Recycled concrete for construction. A Q&A with architect Thomas Juul Andersen

Posted: April 04, 2025

In the old industrial town of Fredericia, in the southeastern part of the Jutland peninsula in Denmark, what was once a landscape dominated by factories and warehouses is now being transformed into a vibrant, sustainable district. At the center of this redevelopment is a unique community house designed to push the boundaries of sustainable construction. The building, part of the Kanalbyen project, a collaborative effort led by the Danish Technological Institute, aims to drastically reduce its environmental impact thanks to advancements in concrete technology.

Thomas Juul Andersen, an architect and researcher at the Danish Technological Institute with nearly two decades of experience in construction innovation, is deeply involved in this initiative. Specializing in digital fabrication and concrete research, he has been working to develop new methods for reducing the carbon footprint of concrete.

In a conversation with Our Industrial Life, Andersen shares insights into the project’s goals, the challenges of making concrete more sustainable, and the innovative techniques used to achieve a lower environmental footprint.

Q: Let’s set the scene. How much concrete is used globally?

Andersen: Concrete is the most widely used man-made material on the planet. Every year, we produce around 30 billion tonnes of it—more than any other building material. The problem is that traditional concrete relies on Portland cement, which is rather carbon-intensive.

At the same time, it's an essential material. It's strong, durable, cheap, and adaptable. That's why the challenge isn't just to use less concrete but to find ways to make it more sustainable.

Q: Is there a strict definition for terms like “climate-neutral concrete” or “sustainable concrete”?

A: Not really—at least not yet. These terms are often used loosely, and their meaning depends on context.

Generally, “recycled concrete” refers to crushed old concrete being reused in new concrete. “Reused concrete” is even better. It means repurposing entire concrete elements without reprocessing.

“Sustainable concrete” might refer to anything from using alternative binders instead of Portland cement to reducing material waste through better design. “Climate-neutral concrete” is trickier because, right now, no concrete is truly climate-neutral. Even the most advanced formulations and digital fabrications still have some emissions, though they might offset them in other ways, such as carbon capture or using recycled materials.

That said, regulatory bodies and industry groups are working on clearer definitions. The goal is to set benchmarks that ensure companies aren’t just making greenwashing claims but actually reducing their environmental impact.

Q: What do you mean by digital fabrication?

Digital fabrication is essentially the use of computer-controlled processes to design and construct buildings with extreme precision. It includes technologies like 3D printing, robotic assembly, and CNC (computer numerical control) milling. The key advantage is that it minimizes waste. Rather than cutting materials down to size after production, we can design exactly what we need from the start.

In construction, digital fabrication opens the door to new architectural possibilities. For example, we can create intricate, optimized structures that use less material while maintaining strength. In the Kanalbyen project, we’re applying this through 3D-printed concrete components, allowing us to experiment with forms and efficiencies that traditional methods couldn’t achieve.

Tell us more about the Kanalbyen project. What is the vision behind it? And what makes this community housing special?

A: Kanalbyen is an urban redevelopment project on a former industrial site in the town of Fredericia, Denmark. The area, now being transformed into a new urban district, includes housing and commercial spaces. My involvement is in a specific aspect of the project, which is the building of the community house. This small structure will serve both local residents and visitors and will be situated in a green corridor at the heart of the district. The aim is to construct the house primarily from concrete while maintaining a very low climate impact. The goal is to limit the carbon footprint to 5 kg of CO₂/m²/year over a 50-year period, which is significantly lower than the typical 10-15 kg CO₂/m²/year for standard concrete structures. Many doubted it was possible, but our calculations suggest we can achieve it. While the house isn’t built yet, some prefabricated elements, like the 3D-printed columns, are ready for installation. The idea originated around four years ago, with a long innovation phase to explore design solutions. The construction phase began last year, and we expect the house to be completed by mid-2025.

Q: What strategies are you using to reduce the CO₂ footprint of concrete in the Kanalbyen project?

A: The biggest strategy is rethinking how we make and use concrete—it’s not just about swapping out materials but redesigning the entire process.

One key approach is using alternative binders to replace a portion of Portland cement. We’re experimenting with supplementary cementitious materials (SCMs) like fly ash, slag, and calcined clay, which significantly reduce emissions. In some cases, we can cut the cement content by 30–50% without compromising performance.

Then there’s 3D printing, which is a game-changer. By precisely layering concrete only where it’s needed, we use significantly less material compared to traditional formwork. Plus, we can create hollow or lightweight structures that maintain strength while reducing weight. This technique not only cuts emissions but also lowers transportation costs—another hidden source of CO₂ in construction.

Finally, we focus on design optimization. Traditional buildings often use more concrete than necessary because of outdated structural norms. But with digital simulation and AI-driven modeling, we can create lighter, more efficient designs without sacrificing durability.

Q: How does low-carbon concrete compare performance to traditional concrete? And are there limitations to using sustainable concrete for larger structures?

A: Generally speaking, low-carbon concrete performs well, but there are tradeoffs. The strength and durability can be comparable to traditional concrete, but factors like setting time and early strength development can vary depending on the materials used.

For larger structures, the challenge is often scalability and standardization. Many low-carbon concrete mixes still need more testing to meet strict building codes, especially for high-rise construction or infrastructure projects like bridges and tunnels. That said, progress is happening fast. New formulations are improving all the time, and industry acceptance is growing.

Q: What are the most promising innovations in sustainable concrete? And what is, ultimately, the best way to reduce the carbon footprint of buildings?

There are several exciting developments. Some companies are developing concrete that absorbs CO₂ during curing, potentially making it carbon-negative. Other companies are developing cement-free concrete alternatives, such as geopolymer concrete, which uses industrial byproducts as a binder, or biological concrete, where bacteria grow calcium-based binders. AI and machine learning help us design concrete mixes with the lowest possible CO₂ footprint while maintaining the required performance.

Each of these innovations brings us closer to more sustainable options. But the most effective approach to reducing carbon footprint is following a hierarchical strategy. First, preserve existing buildings. That means prioritizing renovation and transformation instead of demolition. When buildings must be dismantled, it’s best to reuse structural elements without breaking them down. If reuse isn’t possible, reprocessing materials through recycling can still minimize waste.

For new construction, optimizing concrete formulations by reducing cement content, incorporating SCMs, and refining mix designs is essential. Also, we should keep in mind that concrete isn’t always the best choice, and in some cases, materials like wood or steel may offer lower-carbon alternatives.

However, sustainability must also consider broader environmental impacts; while wood is often seen as an eco-friendly option, large-scale deforestation for timber could harm biodiversity. The key is to use local, low-impact materials suited to each project rather than relying on a single solution for all construction needs.

Additional information

The Kanalbyen project is a collaborative effort involving multiple partners and developers, including AP Ejendomme (the developer and client for the community house), Henning Larsen Architects (responsible for the architectural design), Rambøll (structural design), MT Højgaard (contractors), and the Danish Technological Institute (leading the innovation process for concrete development).