# Pathway to Carbon Neutrality in the Cement Industry: CO2
## 4 Co\({}_{2}\) Capture of RAs
### Concrete Aggregate from Lab Specimens  
To evaluate the maximum CO\({}_{2}\) capture potential without previous carbonation, laboratory concrete samples (CA-L) were analysed. From XRD, CA-L contains portlandite and other major mineralogical compounds, namely gypsum, quartz, calcite and microcline (Figure 5). Figure 4: Evolution of CO\({}_{2}\) content in aggregates RCA-RP1, RCA-RP2, RCA-IS1 and RCA-IW1 with time of forced carbonation, determined from TGA. Figure 5: XRD pattern of uncarbonated CA-L. Notation: G—Gypsum; P—Portlandite; Q—Quartz; C—Calcite; Fk—Microcline. The TG/DTG curves before carbonation and after 5 h of carbonation are presented in Figure 6. After 5 h of carbonation no mass loss due to portlandite decomposition is observed, with the corresponding increase in mass loss between 500 and 900 \({}^{\circ}\)C, attributed to the decarbonation of calcium carbonate. The CA-L was able to capture 4.9% of CO\({}_{2}\) per tonne of RA. As this aggregate contains 11% cement by weight, the cement paste captured 49 kg of CO\({}_{2}\) captured per tonne of aggregate, which corresponds to 491.1 kg of CO\({}_{2}\) per tonne of cement. As expected, this sample was the one with the highest uptake of CO\({}_{2}\), as it corresponds to a noncarbonated RCAs. It can be considered a reference recycled aggregate to evaluate the potential amount of CO\({}_{2}\) that a common concrete can capture. However, this recycled aggregate is not common, unless it comes from concrete elements protected from the external environment. [PERSON] et al. [41] also analysed the CO\({}_{2}\) capture of laboratory-made CA. The authors concluded that CA sequestrated around 7.9 kg of CO\({}_{2}\) by 1 tonne of aggregate. CO\({}_{2}\) Uptake Efficiency of RAs and Calculation of Contribution of Cemertitious Compounds for CO\({}_{2}\) Capture  
Figure 7 presents a summary of the CO\({}_{2}\) uptake efficiency (Equation (7)) of each RA analysed in this study. The samples RCA-IW1 and CA-L differ from the remaining essentially because they are samples that had very low exposure to atmospheric CO\({}_{2}\) and higher carbonation potential. The CO\({}_{2}\) uptake efficiency was calculated according to Equation (10). To obtain the maximum theoretical CO\({}_{2}\) capture, it was assumed that all the cement paste presented in the aggregates is composed of CEM type I with, at least, 95% of Portland clinker. It was also considered that cement is composed of 60% CaO [74; 75] and that all the calcium compounds present in the cement paste can react with CO\({}_{2}\). Taking into account these assumptions, the maximum theoretical CO\({}_{2}\) capture was calculated based on Rc values (Table 2) and considering an average value of 14% for the cement content in all RAs [76], with the exception of CA-L where its composition is known (11% cement). For example, for MRA-RP1, with 42.5% of Rc, the estimated CaO content is 3.5%, which corresponds, according to the stoichiometry, to a maximum theoretical CO\({}_{2}\) capture of 2.8% (Equation (13)). Figure 6: CA-L TGA before (0 h) and after 5 h of forced carbonation. Legend: 1—Dehydration of C-S-H; 2—Dehydroxation of portlandite; 3—Decarbonation of amorphous calcium carbonate; 4—Decarbonation of crystallized calcium carbonate. The maximum experimental CO\({}_{2}\) capture was determined by TGA and corresponds to the maximum value obtained for each RA under forced carbonation conditions. \[CO_{2}\ uptake\ efficiency\ of\ RA\left(\%\right)=\frac{Maximum\ experimental\ CO_{2}\ capture}{Maximum\ theoretical\ CO_{2}\ capture}\times 100 \tag{12}\]  
\[Max_{theoretical}\ CO_{2}\ capture=Rc(\%)\times Cement\ content\ (\%)\times\% CaO\ cement\times\frac{MM_{CO_{2}}}{MM_{ CAO}} \tag{13}\]  
Figure 8 presents the values of efficiency obtained by forced carbonation and the respective contribution of remaining cement compounds in the RAs (portlandite and other cementitious compounds).