# Pathway to Carbon Neutrality in the Cement Industry: CO2
## 4 Co\({}_{2}\) Capture of RAs
### Concrete Aggregate from Lab Specimens
The calculated efficiency value does not demonstrate the real susceptibility of the aggregate to carbonate, because the aggregates were already exposed to environmental conditions, but rather the efficiency of the method to achieve a totally carbonated aggregate. The highest efficiency was obtained in the carbonation of CA-L and RCA-IW1, with 94.6% and 71.3%, respectively. These aggregates have the highest Rc fraction and are also the ones that had less exposure to the atmospheric environment (suffered less natural carbonation). It can be concluded that the reduction in carbonation efficiency is directly  
Figure 8: Carbonation process efficiency and respective CO\({}_{2}\) capture contribution. Figure 7: Maximum amount of CO\({}_{2}\) captured by the different RAs. influenced by the cement content and the level of previous carbonation (during building the environment and after demolition). Unlike MRAs, before carbonation RCAs present a significant portlandite content in their matrix, as well as other hydrated cementitious compounds (e.g., CSH and ettringite). The portlandite was totally consumed during the carbonation of RCA-RP1, RCA-RP2 and CA-L. In RCA-RP1 and RCA-RP2, about 42% and 50% of the produced CaCO\({}_{3}\), respectively, was due to the carbonation of remaining portlandite. In this case, the CO\({}_{2}\) uptake efficiency was 27.2% and 10.6%, respectively. RCA-IW1 and RCA-IS1 have a higher content of portlandite in their initial composition when compared with RCA-RP1 and RCA-RP2. In the case of RCA-IW1 there was a large decrease in portlandite content after carbonation, suggesting that it was completely consumed. The carbonation of portlandite was responsible for 26.1% of the CaCO\({}_{3}\) content formed and the other 73.9% by carbonation of other hydrated cementitious compounds. Based on the results obtained, and the test conditions employed, the carbonation of the RAs was mainly due to the hydrated calcium silicates and hydrated calcium aluminates of the cement. The investigation of CO\({}_{2}\) capture capacity of CDW was carried out in this paper for further application. It aims to use carbonated RAs in mortars and concrete as a substitute for natural aggregates. Note that aggregates represent a high proportion of mortars and concrete constituents, thus the replacement of natural aggregate by recycled aggregates means a high volume of RCA incorporation. Therefore, the percentage presented in this study is much higher if considered the incorporation proportion in mortars and concrete. Additionally, an assessment of life cycle carbon impact was carried out. Mortars with carbonated recycled aggregates were compared with mortars with noncarbonated recycled aggregates and with reference mortars with natural aggregates. The incorporation of recycled aggregates in mortars contribute to the reduction of CO\({}_{2}\) when compared to the use of natural aggregates, depending on the type, origin and proposition of incorporation. Moreover, accelerated carbonation of recycled aggregates reduces even more the CO\({}_{2}\) balance by capturing the CO\({}_{2}\) emissions present.