# Evaluation of the Performance of Previous Concrete Inspired by CO\({}_{2}\)-Curing Technology
## 3 Results and Discussion  
### Concrete Carbonation  
The pH of the potable water used to cure the concrete samples increased from 8.01 to 12.21 due to the dissolution of free lime and Ca(OH)\({}_{2}\) from the porous matrices. When  
Figure 4: Flow of steps in tomography image acquisition and processing. these samples were cured in the CO2 chamber instead of in water, this pH increase was relatively lesser. The average compressive strength of concrete samples cured in water and the CO2 environment was found to reach 27.34 and 31.23 MPa at the end of 28 days. A compressive strength test was carried out on a sulfur-capped concrete sample of 100 mm in diameter and 150 mm in length according to the procedure already explained by [PERSON] and [PERSON] [32]. In this study, there is an increase in concrete strength of up to 14% due to the continuous storage of porous samples in a CO2 environment. There is no significant difference in the rate of percolation of water through these samples as a result of variations in the curing conditions. The average percolation rate was calculated to be 18 L/min/m2. The standard deviation of these measurements was found to be 3.2 L/min/m2. CT images of these samples were analyzed to determine their surface porosities. It is well known that the performance of any pervious medium is inherently dependent on the properties of its pore structure [33]. The total volume and size distribution of the pores and the interconnectivity of the pores phase in pervious concrete determine its performance characteristics [34]. The CT slice shown in Figure 5 reveals the random distribution of pores along the length of the concrete sample in this study. The greyscale value of the CT image differentiates the concrete components. The black regions in this section refer to the voids, the white regions refer to the very dense anhydrous compounds, and the grey regions indicate the formed cement hydrates. The distribution of the pore area fraction (i.e., surface porosity) along the sample length was found to fall within the range of 6-28%. The average pore area fraction of non-carbonated and carbonated samples was calculated to be 16.4% and 14.8%. The difference in these porosities was found to be very low, which explains why the curing condition did not influence the interconnected pore structure. The rate of the percolation of water is the most important characteristic of the performance of pervious concrete [35]. Concrete transport properties are inherently dependent on the characteristics of the pore structure [36]. However, it has been common to relate the permeability of pervious concrete with its porosity, primarily due to the ease with which porosity can be measured in such a porous material [37]. In this study, the concrete sample was divided into two halves using a cutting tool, and a solution of phenolphthalein indicator obtained from Merck was sprayed onto the inner cut surfaces to locate the carbonated regions. Figure 6 shows the fractured surface of the freshly cut pervious concrete samples at the end of CO2 exposure. Spraying the phenolphthalein solution on the cut section of carbonated samples resulted in a colorless effect, indicating a pH less than 8.2. The CO2 gas easily penetrates the macropore channels and seems to carbonate all the cement surfaces prevailing in the interconnected pore structure. The phenolphthalein changed from colorless to pink on water-cured concrete surfaces when the pH was above 9.5. The carbonization process is a chemical reaction that takes place in concrete, involving calcium-containing compounds and CO2, leading to the formation of stable CaCO3 and water [38]. This reaction results in the depletion of  
Figure 5: Porosity distribution along the length of pervious concrete samples. hydroxides, primarily Ca(OH)\({}_{2}\), which reduces the pH of the concrete pore solution [39]. Calcite is 17% larger in size than portlandite, inducing structural changes in the cement matrix [9].