Carbonation has been shown to improve the durability of recycled aggregate concrete compared to untreated forms, offering a more viable alternative to natural aggregate concrete. This finding comes from a recent review published in npj Materials Sustainability, which examines how carbonation affects key durability indicators and explores methods to optimize the process.
Study: Durability performance of concrete incorporating carbonated recycled coarse aggregates: a review. Image Credit: Sundry Photography/Shutterstock.com
Background
Concrete production has long been linked to two major environmental challenges: high carbon emissions and the growing volume of construction and demolition waste (CDW). To address both, there’s a global shift toward circular construction models that reuse demolished concrete in new builds.
The most common approach is to process CDW into recycled aggregates (RAs), which are then used to replace natural aggregates (NAs) in concrete mixes. This not only helps meet increasing construction demand but also reduces mining-related CO2 emissions, conserves natural resources, and limits landfill use.
However, recycled aggregate concrete (RAC) often underperforms in durability. The root cause lies in the physical makeup of RAs—specifically, the presence of old mortar and micro-cracks, which lead to higher porosity. These structural weaknesses reduce the long-term performance of RAC, especially in harsh environments.
To overcome this, researchers have explored carbonation as a method to improve RA quality by densifying its microstructure.
How Carbonation Works
Carbonation involves exposing RAs to elevated levels of CO2 under controlled humidity and temperature for a set duration. Under these conditions, calcium hydroxide within the aggregate reacts with CO2 to form calcium carbonate, effectively densifying the material.
Typical carbonation settings include high CO2 concentrations (20 %–100 %) and optimal relative humidity levels (50 %–80 %), which provide enough moisture for gas diffusion without oversaturating the pores. However, maintaining these parameters consistently at an industrial scale remains a major hurdle. Newer approaches, such as atomized water-droplet humidity control, show promise but are still in early stages of development.
Temperature and pressure also influence the process. While most studies use ambient conditions, elevated settings can accelerate gas diffusion and reaction rates. Likewise, carbonation duration matters: longer exposure allows CO2 to penetrate deeper and more thoroughly, but over-carbonation can weaken the cement matrix and reduce mechanical strength.
Durability of cRAC
The durability of carbonated recycled aggregate concrete (cRAC) is influenced by several factors, including the replacement ratio of NAs with cRAs, the carbonation conditions, and the initial moisture content of the aggregates.
At full replacement levels (100 %), cRAC typically performs below natural aggregate concrete (NAC) across most durability indicators. However, it consistently outperforms non-carbonated RAC—showing improvements between 13 % and 55 %—thanks to its denser microstructure and reduced porosity.
That said, cRAC still inherits some of the limitations of its source material. Old mortar and weak interfacial zones reduce strength, and the presence of impurities such as wood, clay, or brick can further degrade performance. Aggregates with lower impurity levels (below 5%) tend to yield better results.
While some inconsistencies remain, such as contradictory findings on autogenous shrinkage, pre-soaking aggregates can help mitigate these effects. Notably, cRAC shows greater resistance to sulfate attack than RAC, attributed to the reduction in reactive alkalis and improved pore structure after carbonation.
Outlook and Opportunities
Overall, the review positions cRAC as a promising middle ground between RAC and NAC. Though it doesn't yet match the full performance of conventional concrete, carbonation significantly narrows the gap, making cRAC a more viable and sustainable option.
Further improvements hinge on optimizing carbonation parameters. Moisture content, in particular, plays a critical role: cRAs are more porous than NAs and require more water for effective carbonation, yet the ideal moisture level for maximum durability remains unclear. Tailoring carbonation settings—temperature, humidity, CO2 levels, and duration—based on aggregate properties could yield better and more consistent outcomes.
There’s also growing interest in enhancing the process using bio- or nano-materials to increase carbonation efficiency. However, care must be taken to avoid over-carbonation, which can reverse some of the durability gains.
Journal Reference
Zhang, Z., Angst, U., Troian, V., Guo, B., & Zeng, Q. (2025). Durability performance of concrete incorporating carbonated recycled coarse aggregates: a review. npj Materials Sustainability, 3(1). DOI: 10.1038/s44296-025-00071-x
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