Why building with less cement reduces carbon emissions

One of the local resources of most interest to urban development is recycled concrete, since the raw materials from which it is made – gravel, sand and cement – can be partially recovered and reused when an old building is demolished. The first step in the process involves crushing the recuperated concrete. This rough fraction contains gravel and sand that can be extracted and used to produce new concrete. The dust that remains consists mainly of cement, which had hardened in contact with water during the original manufacturing process. “Cement recycling is a more complex process – but it’s crucial,” insists Habert. This is because the manufacturing process for cement generates large quantities of carbon dioxide (CO₂), partly as a result of heating the two main ingredients – limestone and clay – to a temperature of 1,500 °C, and partly due to decarbonation of limestone. The latter step liberates no less than half a tonne of CO₂ per tonne of cement. When the raw material is recycled in a cement factory, it has to be reheated to a high temperature so it can regain its original form, but at least this process doesn’t release any additional CO₂ – provided climate-neutral energy sources do the heating.

Less cement, lower CO₂ emissions

Cement is the ideal binder for concrete. When mixed with water, it binds together gravel and sand. As part of the “Energy Turnaround” national research programme (NFP 70) Habert leads the joint “low energy concrete” project. Other ETH scientists and research groups from EPFL and Empa are also participating in this joint project. Its aim is to develop a product that contains less cement than traditional concrete but still has the same properties. Concrete manufacturers are already using waste from industries such as coal and steel to replace part of the cement in concrete. These waste products have ideal properties and cannot be reused or recycled by the industry that produced them. Nowadays, they replace 30 percent or more of the cement in commercial concrete products.

The researchers want to double the waste content without reducing the mechanical strength of the final product, which must still withstand a force of 30 megapascals. “This would allow buildings to have the same dimensions as today but with a significantly better CO₂ balance,” says Habert. The team’s current work consists of characterising the new concrete with a lower cement content and optimising it still further.

One of the challenges with which the researchers are repeatedly confronted is how to manage the interaction between the low-cement concrete and other materials, including the steel reinforcement bars (rebars) that are cast into the concrete during construction. If the concrete contains too little pure cement, the steel rusts much too quickly. “Such reinforced concrete structures would be less durable, but that’s obviously not the result we’re looking for,” emphasises Habert.

That’s why the scientists are not just optimising the new concrete but also developing alternatives to steel reinforcement. They are working together to find rust-free alternatives such as carbon-fibre-reinforced polymers or synthetic fibres. “My role in this research is to evaluate the environmental impact of these materials,” says Habert. He analyses the CO₂ balance of the various technologies and identifies possible ways of optimising this balance.

One novel solution can be found on the Hönggerberg campus: the House of Natural Ressources, inaugurated this summer in a project led by Andrea Frangi. It serves both as an office building and as a research laboratory for sustainable construction. The concrete slabs used in its construction are reinforced with timber instead of the usual steel. “Concrete and timber make an ideal combination,” says Habert, “because there is absolutely no risk of corrosion.”

Radical approach

Another approach Habert is pursuing is a much more radical departure: concrete made with clay rather than cement. Clay is a far more eco-friendly building material because it is not heated to a high temperature and hence doesn’t trigger any chemical reactions. But this also has its downside: clay-based concrete has a mechanical strength of only 3 megapascals, which is roughly ten times lower than that of conventional cement-based concrete. “Consequently, this type of concrete cannot be used in the same applications,” says Habert. One possible solution would be to restrict the use of cement-free concrete to non-loadbearing walls. This alone could significantly reduce the amount of cement used in construction.

Along with optimising the material per se, another of Habert’s research group’s priorities is to improve the way it is processed at the construction site. After the novel concrete has been poured into the formwork, cracks often appear as it sets. To solve this problem, the researchers are investigating ways of chemically modifying the surface properties of the clay. Their aim is to produce a cement-free concrete that remains workable for three hours – like traditional products. The experimental cement-free concrete sets in 30 minutes, which leaves far too little time for it to be transported and processed. Special porous formwork that allows water to penetrate might also help.

Nonetheless, Habert is well aware that new building materials will only be accepted by the market if they can be used without upsetting conventional construction industry practices. “Very few people are willing to pay more for eco-friendly construction,” says Habert.

Next generation

With a view to transmitting his knowledge to his students, Habert organised a summer school this June on the theme of “Grounded Materials” in cooperation with the Grenoble National School of Architecture (Ensag) and the ETH Department of Environmental Systems Science Transdisciplinarity Lab (TdLab). The ETH students looked into the question of how to promote the use of eco-friendly, locally sourced materials by the construction industry in and around Zurich. Students from the Departments of Architecture, Materials, Environmental Systems Science, and Civil, Environmental and Geomatic Engineering spent two weeks identifying the barriers to more widespread use of local building materials. On the basis of these findings, they developed strategies for making such materials attractive to stakeholders. These strategies focused less on technical characteristics, such as the mechanical strength of concrete or CO₂ emissions, and more on aspects drawn from the fields of sociology, economic science and communication.

Habert is very happy that he has been able to raise the younger generation’s awareness of these issues. “As a geologist, my reflex is to see resources as something physical, found in a specific place in a fixed quantity,” he explains. “But wherever there’s an interface between the natural environment and human society, things begin to get more exciting.” What use can be made of the resource? Where is it situated and where is it needed? Who knows how to use it? How much does it cost? After this year’s summer school, Habert’s students also know that these are important questions.

This article is published in collaboration with ETH Zurich. Publication does not imply endorsement of views by the World Economic Forum.

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Author: Corinne Hodel is a biologist for the Institute of Zoology at the University of Zurich.

Image: Construction cranes and bulldozers operate near Qatar Foundation. REUTERS/Mohammed Dabbous.