# Time Evolution of CO2 Diffusivity of Carbonated Concrete
###### Abstract  
Carbonation of cementitious materials is one of main causes of reinforcement corrosion and CO2 diffusivity influenced by microstructural characteristics of the cementitious materials is a decisive parameter for the carbonation rate. This study focused on establishing a multifactor functional model to calculate the CO2 diffusivity of carbonated cementitious materials. Because CO2 gas flows through carbonated zone, it is necessary to estimate CO2 diffusivity of carbonated concrete. Many factors on the CO2 diffusivity, such as the diffusivity in vapor, tortuosity, microstructural characteristics of cement paste, contribution of aggregate, and reduction of porosity due to carbonation, were considered. Apparent and effective CO2 diffusivity were calculated according to the absence or presence of moisture in the pore system of concrete, and the results were compared with previous research. CO2 diffusivity; carbonation; reinforcement corrosion; moisture +
Footnote †: journal: Applied Sciences  
_Article_  
Time Evolution of CO2 Diffusivity of Carbonated Concrete  
[PERSON] _et al._ (2020) and [PERSON] _et al._ (2020); 1 Construction Information Engineering, Induk University, Seoul 01878, Korea; [EMAIL_ADDRESS]  
2 Civil Engineering, Keimyung University, Daegu 42601, Korea  
Meanwhile, diffusivity of cementitious materials depends on time because ongoing cement hydration of cement can lead to significant densification of pore system of the cementitious materials [8]. Moreover, diffusivity of concrete is affected by various factors such as (a) pore structure system with the type of cement; (b) all mixing proportional properties of concrete; (c) interfacial transition zone between cement paste and aggregate; (d) environmental conditions such as temperature and humidity, and so on. Therefore, it is difficult to define the diffusivity of concrete systematically. In fact, in recent decades, various methodologies for defining the diffusivity of concrete have been proposed from empirical solution to computational model [5; 6]. Practical and realistic model can be realized by means of a simple formulation with affecting multi-factors. However, the relationship between each factor and the diffusivity or interference effect was not clear in previous research [9; 10]. CO\({}_{2}\) diffusivity of concrete has a functional relationship with the pore system of cement paste during the time when the diffusion of CO\({}_{2}\) takes place through already carbonated concrete. It is usually assumed that carbonation front progresses after all Ca(OH)\({}_{2}\) has been transformed. It is important to examine knowledge about the diffusion properties of the carbonated concrete. Carbonation process is assumed to consist of a diffusion of CO\({}_{2}\) through carbonated zone to a front where all of the CO\({}_{2}\) is assumed by reaction with cement hydrates. The gradient for the diffusion process is the concentration gradient of free CO\({}_{2}\) from surface concrete to carbonation depth, as shown in Figure 1. According to the figure, CO\({}_{2}\) concentration is zero at the carbonation front, while CO\({}_{2}\) gas reacts with cement hydrates at carbonation zone and CO\({}_{2}\) gas does not exist at noncarbonated zone. This implies that CO\({}_{2}\) flows through the carbonation zone by carbonation rate depicted with CO\({}_{2}\) diffusivity, reaches the noncarbonated zone, and reacts with the cement hydrates. Therefore, CO\({}_{2}\) diffusivity should be estimated, based on the microstructure characteristics and changed system of microstructure due to carbonation. Figure 1: Importance of CO\({}_{2}\) diffusivity as a decisive parameter for controlling carbonation rate of concrete. In this figure, the degree of carbonation of concrete is expressed as the hydroxyl concentration of cement hydrates. In the carbonated area, the pH of the pore solution is greatly reduced, and the rate depends on CO\({}_{2}\) concentration in the atmosphere as an environment response. For noncarbonated area, it has an original pH value.