Black Concrete, Green Vision: How Biochar could cut the CO₂ Footprint

At ETH Zurich, researchers are working on concrete that helps reduce carbon emissions. In the Department of Civil, Environmental and Geomatic Engineering, postdoctoral researcher Mareike Thiedeitz is developing and testing biochar concrete. This material stores carbon instead of releasing it during production. The project is funded by InnoSuisse and brings together partners from research and industry. Their aim is to reduce the environmental impact of construction while enabling automated processing of new materials.

Mareike, we saw you in the construction hall pouring a black mixture into a white, 3D-printed concrete column. What exactly was happening?

That was a materials test involving two very different types of concrete. The white column is made from a high-performance 3D-printed concrete designed for precision, stability and colour. The black material I poured in is known as biochar concrete. The aim was to develop a mix that’s easy to process—flows well, for example—while also improving the CO₂ balance of the building material, ideally even making it carbon-negative.

How did the collaboration with other researchers come about, and who is involved?

This is part of an InnoSuisse project—a collaboration between research and industry. I worked closely with several researchers from the NCCR Digital Fabrication network. It’s not just about developing materials but also about design, structural engineering and automation. From architecture, there’s Dr. Ana Anton and Professor Benjamin Dillenburger—Dr. Anton led the fabrication of the 3D-printed Tor Alva tower. Then we have structural engineers like Dr. Lucia Licciardello, Dr. Alejandro Giraldo Soto, and Professor Walter Kaufmann, who focus on reinforcement and structural requirements. In our materials group, I work with Dr. Tim Wangler and Professor Robert Flatt. It’s a truly interdisciplinary team covering everything from design and mechanical performance to automation.

Researchers from the Chair of Structural Engineering and form the Chair of Physical Chemistry of Building Materials gathering around the column. Photo by NCCR Digital Fabrication, Lea Keller

Why is the CO₂ footprint such a big issue in construction?

Concrete is one of the biggest CO₂ emitters worldwide. Its binding agent—cement—releases a huge amount of CO₂ during production, around 8% of global emissions. That might sound small, but considering that concrete accounts for over 50% of the materials we use globally, we urgently need to reduce emissions at scale. There are many strategies being explored—some of which are combined—to create more climate-friendly concrete. However, trade-offs exist: 3D printing concrete can reduce material use, but the printed material often has a higher carbon footprint than standard concrete. That’s why we’re working on ways to make the material itself more climate-friendly—such as binding CO₂ through carbonation or by using CO₂-negative additives.

How can concrete store CO₂—and what role does biochar play?

Carbonation is the process in which the hardened cementitious binder reacts with CO₂ from the air, which leads, among other things, to the precipitation of calcite, i.e. limestone - one of the raw materials used in cement production. Normally, carbonation can have a negative impact on the durability of concrete, as it increases the risk of corrosion of the reinforcing steel. In the Tor Alva design, the risk of corrosion to the reinforcement does not play a role, as long as we use stainless reinforcing steel or the reinforcement is embedded in standard concrete. My colleagues therefore include this process positively in the CO₂ balance because CO₂ is permanently bound in the process. In another approach, we fill 3D-printed concrete with a concrete whose calculated carbon footprint can be massively reduced by adding biochar. These are the experiments we have now carried out in the construction hall.

By adding biochar to the 3D-printed concrete, the carbon footprint can be massively reduced. Photo by NCCR Digital Fabrication, Lea Keller

What are the challenges when working with this type of concrete?

Biochar is a carbon-rich material produced by pyrolysis—heating organic waste like wood scraps or agricultural residues in the absence of oxygen. When we incorporate biochar into concrete, the carbon it contains remains locked in for the long term. This prevents CO₂ from entering the atmosphere. Biochar is very fine and porous, like a sponge. It rapidly absorbs water. This makes the concrete stiffer, as the water is no longer available to improve flow. To keep the mix workable, we add more water and, more importantly, chemical admixtures. Think of these like spices in cooking: small amounts have a big impact. For instance, 1% superplasticiser relative to the cement weight can significantly improve flowability. But such admixtures can also increase viscosity when water content is low, resulting in a thick, lava-like consistency. This is problematic for processes like pumping, digital casting or 3D printing, as existing pump systems can’t handle such viscous material. That’s why I’m working on optimising the mix—by pre-saturating the biochar, fine-tuning the admixtures, and improving the particle size distribution.

What’s your approach to developing the concrete mix?

A bit like cooking! I blend aggregates such as sand and gravel with cement and biochar. The biochar I use is ground to a similar particle size as cement and greatly influences the mix. In small amounts, its fine structure can even improve the microstructure of concrete. But since our goal is a better CO₂ balance, we work with higher proportions of biochar. The challenge is to create a mixture composition with a high biochar content that still flows well and doesn’t segregate—so all components stay evenly distributed. It’s a constant balancing act between environmental sustainability, rheology and performance.

The material components are used on a small scale for testing. Photo by Mareike Thiedeitz

What are the next steps in your research?

There are many unanswered questions regarding the workability and durability of concrete with biochar. How does the pre-saturation of biochar affect the long-term properties of the concrete? Does it release the water again? Does this change the strength or durability? In addition, there is currently a lot of discussion about the extent to which biochar can contribute negatively to the CO2 balance at all, because after the calculated service life of concrete - in Switzerland this is 60 years - it is unclear how concrete can be recycled with biochar.

However, I am now switching to my own research project. I have been granted an ETH Postdoctoral Fellowship, as part of which I will be working on the multi-scale utilization of organic waste materials as additives in concrete. This is a very exciting step for me, as I will continue to investigate biochar and agricultural waste ashes regarding their potential as sustainable and affordable building materials - especially with a view to emerging countries. As part of this, I am also changing institutes: I am moving from materials science-based research at the Institute of Building Materials to the Chair of Sustainable Construction at the Institute of Construction and Infrastructure Management under Professor Guillaume Habert.

I will now investigate the actual sustainability in a wider context: What about resource availability? Where can material value chains be created to achieve not only ecological, but also social and economic benefits for communities? The aim is not only to develop material solutions for capital-intensive markets, but in particular to develop low-threshold (“grassroots”) solutions that can be implemented in remote regions.

The Innosuisse project has laid a valuable foundation for this and revealed the most important research questions. I am very grateful for the opportunity to be able to explore the topic in a broader context and from new perspectives.