# Evaluation of the Performance of Previous Concrete Inspired by CO\({}_{2}\)-Curing Technology
## 3 Results and Discussion
### Concrete Carbonation
It is worth mentioning that cement-based materials cured in a CO\({}_{2}\) environment are different from concrete carbonation, and their main reactions and influencing factors are naturally different [40]. Carbonation curing is related to the development of strength, microstructure densification, and mitigation of chemical attack, but the decrease in alkalinity raises concerns about the corrosion issue of steel bars [40]. Pure CSH, portlandite, and ettringite in powder form rapidly carbonate at high relative humidity, while the pore structure, relative humidity, drying state, and degree of water saturation inside the concrete pore system play a significant role in the carbonation of solid concrete components [41]. The bonding of the CSH surfaces to the calcium ions in the calcite is very strong, thus improving the compressive strength [42]. Calcite crystals of significant volume are usually seen in the microstructure of CO\({}_{2}\)-cured concrete but not in water-cured concrete. Three types of CaCO\({}_{3}\), including calcite, vaterite, and aragonite, form in concrete exposed to CO\({}_{2}\) at an early stage. The prolongation of this CO\({}_{2}\) exposure transforms vaterite and aragonite into more stable calcite [42]. The SEM and XRD results shown in Figure 6 provide morphological and mineralogical information on the pervious concrete surfaces cured with water and CO\({}_{2}\). CSH with sheet morphology was observed in the microstructure of the water-cured sample, while rhombohedral CaCO\({}_{3}\) crystals (i.e., calcite) were found in the CO\({}_{2}\)-exposed sample. The major X-ray peaks at 18.1\({}^{\circ}\), 26.6\({}^{\circ}\), 27.7\({}^{\circ}\), and 29.4\({}^{\circ}\) 20 positions are attributed to portlandite (Ca(OH)\({}_{2}\)), quartz (SiO\({}_{2}\)), calcium silicates (i.e., unhydrated cement), and calcite (CaCO\({}_{3}\)), and their presence was observed in the XRD pattern of concrete samples. It is worth mentioning that the powder diffraction file numbers of quartz, calcite, and portlandite crystals, according to the International Center for Diffraction Data, are 46-1045, 5-586, and 5-733. The X-ray peak attributed to portlandite was not found in the carbonated sample XRD result, indicating the conversion of portlandite to calcite during the hardening stage of the sample cured in a CO\({}_{2}\) environment. These findings highlight that CO\({}_{2}\) exposure did not alter the hydraulic conductivity of pervious concrete but helped to form stable carbonates that improved compressive strength. Carbonation involves the change of CO\({}_{2}\) from the gaseous to the aqueous form. The reaction of CO\({}_{2}\) with water led to carbonic acid, which dissociated into protons and carbonates, depending on the pH of the solution [43]. After the dissolution of gaseous CO\({}_{2}\), the precipitation of carbonates with metal cations remained in the solution, provided that the solubility of the metal carbonates was exceeded. The carbonation reaction is very slow in water-saturated concrete because CO\({}_{2}\) diffusion is affected by free water that is filled in the pores. This reaction is also slower in completely  
Figure 6: SEM and XRD results, which reveal the mineralogy of carbonated concrete. dry pores because the formation of metal carbonates occurs in an aqueous form, often in the film of free water occupying the pores [44].