CO2 uptake of recycled aggregates: a way to reduce the environmental impacts of recycled aggregates concrete

By N. TAZI, R. IDIR and A. BEN FRAJ
Cerema, Project team DIMA

Construction is essential to the development of countries, even during a global health crisis. Billions of tons of materials are continuously consumed to satisfy the world’s insatiable demand. However, this trend in materials use leads to a significant depletion of natural resources and will converge towards a tipping point. Fortunately, solutions exist nowadays to substitute natural resources with recycled materials. These solutions address the challenge of resource depletion as well as the valorization of waste from the construction sector. Cerema, within the SeRaMCo Interreg project, is working on several paths to use recycled concrete made of recycled materials and thus reduce its environmental impact. This project aims at increasing the use of recycled aggregates and fines in a real circular economy. This article resumes some of these ways addressed in our previous research article and focuses on the effects of CO₂ uptake of recycled aggregates.

Mitigating the environmental impact of recycled concrete could be achieved by optimizing the recycling process of construction & demolition wastes, considering the preservation of natural resources by using recycled materials or by considering the Carbone uptake of recycled aggregates. The latter is performed by taking into account the carbonatation of recycled aggregates. Recent results estimate up to 0.05-ton CO₂/ton uptake of recycled aggregates, which would reduce consequently the “global warming” indicator when performing a life cycle analysis of these aggregates. While the environmental impacts of recycled aggregates have been assessed previously (Figure 1), an extra potential reduction of these environmental effects could be achieved when considering the following benefits of using recycled aggregates:

• Valorizing trivial industrial wastes in the recycling process;
• Considering the CO₂ uptake and how it affects the “global warming” indicator of recycled aggregates.

Thus, as shown in Figure 2, the environmental impacts of recycled aggregates are mitigated, and it is then suitable to use these recycled materials as a substitution to natural aggregates.

Figure 1: Environmental impacts from the production of 1 ton of recycled aggregates without considering the valorization of wastes and the uptake of CO2 

Figure 2: Environmental impacts from the production of 1 ton of natural aggregates considering the valorization of wastes and the uptake of CO2 

What would also be interesting is how the CO₂ uptake affects the ecological profitability distance of aggregates, which corresponds to the delivery distance of natural aggregates, from which the global warming indicator of Natural Aggregates Concrete (NAC) exceeds that of Recycled aggregates one (RAC). Let us take the example of a construction site in Paris in need of aggregates for its concrete with 50% of NA and 50% of RA. If we consider multiple natural aggregates suppliers in a radius distance from 22 to 250 km from the construction site, one could notice (see figure 3) that natural aggregates supply is ecologically better if suppliers are in a radius distance less than 50 km. Exceeding this optimal distance, it will be ecological advantageous to use recycled aggregates. However, when considering the CO₂ uptake of recycled aggregates (green curve in Figure 3), the use of these aggregates is always profitable from the environmental point of view, regardless of the delivery distance of NA.

Figure 3: Ecological profitability distance for recycled aggregates, compared to natural ones 

This alternative not only leads the way to use recycled aggregates and thus recycled concrete from the environmental point of view but paves also the way to implement circular economy business models in the construction sector to mitigate its climate change contribution. Isn’t that great?