Factors that Influence the Carbonation Rate

The rate of carbonation depends on both environmental factors (humidity, temperature, concentration of carbon dioxide) and factors related to the concrete (mainly its alkalinity and permeability). 

Humidity. The rate of carbonation varies with humidity of concrete for two reasons. First, as already seen in Chapter 2, diffusion of carbon dioxide within concrete is facilitated through the aerated pores, but it is very slow through those filled with water (the diffusion of CO2 in water is four orders of magnitude slower than in air). 

The rate of diffusion of CO2 consequently decreases with an increase in humidity of the concrete until it becomes zero in water-saturated concrete. This means that when the concrete is wet, CO2 does not penetrate it On the other hand, the carbonation reaction occurs only in the presence of water so that it becomes negligible in dry concrete. 

CO2 concentration. The concentration of carbon dioxide in the atmosphere may vary from 0.03% in rural environments to more than 0.1% in urban environments. Comparatively high concentrations can be reached under specific exposure conditions, such as inside motor vehicle tunnels. As the CO2 content in the air increases, the carbonation rate increases. Accelerated tests carried out in the laboratory to compare the resistance to carbonation in different types of concrete show that, indicatively, one week of exposure to an atmosphere containing 4% CO2 will cause the same penetration of carbonation as a year of exposure to a normal atmosphere [8]. Some researchers suggest that with a high concentration of CO2 the porosity of carbonated concrete is higher than that obtained by exposure to a natural atmosphere, particularly if the concrete has been made with blended cement or has a high cement content However, this is controversial, since it was shown that even 100 % CO2 under increased pressure, produced the same microstructure as natural carbonation [9]. 

Temperature. All other conditions being equal, especially that of humidity, which is, in general, the most important single parameter, an increase in temperature will raise the rate of carbonation. 

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