# Pathway to Carbon Neutrality in the Cement Industry: CO2
## 2 Research Significance
### Effect of Carbonation on RCA Properties  
Carbonation is reported to modify the microstructure of recycled concrete aggregates (RCAs), porosity and related properties. In well-hydrated cement-based materials, CO2 reacts with calcium hydroxide, as well as with calcium silicate and calcium aluminate hydrates, precipitating calcite in the pores [33]. This decreases the total porosity of RCAs [14; 34; 35], by reducing the pores diameter. The influence of carbonation in RCA microstructure varies according to the initial porosity of the aggregates. [PERSON] et al. [34] investigated the carbonation of two samples with low and high porosity, respectively. From XRD analysis, the high-porosity samples showed a higher amount of CaCO3 formed than the low-porosity samples. In those with low porosity, a dense CaCO3 layer was formed near the surface and the CO2 could not penetrate the sample in depth. On the other hand, the carbonation in high-porosity specimens also occurred in deeper pores, because CO2 easily flowed through the pore system, forming a CaCO3 film along open pores. In this case, the porosity decreases and makes the CO2 diffusion more difficult, slowing down the carbonation reaction [34]. Some studies found in the literature point out a reduction of porosity with carbonation in cement-based materials. The reduction of porosity due to carbonation naturally increases the density of RCAs [14; 16; 17; 35; 36; 37; 38; 39; 40; 41]. The increase in density depends on the initial characteristics of RCAs, namely porosity and carbonation potential. About 5-8% increase is reported when compared to the density of uncarbonated RCAs [42; 43]. The increase in density will lead to lower absorption and higher mechanical strength of RCAs, bringing them closer to the characteristics of natural aggregates (NA). In fact, usual RCAs are more porous than NA, which leads to higher water absorptions [44; 45]. However, as mentioned, the porosity and water absorption of RCAs are reduced after carbonation [35; 42; 43; 46; 47]. Depending on the initial characteristics of RCAs, some researchers report a reduction of 20-28% in water absorption, after carbonation [17; 42; 43]. This reduction allows to reduce the amount of mixing water in cement-based materials and better control of their fresh properties. [PERSON] et al. [36] studied the carbonation of RCAs with different-sized particles under 2.5 mm. The authors verified that the smaller particles carbonated faster than the larger ones due to the higher surface area of the aggregate. Additionally, it was reported that after carbonation, both CaCO3 and silica gel were formed, which increased the density of the RCAs and reduced their water absorption. In this work, the carbonation potential was not measured since the purpose of the study was to enhance the mechanical properties of RCAs. [PERSON] et al. [48] also investigated the carbonation of RCAs, aiming to improve their mechanical properties. Again, these authors did not focus on the CO2 uptake of the RCAs, but on improving RCA performance. In the study of [PERSON] et al. [41], an accelerated carbonation technique was used to enhance the quality of recycled concrete aggregate. The authors studied the CO\({}_{2}\) capture of two RCAs, one produced in the laboratory and another one from demolition concrete debris. The RCAs from the laboratory with particle sizes between 5 mm and 20 mm captured on average 7.9 kg of CO\({}_{2}\) per tonne of aggregate. The RCAs from crushing debris sequestrated 5.2 kg of CO\({}_{2}\) per tonne of aggregate. This reduction of RCA CO\({}_{2}\) uptake was attributed to their previous natural carbonation during their service life. The interfacial transition zone (ITZ) between the natural aggregates and the cement paste is a weak region of the RCAs. When the adhered old mortar shrinks may introduce microcracking in this ITZ [49], which reduces the mechanical strength of RCAs compared with natural aggregates. In this context, according to [PERSON] et al. [42], carbonation can improve the ITZ of RCAs.