Carbon dioxide, concrete affected

Effects of Carbon Dioxide. Concrete is known to be affected by the take-up of CO2 from ambient air, i.e. carbonation (32,34-37). 

Carbon dioxide is, of course, fundamentally important to plants because of photosynthesis. Most plant cell cultures are heterotrophic, non-photosynthetic and use a chemical energy source. It is reasonable to suspect, however, that some of the control mechanisms for the photosynthetic dark reactions would be regulated by C02 concentration. This could affect both cell growth and, indirectly, production of useful compounds. More concretely, C02 is known to promote synthesis of ethylene [38] on the other hand, C02 concentrations of 5-10% inhibit many ethylene effects [53]. 

Woods (32) states "The reaction between atmospheric carbon dioxide and dense hardened concrete is very slow, and even after a considerable number of years, may affect only a thin layer nearest the exposed surfaces. A principal product of the reaction is calcium carbonate, the presence of which may enhance the early resistance of concrete to attack by some chemicals in solution, such as sulfates. In practice, however, any beneficial effect that may exist appears to be of relatively small moment." The harmful effect of carbonation arises when the carbonated layer created on the surface of reinforced concrete over the years reaches the steel reinforcement. The alkaline protective layer is then considerably less alkaline, and the steel bars may start to rust. 

CCPs have many economic and environmentally safe uses (Table 14.14). For example, in construction, a metric ton of fly ash used in cement and concrete can save the equivalent of a barrel of oil and can reduce carbon dioxide releases that may affect global warming. The use of CCP saves landfill space and can replace clay, sand, limestone, gravel, and natural gypsum. 

Several variables affect the rate of carbonation. In general, low-permeability concrete is more resistant. Carbonation tends to proceed most rapidly at relative humidity levels between 50 and 75 percent. At lower humidity levels, carbon dioxide can penetrate into the concrete relatively rapidly, but little calcium hydroxide is available in the dissolved state for reaction with it. At higher humidity levels, the water-filled pore structure is a more effective barrier to the ingress of carbon dioxide. Clearly, environmental cycles of alternate dry and wet conditions will be associated with rapid carbonation damage. 

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