# Pathway to Carbon Neutrality in the Cement Industry: CO2
## 6 Conclusions  
Recycled aggregates from construction and demolition waste have been submitted to forced carbonation. Several types of RAs were investigated at different periods of carbonation. The quantification of CO\({}_{2}\) uptake of RAs was carried out through thermogravimetric analysis. The following conclusions are drawn:  
* CO\({}_{2}\) capture depends on the type of CDW (mixed or mainly concrete). Indeed, the maximum CO\({}_{2}\) uptake is related to the cementitious materials content and to the degree of natural carbonation. In short, CDW captured from 5 wt.% to 35 wt.% of CO\({}_{2}\) per tonne of cement paste, which corresponds to 0.6% to 4.1% per tonne of aggregate. * The lowest-captured CO\({}_{2}\) value was 0.6% for MRA-RP3 and RCA-RP2 (corresponding to 6.0 kg per tonne of aggregate) and the highest was obtained in CA-L concrete produced in the laboratory with 4.9% (49 kg per tonne of aggregate). Considering that the MRAs did not present portlandite content, it was verified that the carbonation occurred by the reaction of CO\({}_{2}\) and C-S-H.
* Taking into account that only 13% to 16% of Rc fraction is composed of cement paste, forced carbonation allowed the aggregates to absorb between 20% and 27% of cement process emissions (the ones that refer to decarbonisation of calcium carbonate). * Forced carbonation does not continuously increase or stabilize over time. It reaches a maximum value of captured CO\({}_{2}\) and then it goes down. After a given time, an acid attack occurs that reduces the calcium carbonate in the sample. The tested MRAs reached the maximum capture value after 5 h and the RCAs after 12 h.
* It was also verified that long periods of exposure to CO\({}_{2}\) do not contribute to the increase in CO\({}_{2}\) capture by RAs. An acid attack occurs at longer carbonation periods. This attack promotes the dissolution of CaCO\({}_{3}\), decreasing the potential for CO\({}_{2}\) capture for long carbonation periods. * It was concluded that RAs, when submitted to forced carbonation, capture from 52 to 491 kg of CO\({}_{2}\) per tonne of cement paste. Therefore, the forced carbonation of RAs, independently of the building life cycle (time and conditions of environmental exposure) and demolition carbonation phases, still has the potential to capture CO\({}_{2}\). * The forced carbonation of recycled aggregates has a great influence on their characteristics. The characterization of the aggregates before and after their carbonation made it possible to analyse the modification in their physical properties, namely water absorption and particle density. During the carbonation, the precipitation of calcium carbonate inside the pores causes an increment in the density of the aggregate and decreases the water absorption. The physical properties are related to the carbonation potential of the aggregate. Overall, this research highlights that recycled aggregates from construction and demolition wastes can be seen as sources of CO\({}_{2}\) capture and storage. This contribution leads to the reduction of the carbon footprint of the cement industry and the more effective reuse of CDW. **Author Contributions:** D.B. : Conceptualization, Investigation, Formal analysis, Writing--Original draft C.B.F. : Conceptualization, Supervision, Formal analysis, Writing--Reviewing and Editing C.M.P. : Supervision, Formal analysis, Writing-Reviewing and Editing R.V. : Supervision, Writing--Reviewing and Editing J.A.B. : Writing--Reviewing and Editing R.I.G. : Investigation A.S.S. : Supervision, Writing--Reviewing and Editing. All authors have read and agreed to the published version of the manuscript. This research was funded within the research project AL7.1 by c5 Lab (Interface Mission 01/C05-i02/2022.P264). Not applicable. **Data Availability Statement:** Data is contained within the article. The authors acknowledge for the financial support provided by the Interface Program, which plays a crucial role in sustaining the operations and research initiatives of c5 Lab--Sustainable Construction Materials Association.