Effect of carbonation process on the engineering properties of cement concrete.
Carbonation of concrete is associated with the corrosion of steel reinforcement and with shrinkage. However, it also increases both the compressive and tensile strength of concrete, so not all of its effects on concrete are bad.
Carbonation is the result of the dissolution of CO2 in the concrete pore fluid and this reacts with calcium from calcium hydroxide and calcium silicate hydrate to form calcite (CaCO3). Aragonite may form in hot conditions.
Within a few hours, or a day or two at most, the surface of fresh concrete will have reacted with CO2 from the air. Gradually, the process penetrates deeper into the concrete at a rate proportional to the square root of time. After a year or so it may typically have reached a depth of perhaps 1 mm for dense concrete of low permeability made with a low water/cement ratio, or up to 5 mm or more for more porous and permeable concrete made using a high water/cement ratio.
Hi! These articles are excellent about durability and effect of carbonation process:
Article Effect of carbonation on strength development of cement-trea...
http://iopscience.iop.org/article/10.1088/1757-899X/263/3/032011/pdf
Article Effects of Carbonation on the Microstructure of Cement Mater...
Carbonation decease the PH of concrete so that it will prepare the suitable zone for corrosion process
Carbonation of concrete is associated with the corrosion of steel reinforcement and with shrinkage. However, it also increases both the compressive and tensile strength of concrete, so not all of its effects on concrete are bad.
Carbonation is the result of the dissolution of CO2 in the concrete pore fluid and this reacts with calcium from calcium hydroxide and calcium silicate hydrate to form calcite (CaCO3). Aragonite may form in hot conditions.
Within a few hours, or a day or two at most, the surface of fresh concrete will have reacted with CO2 from the air. Gradually, the process penetrates deeper into the concrete at a rate proportional to the square root of time. After a year or so it may typically have reached a depth of perhaps 1 mm for dense concrete of low permeability made with a low water/cement ratio, or up to 5 mm or more for more porous and permeable concrete made using a high water/cement ratio.
The hydrated concrete has a tendency of combining with carbon dioxide, CO2 present in the atmosphere and forming carbonates, which partly neutralizes the alkaline nature of concrete. This process is known as carbonation. When carbonation depth exceeds depth of cover to the reinforcement, the salt ions find a suitable environment leading to greater corrosion.
it can increase the compressive strength, since the new formed product CaCO3 has finer particle size, which act as filler.
but i am not fully convinced that it increases the tensile strength for the reinforment concrete.
for the reinforcement concrete, carbonation results in decreaseing the pH fron 12-13 to around 9, which make the enviromnmet more acidic,this results in corrosion and later decreasing the tensile strength.
many factors control the carbonation process, i guess the above mentioned papers would have more details.
carbonation is a beneficial phenomenon for concrete alone, because it densifies the cement matrix which leads to decrease the porosity of the concrete therefore to increase the compressive strength. but it is bad for a reinforced concrete, because it attacks the steel bars and causes their corrosion by decreasing the pH of the interstitial solution, which causes the deterioration of the reinforced concrete structures and then decrease their service life.
When carbonation depth exceeds depth of cover to the reinforcement, the salt ions find a suitable environment leading to greater corrosion.
Surface hardness and compressive strength of concrete increase, while pH decreases.
Carbonation makes it more permeable the concrete and reduces pH (reduces ph), therefore, the risk of corrosion is high if carbonation reaches the reinforcement
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