that can withstand Greenlandic wind conditions, the two researchers find it relevant to continue the development of their ideas, as the whole world can benefit from them. The researchers also see relevance linked to a future where there is talk of 3D printing habitats on the Moon or other planets. Whether on Earth or in space, scientists believe humans can become better at reusing and utilizing resources that can be found locally. Holger Koss elaborates:
“A cement-free concrete will always be interesting, as it is a more sustainable solution. It’s also relevant for many countries to use resources locally—not just for Greenland. Greenland was just our starting point. The use and recycling of local materials and building optimal constructions with as few materials as possible is relevant for the whole world.”