## 4 Activation Methods of RP
### \(Co_{2}\) Activation
#### 4.2.1 Direct Carbonation Processes  
Direct carbonation is a straightforward chemical process which involves the direct reaction of untreated RP with CO\({}_{2}\) to produce carbonates. Carbonation can be directly carried out in either a gas-solid system or an aqueous system [169; 221; 257; 259; 260; 262]. The gas-solid carbonation methods are developed by increasing the CO\({}_{2}\) concentration, pressure, and temperature, along with maintaining a suitable relative humidity. Common gas-solid methods include normal CO\({}_{2}\) curing, flow-through carbonation, and pressurized carbonation [221; 257]. In the dry process, water exists in the form of vapor, as a coating of absorbed H\({}_{2}\)O on the particle surface, and within the pores between the particles [221] and the process involves the reaction of RP with CO\({}_{2}\) in the absence of any liquid [221; 259; 260; 262]. Aqueous carbonation (AC) is an alternative method of carbonation that relies on interactions between a liquid and a solid. The process involves submerging the solid materials in a bulk liquid (i.e., at high liquid-to-solid ratios) and introducing CO\({}_{2}\) by injection. Upon contact with the bulk liquid, the gaseous CO\({}_{2}\) undergoes a transformation into aqueous carbonate species [80; 169; 221; 260; 267; 268]. The process of carbonating RP in water-based systems is a widely used and accepted method [80; 269]. This process not only simplifies the conversion of CH in RP into CaCO\({}_{3}\), but also generates nano-silica as a result of the decomposition of C-S-H gel [80; 221; 261]. Regarding the carbonation time, [PERSON] et al. [219] found it to be 90 h with a water-to-solid ratio (w/s) of 0.4. They argued that an excessively high or low w/s hinders the efficiency of the carbonation conversion process. However, extending the carbonation duration enhances the carbonation reaction of WCP even more. They reported that the presence of reactive components in WCP allows for efficient reaction with CO\({}_{2}\), leading to the formation of additional calcite crystals during the carbonation process. [PERSON] et al. [117] carried out a parametric study of accelerated carbonation on recycled fine (RF), with varying parameters of water content (1, 7, and 25%), powder size (50 \(\upmu\)m and 80 \(\upmu\)m), carbonation duration (1, 3, 8, 14, 17, and 24 h), CO\({}_{2}\) concentration (18 and 100%), and heating temperature (500 \({}^{\circ}\)C). They found that the impact of particle size on the extent of carbonation is substantial. A lower particle size (50 \(\upmu\)m) exhibits a greater degree of carbonation in comparison to a larger particle size (80 \(\upmu\)m). The duration of carbonation, ranging from 1 h to 24 h, affects the absorption of CO\({}_{2}\). The absorption of CO\({}_{2}\) occurs rapidly for 17 h.  
[PERSON] et al. [91] evaluated the impact of temperature ranging from 20 \({}^{\circ}\)C to 140 \({}^{\circ}\)C on the carbonation efficiency of RCF. They reported that elevated temperature, in combination with a 20% concentration of CO\({}_{2}\), enhances the carbonation process of RCF and the recommended optimal carbonation temperature is 100 \({}^{\circ}\)C. They also stated that the carbonation temperature has a significant impact on the polymorph and morphology of calcium CaCO\({}_{3}\) precipitated in carbonated RCF. [PERSON] et al. [270] stated that the surface of the carbonated RP is covered with a coating of amorphous silica gel and the produced CaCO\({}_{3}\) was enveloped by silica gel. They also reported that, during the process of dry carbonation, the RP particles were in close proximity to one other because they were not well dispersed. As a result, when calcium dissolved in the moisture on the surface, the calcium carbonate (Cc) precipitated. [PERSON] et al. [259] reported that the moisture conditions during carbonation had a significant impact on the carbonation process.