CO₂-reduced concrete

* Updated version of the 2023 article.

Definition

Virtually all new construction projects make use of concrete to some degree. Whether roads, dams or bridges – concrete is the world’s most widely processed material by weight. The manufacturing process generates substantial greenhouse gas emissions.  

Around 60 percent of emissions are generated as a by-product of the chemical reaction when limestone and other ingredients are fired to produce cement, the main component of concrete. The remaining 40 percent of greenhouse gases come from the energy generated to operate blast furnaces. Transforming limestone into cement requires temperatures exceeding 1,400 degrees Celsius for running mills and transporting finished cement. Overall, the cement and concrete industry is responsible for 8 percent of global CO₂ emissions – around three times as much as international aviation.  

Given these figures, it is clear that the concrete industry must reduce its greenhouse gas emissions.  Switzerland’s cement and concrete industry has therefore committed to becoming carbon neutral by 2050. Various stakeholders are taking different approaches to achieve this.  

Current applications and opportunities

One obvious way to reduce CO₂ emissions in concrete production involves changing its composition by adding additional materials so that concrete contains less cement. So less CO₂ would be generated during production. Countries with large steel or coal industries have pursued this path for years.  Slag or fly ash are mixed into the concrete, although they have become scarce in many places recently.  

Zindel United is taking an alternative approach, adding Klark biochar to its concrete. The carbon bound in plants is thus stored in concrete and cannot escape into the atmosphere as CO₂. Overall, net emissions are significantly reduced, as the concrete acts as a store for the carbon bound in the biochar, while the biochar acts as a kind of negative emission technology incorporated into the concrete. 

At EPFL, a cement that works in yet another way is being developed. This cement, known as “Limestone Calcined Clay Cement” (or “LC3” for short), reduces concrete’s greenhouse gas emissions by up to 40 percent. Instead of comprising 95 percent clinker and 5 percent gypsum like conventional cement, LC3 uses a mixture of 50 percent clinker, 30 percent calcined clay, 15 percent limestone and 5 percent gypsum. Experience with this material shows that LC3 achieves similar strength and stability properties to conventional Portland cement. 

Concrete demolition material is another lever. Concrete is deposited in landfills at the end of its product lifecycle. However, research is now being conducted to determine whether and to what extent crushed demolition concrete can be reused as a substitute for gravel in concrete. Another approach involves enriching ground concrete with CO₂, turning it into a limestone-like raw material. This material could then be mixed into new materials or used as a filler in the likes of the synthetic resin industry.  

Challenges

The construction industry is known for its cautious nature, meaning that it is slow to adopt new products such as concrete that stores CO₂. As long as the applicable standards do not require it to be used, widespread breakthrough in the industry is unlikely. 

With appropriate standards and laws, plus requirements that certain amounts of CO₂ be stored, Switzerland could establish itself as a pioneer in this field. As soon as it can be demonstrated that such concrete is also economically viable, through the likes of higher carbon taxes, such processes and products will become established naturally. However, the industry still has a long way to go, with major challenges ahead in research, development and certification. 

Focus on industry

A more sustainable approach to construction cannot be achieved through a single measure. Repurposing and renovations are, if not necessarily cheaper, usually much more environmentally friendly than new builds.  

Materials play a key role in making the construction industry greener. This requires all the stakeholders along the entire construction industry value chain to get involved. A rethink is needed, from property developers and architects to the construction companies that ultimately purchase materials and build structures. 

More sustainable products are often more expensive at present. Even additional costs of just 2 percent for CO₂-reduced concrete can act as a deterrent, especially if there is a lack of empirical data and no standards for planning and implementation have been established. 

This is why rules that set targets rather than prescribing specific products or solutions are needed. Such legislation would encourage the construction industry to seek other affordable and innovative solutions – while creating legal certainty and thus planning security for all parties involved at the same time.   

International perspective

Switzerland is one of the world’s leading countries in the development and commercialisation of CO₂-reduced concrete. EPFL, ETH Zurich and Swiss universities of applied sciences have a wealth of theoretical and practical knowledge. With Holcim and Sika, two leading companies in the construction chemicals industry, Switzerland has a broad spectrum of theoretical and practical knowledge in the field of CO₂-reduced concrete. At the same time, international commercialisation is proving to be more complicated than in other industries, as construction practices are strongly influenced by local customs and regulations.  

Future applications

Other materials could also replace cement in the future. Researchers at Empa are studying magnesium silicates, for example. Magnesium oxide is extracted from these magnesium silicates and enriched with water and CO₂ during cement production. This allows the material to actively absorb and store CO₂. The challenge with such cement-like building materials is that most of them are less alkaline than cement and thus protect steel less effectively against corrosion than concrete. 

Emissions from the firing process are unavoidable as long as cement is used. To prevent them from entering the atmosphere directly and having a harmful effect on the climate, the CO₂ released from the exhaust gases can be captured, stored or used for other industrial processes (negative emission technologies). 

If the construction sector is successful in reducing its CO₂ emissions, it is conceivable that it may in the future participate in certificate trading and benefit greatly from this. This could open up new business models for property owners and operators. Innovative financing concepts in the building sector are also conceivable. But for this to become a reality, the buildings’ carbon footprint of buildings must be reduced both sustainably and comprehensively.