Stone on stone, fungus, or wood?

Stadtbewohner:innen erleben manche Folgen des Klimawandels in besonderer Härte. Weil das Wasser durch die versiegelten Böden nur schlecht versickert, kann es nach starken Regenfällen zu verheerenden Überschwemmungen mit weitreichenden Folgen für Mensch und Infrastruktur kommen. Viele Baustoffe absorbieren Sonnenstrahlung. Während sommerlicher Hitzewellen heizen sie sich auf wie Backöfen. In den dicht bebauten Städten kommt es an heißen Tagen zum Hitzeinseleffekt: Hier kühlt es selbst nachts nicht mehr genügend ab. Gerade ältere Menschen leiden gesundheitlich unter solch einem Hitzestau. Gebaut wird unterdessen weiter - weltweit, vom Einfamilienhaus mit Homeoffice auf dem Land über das neue Parkhaus bis hin zum urbanen Wolkenkratzer. Hunderte Millionen Menschen werden laut Vorhersagen der Vereinten Nationen in den kommenden Jahrzehnten in Städte umsiedeln.

Klassiker Betonbau: CO₂-Ausstoß als Wertstoffquelle?

Die Forschung an Materialien und Methoden für das Bauen von morgen verfolgt daher zwei Ziele: Die Materialien sollen zum einen möglichst klima- und ressourcenschonend hergestellt werden. Zum anderen sollen die entstehenden Bauten und Infrastrukturen möglichst resilient gegen Auswirkungen des Klimawandels sein.

Baustoff Nummer Eins ist heute Beton. Er ist weltweit das am häufigsten verwendete Material nicht-natürlichen Ursprungs. Die Bestandteile des Baustoffs - Sand, Kies und Wasser – werden mit Zement gebunden. Und dessen Herstellung ist für etwa sechs bis acht Prozent des globalen CO₂-Ausstoßes verantwortlich. Am Karlsruher Institut für Technologie (KIT) erforschen Wissenschaftler:innen um Professor Frank Dehn gemeinsam mit außeruniversitären Forschungseinrichtungen die Möglichkeit, kleinste Partikel aus Altbeton zu sortieren, zu aktivieren und wieder als Bindemittel für frisches Baumaterial einzusetzen. „Das wäre ein beeindruckendes Beispiel von Upcycling“, sagt der Leiter des Instituts für Massivbau und Baustofftechnologie. Auf diesem Weg könnten bei der Zementproduktion große Mengen CO₂ eingespart werden. Bisher findet höchstens ein Downcycling statt: Alter Beton wird teilweise im Straßen- oder Tiefbau weiterverwendet.

“Doing without cement and concrete won’t be an option in the future,” says Arne Stecher, who is responsible for the German decarbonization strategy at the cement manufacturer Holcim. As one of the largest manufacturers of building materials, Holcim has annual CO₂ emissions in excess of 100 million tons worldwide. “Our task is to remove CO₂ from the manufacturing process and to further use it as a raw material,” Stecher explained at the 2021 dialogue conference of the Helmholtz Climate Initiative. At the Helmholtz Centre for Materials and Coastal Research (Hereon), researchers led by Dr. Torsten Brinkmann have developed a membrane separation process to separate CO₂ from flue gas. Holcim will be testing such a carbon capture technology in a prototype from the beginning of 2022. This could make it possible in the future to further process the collected and processed CO₂, for example as a raw material for synthetic fuels.

Generally, concrete is used in conjunction with steel. The production of steel generates just as much CO₂ worldwide as cement production. Obtained exclusively via renewable energy, green hydrogen is intended to make the process of melting steel more climate-friendly. “If steel plants have supplies of hydrogen, it is, technically speaking, not a problem to produce high-quality steel,” says Professor Olena Volkova from the Freiberg University of Mining and Technology. Up until now, however, the production capacities for green hydrogen and the infrastructure for transporting it have been lacking. That said, the steel manufacturer Thyssen Krupp has set itself the goal of achieving carbon-neutral production by 2050, with the help of hydrogen and carbon capture.

Wood: on top and all around

Right now, more and more attention is turning to locally sourced construction materials: Clay, which regulates humidity and thus ensures a comfortable climate in interior spaces; or sheep's wool, which is as an agricultural by-product and can be used for thermal insulation. Wood is another highly praised building material. Given that trees store CO₂ and do not release it for a long time during the installation process, the building material is sometimes referred to as a CO₂ storage unit. This is somewhat misleading because the crucial CO₂ reservoirs are living forests. And they suffer from events such as fires or infestations of insects, which are in turn intensified by climate change. Excessive logging on top of that would be far from climate-friendly. Not to mention: the wood reserves available are limited.

Furthermore, as a material, it is sensitive to moisture, which must be taken into account when opting to construct buildings out of wood. And also if there are construction methods with load-bearing elements made of wood: “We still have little experience with tall buildings,” says Dr. Carmen Sandhaas, a KIT specialist for timber construction and building structures. “Planning, designing, and constructing such buildings is currently even more complex than for concrete buildings. In this respect, we need more practice.” Given the rapid rate of growth currently being witnessed in urban environments, tall structures are simply inevitable in order to avoid covering large surfaces with concrete and paving long roads. “Of course we’re not likely to build a Burj Dubai out of wood,” says Sandhaas. Nonetheless, wood does have its own advantages as a building material: “Thanks to its light weight, it’s possible, for example, to create additional living space,” Sandhaas explains. This can be done by simply adding on to the top of old buildings, where the static design would not allow for the weight of other floors made of stone or concrete.

Combining and separating materials

At KIT, a particular concern of the materials researchers is combining different materials so as to optimally bring to bear their respective advantages. Sometimes, most poignantly so: For example, the KIT specialists are experimenting with wood as sheathing for steel beams, to make them more stable against pressure from above. “There is still much research to be done when it comes to connecting wood and concrete elements,” says Carmen Sandhaas.

The required materials should be installed in such a way that they are as completely recyclable as possible – in a manner that is modular and unmixed. In an optimal scenario, an entire (concrete) wall can be transplanted from an old building to a new one. The basic idea is to use materials in as unmixed a fashion as possible – i.e. to design, for example, load-bearing structures separately from thermal insulation. This calls for optimized and innovative techniques for joining various materials. Professor Dirk Hebel, Dean of the KIT Faculty for Architecture, refers to this as “a technological challenge.” As for the parties performing the construction, Hebel says: “We always have to ask ourselves: Can what I’m designing be dismantled and reused for new tasks? If it will end up as waste instead, then we haven’t designed it the right way!” Or will it simply end up in the compost heap?

In addition to wood, Hebel and his collaborators are experimenting with other natural building materials that might seem exotic at first glance: They are growing construction elements from fine, spongy fungal cells – i.e. the mycelium. By combining these with grasses such as bamboo, bending forces can be absorbed in a manner similar to the combination of steel and concrete. “In a manner of speaking, our vision is to grow houses in the future,” says Dirk Hebel. The fungi's fine cell network quickly assumes any conceivable shape that is imposed on it. Together with other experts, Hebel and his team are developing novel structures that can absorb compressive and tensile forces and thus constitute an alternative to established systems.

Rechtliche Hürden abbauen

Eine weitere Möglichkeit, Fassaden und Dächer nutzbringend und klimafreundlich zu gestalten, bieten Solarzellen. Eine Studie im Rahmen des EU-geförderten Projektes BIPVboost ergab: Bauintegrierte Photovoltaikanlagen (BIPV) könnten bis zu einem Viertel des privaten Strombedarfs in Deutschland decken. Die Gestaltungsmöglichkeiten für Fassaden mit modernen Solarmodulen sind vielfältig: Sie können verschiedene Farben haben und ganz unterschiedliche Formen annehmen. Eine interessante Option ist künftig auch die Beschattung von Fenstern und Glasfassaden mit teiltransparenten - zum Beispiel organischen - Solarzellen. Auch mit Pflanzen auf dem Dach vertragen sich die Module. Ihre Effizienz kann durch deren kühlenden Effekt sogar gesteigert werden, wie Bender und seine Kolleg:innen in ihrem Synthesebericht zur Baubegrünung  beschreiben.