# Time Evolution of CO2 Diffusivity of Carbonated Concrete
## 3 Results and Discussion
### CO\({}_{2}\) Diffusivity of Concrete after Carbonation  
Figure 7 shows the result of estimating CO\({}_{2}\) diffusivity of carbonated concrete with w/c ratio. The result reflected the reduction of pore due to carbonation. Among the four main functions of Equation (2), (a) CO\({}_{2}\) diffusivity in bulk fluid is constant because it has nothing to do with cementitious material. However, (b) pore structural characterization; (c) tortuosity of streamline, and (d) disturbed effect are influenced by carbonation. That is, the carbonation of concrete can make the flow characteristics of the fluid more complicated by reducing the pore system of the concrete. In particular, the CO\({}_{2}\) diffusivity of early carbonated concrete decreased significantly. However, carbonation leaded to the reduction of CO\({}_{2}\) diffusivity of concrete regardless of age, and this tendency was obvious for all concrete.  
[PERSON] et al. suggested that chloride diffusivity of OPC concrete was significantly decreased after carbonation, while blended concrete showed different results depended on admixture type or the replacement ratio [26]. That is, chloride diffusivity of concrete with fly ash decreased, while that of concrete with granulated blast furnace slag increased. As Ca(OH)\({}_{2}\) is transferred to CaCO\({}_{3}\) due to carbonation, the microstructure characteristics of the concrete is bound to change because of the difference in molar volumetric expansion of the two materials [27]. However, this is not the case for all concrete and pore of carbonated concrete with blast furnace slag rather increased [28].  
Figure 6: Comparison of effective CO\({}_{2}\) diffusivity of concrete (D\({}_{\rm e}\)) and apparent CO\({}_{2}\) diffusivity (D\({}_{\rm a}\)) before carbonation.  
Figure 8 illustrates the difference between effective CO2 diffusivity and apparent diffusivity of concrete after carbonation. Like the comparative ratio in Figure 5, the ratio of carbonated concrete showed a similar trend.  
Figure 9 shows the result of comparing the diffusivity concrete before and after carbonation. The ratio of the two diffusivities decreased as time passed. and the width of the w/c ratio decreased with elapsed time. In concrete with a high w/c ratio, the trend of decreasing the diffusivity due to carbonation with time was pronounced. Carbonation led to reducing the effective diffusivity of concrete to the level of 47-69%. This reduction ratio tended to decrease as w/c ratio of concrete was low and time elapsed.  
Figure 8: Comparison of effective CO2 diffusivity of concrete (De) and apparent CO2 diffusivity (Da) after carbonation.  
Figure 7: Effect of carbonation on CO2 effective diffusivity. (a) w/c 0.45; (b) w/c 0.50; (c) w/c 0.55.  
Based on all results above, this study is believed to be useful in expressing the CO\({}_{2}\) diffusivity as a function of the mixing conditions of concrete and degree of cement hydration. As a material parameter plays a decisive role in the accuracy of durability design system for concrete structures, CO\({}_{2}\) diffusivity with time is very important to calculate the service life of the concrete in detail. This work will be combined with the system in the future.