Carbstone: the first concrete block to absorb CO2
Fair is fair: manufacturing concrete blocks is not exactly an environmentally friendly activity. This is mainly because the production of cement (one of its main components) releases a lot of CO2. “Can’t anything be done about that?”, they thought at the Maasmechelen concrete manufacturer Gubbels. And suddenly the company had a world first.
That world first is the Carbstone, the world’s first concrete block that does not use cement as a binding agent, but … CO2 itself. This is a double win: not only is CO2-intensive cement not used, the CO2 is actively used as an ingredient. “The development of the Carbstone began back in 2004,” says director Bjorn Gubbels. “The basic idea comes
from the Genk-based company Orbix. They discovered that ground and purified steel slag, a residue from steel production, hardens when in contact with CO2 from the atmosphere. From that grew the idea of using those steel slag as a substitute for cement in concrete production.”
That idea in itself is quite simple. “In our ‘curing’ chamber, the blocks of sand, gravel and steel slag scraps are exposed to CO2 and so they absorb the greenhouse gas,” Gubbels explains. “The CO2 allows the block to harden; after only 24 hours it has reached its final strength and can be used. Each block contains about two kilograms of CO2.
We collect that CO2 from the Dutch chemical cluster Chemelot, just across the Belgian border. For them, that is also an unwanted byproduct that is now put to good use. Ditto for the steel industry: steel slag is also considered waste, but so now we can reuse it.”
"The CO2 allows the block to harden; after only 24 hours it has reached its final strength and can be used."
Long-term work
The explanation may be simple, but the actual implementation involved a great deal of trial and error. “All in all, we worked on it for almost 20 years,” Gubbels says. “We converted a traditional drying chamber into a curing chamber in the greatest secrecy, and that was a lot more difficult than we thought, really pioneering work. I had estimated that it would cost us about 300,000 euros, ten times what an ordinary drying chamber costs. But in the end we spent 1.2 million. Anyway, that’s learning money, because now we know perfectly how to build such a chamber from scratch, at a reasonable cost.”
The Carbstone looks exactly the same as a “regular” concrete block, is just as strong and can support just as much weight. It is also fully recyclable and reusable. In terms of frost resistance, it even scores slightly better than its traditional brother. “Also, the price is exactly the same,” Gubbels said. “So for architects and building owners who want to build circularly, and there are more and more of them, the choice is quickly made. For the site workers there is also absolutely no learning curve involved. The fact that it is ready in 24 hours is also a huge advantage. An ordinary concrete block takes about 28 days to dry out completely before it can go to the building site. That makes us much more flexible in production and much more responsive to market demand.”
For now, however, that problem is far from posing itself. The first curing chamber in Maasmechelen runs 24 hours a day, but demand for the green concrete block is many times greater than what the plant can currently produce. Gubbels: “Currently, about 15% of the production consists of CO2-negative Carbstones, but the absolute intention is to increase that to 100% in the long run. The plans for that are ready.”
"The Carbstone looks exactly the same as a 'regular' concrete block, is just as strong and can support just as much weight. It is also fully recyclable and reusable. In terms of frost resistance, it even scores slightly better than its traditional brother. The price is also exactly the same."
Is cement becoming a luxury product?
Gubbels therefore bluntly calls the Carbstone “the path to the future” for “his” concrete plant. “With increased energy costs and all the costs that CO2 brings, cement will become unaffordable in the future. It will become a luxury product. Almost all other concrete producers have now only begun to think how to deal with that. In Canada and Japan there are a few experiments going on with the same technology as ours. But those people are today where we were ten years ago.”
At the same time, Gubbels doesn’t want to shield his product too much either. “On a global scale, all in all, we are just a small shrimp,” he says. “The demand curve for this product is going to rise tremendously; I can’t meet that alone. If other manufacturers want to cooperate, we are certainly open to that. That will benefit the growth of the entire market.”
In fact, those first collaborations are already fairly specific. “For example, we already have a ongoing project with the Belgian brick manufacturer Vandersanden. They make a CO2-negative façade, we make a CO2-negative interior façade. Add environmentally friendly insulation and you have a perfectly circular prefabricated product. That’s also the beauty of the Carbstone. I see a lot of small start-ups with good ideas around environmental friendliness and circularity, but often they stay small-scale because it’s so hard to scale up. The Carbstone can be manufactured on an industrial scale immediately, for maximum impact.” Carbstone: the first concrete block to absorb CO2
Source: Fokus, March 2024 (content: Smart Media Agency)