Atmospheric CO2 penetrates the concrete's pores and, in the presence of moisture, forms carbonic acid, which then reacts with calcium hydroxide in the hydrated cement, forming calcium carbonate. This process reduces the concrete's volume and is termed carbonation shrinkage.
The concrete's permeability is slightly reduced as calcium carbonate produced during the reaction fills its pores. Furthermore, its strength is slightly enhanced as the water released during the reaction facilitates the hydration of the unreacted cement. However, carbonation has a more concerning effect by neutralizing the alkaline nature of the cement paste, and the risk of corrosion increases if carbonation reaches the reinforcement steel and allows moisture and oxygen to interact with it.
Carbonation is more significant in environments where concrete is shielded from direct rainfall yet exposed to moist air, compared to that which is periodically rinsed by rain. This is because the water-filled pores slow down CO2 diffusion. Carbonation typically progresses from the surface inward, and its rate is influenced by the concrete's permeability, moisture content, CO2 levels, and ambient relative humidity.
To measure carbonation depth, a freshly broken surface of concrete is treated with a phenolphthalein indicator. The non-carbonated areas on the treated surface turn pink due to the presence of calcium hydroxide, while carbonated areas do not change their color.
In the presence of moisture, atmospheric carbon dioxide entering the concrete surface transforms into carbonic acid, which chemically reacts with the calcium hydroxide in the hydrated cement to form calcium carbonate and water.
The calcium carbonate produced during the reaction deposits in the concrete pores, while the released water aids the hydration of unreacted cement.
This results in the contraction of the concrete, known as carbonation shrinkage.
Carbonation starts at the concrete's surface and slowly moves inward.
Its progress depends on the concrete's permeability, moisture level, carbon dioxide content, and relative humidity of the ambient air.
As carbonation advances, the alkaline nature of the hydrated cement paste neutralizes.
Consequently, if oxygen and moisture penetrate the concrete, corrosion of the steel reinforcement embedded within it is likely to occur.
Commonly, to assess the extent of carbonation in concrete, a freshly broken surface is sprayed with phenolphthalein.
Areas that turn pink reveal the presence of free calcium hydroxide, indicating the absence of carbonation, while the areas that retain their original color indicate carbonation.
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