Fascination Technology How bio-concrete is made from blue-green bacteria

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Fraunhofer-Institut für Keramische Technologien und Systeme IKTS

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In our "Fascination Technology" section, we present impressive projects from research and development to designers every week. Today: how biogenic building materials are created from cyanobacteria.

The construction industry has a problem: A lot of CO₂ is emitted during the production of cement, the main component for concrete. According to the German Federal Environmental Agency, this resulted in around 20 million tons of CO₂ in Germany alone in 2018. This corresponds to around ten percent of industrial emissions.

Bacterial cultures form rock-like structures under photosynthesis.

Researchers from the Fraunhofer Institute for Ceramic Technologies and Systems IKTS and the Fraunhofer Institute for Electron Beam and Plasma Technology FEP are presenting an environmentally friendly, biologically induced process for the production of biogenic building materials in the "BioCarboBeton" project. The carbon dioxide is used for the process and bound in the material.

The basis of the process are cyanobacteria, also known as blue-green bacteria. These photosynthetic bacteria cultures form limestone in the interplay of light, humidity, and temperature, forming stromatolites in the process. These rock-like structures existed 3.5 billion years ago, demonstrating the robustness of this biological process. As was the case back then, during the process of mineralization CO₂ is fixed from the atmosphere and bound in the biogenic rock.

The Fraunhofer researchers have succeeded in replicating this natural process in a technical procedure. Under the project leadership of the initiator and idea generator Dr. Matthias Ahlhelm, the Fraunhofer IKTS is developing materials and processes, selecting possible fillers and binders, and shaping and structuring. Under the leadership of Dr. Ulla König, the Fraunhofer FEP establishes the cultivation of cyanobacteria, the complementary microbiological analysis and the scaling of the biomass production to be gained.

The bacterial solution becomes a solid.

In the first step, the light-sensitive cyanobacteria are cultivated in a nutrient solution to generate biomass, with the intensity and color of the light source used influencing their photosynthesis and metabolism. To allow mineralization in the bacterial solution based on the model of stromatolites, suppliers of calcium such as calcium chloride are added. Then the researchers produce a mixture of hydrogels and various fillers, for example different types of sand such as sea or quartz sand. Additional feeding of CO₂ increases the content of dissolved carbon dioxide and supports the process.

The bacteria-mixed material mineralizes in molds.

The homogeneously stirred bacterial mixed material is now given structure by, for example, being poured into molds. These are preferably translucent so that the bacteria's metabolism and photosynthesis can continue. The subsequent mineralization then leads to the final solidification. The bacterial mixed mass can also be shaped by spraying, foaming, extrusion or additive manufacturing, in which it then mineralizes.

The resulting solid is still porous during the process, allowing light to penetrate into the interior and promoting CO2 fixation through limestone mineralization.

Matthias Ahlhelm

Alternatively, porous substrates can also be produced, which are subsequently treated with the cyanobacteria culture: "The emerging solid is still porous during the process, so light penetrates inside and drives forward the CO₂ fixation through limestone mineralization. We stop the process by withdrawing light and moisture or by changing the temperature,” explains Matthias Ahlhelm. All bacteria then die completely. This creates a solid product based on biogenic calcium carbonate and fillers, which can be used as a brick, for example. The bio-materials from cyanobacteria contain no toxic substances.

Sources of carbon dioxide from industrial waste gases.

One goal of the "BioCarboBeton" project is to determine the possible material and strength properties of the solids to be produced and to upscale processes. In doing so, the researchers are already thinking of a circular process management. For instance, the sources of carbon dioxide could be drawn from industrial waste gases. Currently, biogas is being used. As calcium sources, basalts and mine waste could serve, but also milk residues from dairies. And as a filler, besides sand, crushed construction waste or renewable resources are also usable.

From insulation material to mortar.

By carefully selecting the fillers and controlling the process and mineralization parameters, products can be produced for different application scenarios. These potentially range from insulation material to bricks and formwork fill to mortar or facade plaster that hardens after application.

After the team of researchers has established and tested the process at the Fraunhofer IKTS and the Fraunhofer FEP, they are now working on scaling the quantities and determining the desired solid properties. The goal is to enable manufacturers to produce the environmentally friendly bio-building materials quickly and economically in the required quantities.

Matthias Ahlhelm and Ulla König are convinced: "The process shows the enormous potential in the biologization of technology. Overall, our 'BioCarboBeton' project offers not only for the construction industry the chance to take a big step towards a circular economy."