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Recycled Concrete: A Sustainable Solution for Construction?

Recycled Concrete: A Sustainable Solution for Construction?
In France, SEQENS, a subsidiary of the Action Logement Group, and HOLCIM, a construction materials producer, have joined forces to build the first building made entirely of recycled concrete. The residence, currently under construction in Gennevilliers, a few kilometers from Paris, will be completed in 2024. (Credit: SEQENS / HOLCIM)

The construction industry faces significant environmental challenges, particularly in terms of CO2 emissions and the use of natural resources. Concrete is widely used on construction sites, but its carbon footprint is alarming. Could recycled concrete make tomorrow’s buildings more sustainable?

Concrete is one of the most commonly used construction materials worldwide, appreciated for its durability, strength, and versatility. In fact, two-thirds of all structures on the planet are made of concrete.

However, while being the most commonly used building material, it is also a significant source of greenhouse gas emissions.

Concrete: An Environmental Adversary

Cement, the main component of concrete, is produced by heating limestone at high temperatures, a process that releases large amounts of CO2. Concrete also contains substantial amounts of sand, which must be extracted. According to estimates, it takes 200 tonnes of sand to build a single-family home, 30,000 tonnes for one kilometer of road, and 12 million tonnes for a nuclear power plant.

The problem is that sand resources are declining year after year, leading to an impending shortage. Moreover, the environmental cost of transportation should be considered since sand often needs to be transported from far away, further adding to the overall carbon footprint of concrete. Furthermore, sand mining from coasts accelerates erosion and leads to ecological disasters worldwide.

More Eco-Friendly Concretes

New types of concrete are therefore being developed to combine the material’s efficiency and durability with environmental sustainability.

Wood-based concrete is one of these innovative construction materials. Last year, we interviewed CCB Greentech, a company that claims their wood concrete has a negative carbon footprint.

Another alternative is recycled concrete. In France, SEQENS, a subsidiary of the Action Logement Group, and HOLCIM, a construction materials producer, have joined forces to build the first building made entirely of recycled concrete. The residence, currently under construction in Gennevilliers, a few kilometers from Paris, will be completed in 2024. The 2,200 tonnes of 100% recycled concrete that will make up the building have already been produced by French cement manufacturer Lafarge. This is a world first.

All the components of the concrete come from construction and demolition waste, unused hardened concrete from construction sites, and demolition site concrete. (Credit: SEQENS / HOLCIM)

100% Recycled Concrete

Traditionally, concrete production requires 80% aggregates (gravel and sand), 11% cement, and 9% water. But for this project, all the components of the concrete are sourced from recycled materials, explains Mouloud Behloul, Director of Innovation and Sustainable Construction at Lafarge Cement France.

“This represents a significant technical achievement. The clinker, which is the active base of cement, the aggregates, and the sand come from construction and demolition waste, unused hardened concrete from construction sites, and demolition site concrete. The water used is also recycled and comes from the process water circuit. We have developed a specific recipe using these different recycled materials to achieve the same results as traditional concrete.”

As a result, the world’s first 2,200 tonnes of recycled concrete have been produced at Lafarge’s cement plant in Altkirch, eastern France.

Technical Achievement

Making 100% recycled concrete was not easy. The first technical achievement concerns clinker. Clinker is the active base of cement. It is obtained by heating limestone and clay to a temperature of 1430°C to obtain a granulate which is then crushed and mixed with gypsum and limestone additives.

“Traditionally, to produce one ton of clinker, 1.6 tons of limestone and clay need to be extracted from a quarry. In our sustainability approach, we sought to replace raw materials from quarries with recycled materials. Therefore, we have recovered construction waste from demolition concrete and waste from the steel industry. Thanks to this, we have succeeded in formulating ingredients similar to those found in clay and limestone, namely alumina and silica from clay.”

The second technical achievement concerns sand. While sand can be recycled, it requires special preparation to regain all its properties.

100% recycled clinker pre-firing materials (Credit: Lafarge)
Recycling of demolition concrete (Credit: Lafarge)
Analysis of recycled clinker (Credit: Lafarge)

“When we crush sand from demolition, its shape is not spherical but acicular. Therefore, if we use it as is, the concrete loses performance. Hence, we have developed a technique to adapt the concrete formula to recycled sand. We have worked on granular stacking, proportion, and water quantity.”

Lafarge has conducted durability and strength tests that now allow the group to assert that its recycled concrete has properties equivalent to traditional concrete in terms of durability, fire resistance, porosity, and service life.

“We prefer to refer to it as new concrete with recycled elements.”

According to Mouloud Behloul, the implementation of recycled concrete does not require major changes in construction processes. The equipment and casting techniques remain the same.

However, it is necessary to isolate a specific batch of recycled aggregates and sand to ensure traceability and compliance with regulatory requirements. This is called characterization.

“According to the law, up to 30% of recycled elements do not require characterization, so it’s like making normal concrete, there is no constraint. Beyond that, we need to perform durability and water porosity tests. We need to conduct standard tests to observe concrete behavior such as shrinkage and creep. This is an important quality constraint that lengthens the manufacturing process compared to traditional concrete.”

Testing recycled concrete (Lafarge)
Testing recycled concrete (Lafarge)

Environmental Benefits?

What are the environmental benefits of recycled concrete compared to traditional concrete? According to Mouloud Behloul, the main focus is on circular economy and recycling, not carbon footprint reduction. And for a good reason, the production process remains unchanged, as high-temperature kilns are still required to manufacture clinker.

“It’s not low-carbon; it has a lower carbon impact. The objective is to recycle and our primary goal is to demonstrate that 100% recycled concrete is possible.”

As a small consolation, the waste used comes from local demolition sites, which slightly reduces the carbon footprint by 10%, Mr. Belhoul said.

“We only use locally sourced waste, in the Île-de-France region, near the construction site of the future complex. There is no point in sourcing waste from the other end of France.”

The Challenging Democratization of Recycled Concrete

Could 100% recycled new concrete become prevalent on construction sites? For Lafarge, the goal is not to launch a 100% recycled concrete industry because the demand is not there yet. 

“Our clients are currently not the driving force. It is necessary to popularize the idea. The construction industry is conservative. When we build, it is for 1 or 2 generations, which encourages conservatism and building in the same way. When we introduce an innovation, there is always resistance from industry professionals who wonder if it will hold up for 100 years. People only believe when we provide the evidence. Therefore, more tests are needed to reassure. With this future project, we want to demonstrate that it is possible to have a construction site with 100% recycled concrete. By doing so, we hope to integrate at least 20% to 30% of recycled components into future concrete. That’s how we can democratize it.”

The existing concrete standards already allow for the integration of 10% to 100% of recycled elements in concrete. In the future, Lafarge expects to incorporate at least 15% of recycled clinker into all its cement plants.

However, the question of the price of recycled concrete remains. There will be an additional cost, but for now, it is impossible to predict how much a ton of new concrete with recycled components would cost.

“The additional cost compared to traditional concrete must be reasonable, not twice as expensive, for example.”

The complex will be delivered in 2024, and the first recycled concrete will be poured next month.