## 4 Activation Methods of RP
### \(Co_{2}\) Activation
#### 4.2.1 Direct Carbonation Processes
According to the report, when RCP was carbonated at a relative humidity of 80%, the resulting Cc consisted of 26% calcite, 45% vaterite, 3% aragonite, and 36% amorphous Cc. [PERSON] et al. [264] examined the process of directly carbonating waste concrete fine (WCF) in an aqueous solution under atmospheric pressure and with a low concentration of CO\({}_{2}\). They observed that the level of carbonation increases proportionally with the increase in the solid-liquid ratio, aligning with the observed CO\({}_{2}\) absorption outcomes. [PERSON] et al. [257] reported that the carbonation kinetics of RCFs in an aqueous environment exhibited a characteristic dependence on particle size. Particles with a greater size were coated with a reactive shell on their outer surface, whereas smaller particles disintegrated during accelerated carbonation. They also observed that during the process of cement hydration, carbonated RCFs had a role in initiating and stabilizing C-S-H gels, which led to the fast hydration of cement. [PERSON] et al. [261] examined the carbonation mechanism of RCF in pure water. They observed that the Cc was the primary product of carbonation in the carbonated RCF. Two distinct types of Cc were identified: calcite and amorphous calcium carbonate (ACc). Both types of Cc were generated at the same time within the first 5 min. However, the amount of calcite increased as the carbonation duration increased, whereas the amount of ACc declined and totally disappeared after 30 min. In general, direct carbonation increases the particle size and SSA of RP [169; 259; 261; 270; 219; 263; 270] due to the particle agglomeration and chemical reaction of hydration products, and the carbonated RP exhibits a denser and smoother surface topology due to the carbonation of the hydration products, which fills the micropores and voids with CaCO\({}_{3}\) and silica gel [61; 91; 167; 169; 226; 261]. [PERSON] et al. [219] observed that D50 of raw WCP is 26.49 \(\upmu\)m. After the carbonation process, it increased to 38.41 \(\upmu\)m. The D50 of carbonated concrete fine (CF) was 57.00 \(\upmu\)m, whereas the D50 of untreated CF was 24.50 \(\upmu\)m. [PERSON] et al. [270] measured the D50 of carbonated RP as 49.00 \(\upmu\)m, whereas the D50 of untreated RP was 19 \(\upmu\)m. In contrast to the results observed with dry carbonation, [PERSON] et al. [262] discovered that RP exhibited enhanced fineness and reduced average particle sizes following liquid-solid carbonation. Both carbonation procedures can effectively generate a suitable level of carbonation in RP. The efficacy of gas-solid carbonation is significantly influenced by the concentration of CO\({}_{2}\), pressure, relative humidity, duration of carbonation, and moisture content of the parent materials. To ensure a satisfactory carbonation rate, it is necessary to carefully control the internal and ambient humidity of the samples [261; 271]. The literature demonstrated that the process of gas-solid carbonation exhibited the most rapid initiation when the relative humidity was between 50% and 70% [261]. On the other hand, liquid-solid carbonation was found to be more effective in completing the carbonation process within a timeframe of 6 h, whereas the carbonation time for gas-solid carbonation was 24 h [80; 257; 262]. Additional pressure was necessary to expedite the diffusion of CO\({}_{2}\) and increase the rate of gas-solid carbonation [261; 272]. Experimental studies have shown that the carbonation rate can be enhanced by up to three times using the liquid-solid method compared to the gas-solid method [259; 262], and almost total carbonation could be obtained within a few hours [261; 262]. Furthermore, the maximal mass output of Cc was enhanced to more than 60%, indicating a greater level of carbonation [261; 262]. During liquid-solid carbonation, the majority of Cc is formed as calcite, whereas the presence of other polymorphs, including amorphous Cc, is significantly reduced compared to gas--solid carbonation.