New strategies to boost the net CO2 benefit of recycled aggregate concrete

A recent study published in Engineering titled “Strategies to Improve the Life Cycle Net CO₂ Benefit of Recycled Aggregate Concrete” offers valuable insights into enhancing the environmental sustainability of recycled aggregate concrete (RAC) and carbonated recycled aggregate concrete (CRAC). The research, conducted by Rui Hu, Yingwu Zhou, and Feng Xing, explores the life-cycle CO₂ emissions of these materials, considering various factors and scenarios to maximize their net CO₂ benefits.

The study begins by acknowledging the potential of CRAC, which involves recycling concrete waste and fixing CO₂, to achieve a positive net CO₂ benefit. However, the application of recycled aggregates and carbonation modification presents both advantages and disadvantages in terms of net climate benefit. To quantify these effects, the researchers analyzed 122 datasets, including 37 for RAC and 85 for CRAC, considering factors like mix proportions, strength ratios, and carbonation treatment processes.

The life-cycle CO₂ emissions were calculated based on 14 processes, ranging from raw material production to demolition waste transportation. The study considered the uncertainty and variability of CO₂ emissions from material production and analyzed three scenarios: transporting CO₂ through pipelines, producing recycled concrete on site, and using clean-energy transportation. The findings reveal that the net CO₂ benefit of RAC and CRAC is significantly influenced by factors such as the efficiency of carbonation treatment, concrete strength, and the duration of CO₂ curing.

One key finding is that the production of recycled aggregates (RAs) contributes to the greatest negative CO₂ benefit due to the additional steps involved in breaking down concrete waste. However, carbonation treatment can absorb CO₂ and improve the strength and durability of recycled concrete, offsetting some of these negative impacts. The study suggests that an effective carbonation method, particularly when the CO₂ curing duration is less than 48 hours and the strength ratio is greater than 0.95, can significantly increase the possibility of a positive net CO₂ benefit.

The researchers also explored the impact of different scenarios on the net CO₂ benefit. For instance, transporting CO₂ through pipelines and producing recycled concrete on site can enhance the net CO₂ benefit by reducing transportation emissions. Additionally, using clean-energy transportation for all transportation processes can significantly boost the net CO₂ benefits of RAC and CRAC.

To facilitate practical engineering design, the study employed a machine learning model to predict the possibility of a positive net CO₂ benefit. The model, based on the extreme gradient boosting machine, was trained using the collected data and achieved a high accuracy with a correlation coefficient of 0.987. This tool can efficiently pre-estimate whether RAC/CRAC can produce a positive net CO₂ benefit in real-world applications.

The research concludes with several recommendations to improve the net CO₂ benefit of RAC and CRAC. These include adopting carbonation technology when using recycled aggregates, ensuring high strength and good durability of CRAC, limiting energy consumption during carbonation treatment, and combining clean-energy transportation with high-efficiency carbonized RAs. These strategies hold promise for achieving better life-cycle CO₂ benefits in the construction industry.

This study provides a comprehensive analysis and practical guidance for enhancing the sustainability of recycled aggregate concrete, offering valuable insights for both researchers and practitioners in the field of sustainable construction materials.

The paper “Strategies to Improve the Life Cycle Net CO₂ Benefit of Recycled Aggregate Concrete,” is authored by Rui Hu, Yingwu Zhou, Feng Xing. Full text of the open access paper: https://doi.org/10.1016/j.eng.2024.11.040. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.