Carbonatation

Carbonatation is a chemical reaction in which calcium hydroxide reacts with carbon dioxide and forms insoluble calcium carbonate:

The process of forming a carbonate is sometimes referred to as "carbonation", although this term usually refers to the process of dissolving carbon dioxide in water.

Carbonatation is a slow process that occurs in concrete where lime (calcium hydroxide) in the cement reacts with carbon dioxide from the air and forms calcium carbonate.

The water in the pores of Portland cement concrete is normally alkaline with a pH in the range of 12.5 to 13.5. This highly alkaline environment is one in which the steel rebar is passivated and is protected from corrosion. According to the Pourbaix diagram for iron, the metal is passive when the pH is above 9.5.

The carbon dioxide in the air reacts with the alkali in the cement and makes the pore water more acidic, thus lowering the pH. Carbon dioxide will start to carbonatate the cement in the concrete from the moment the object is made. This carbonatation process will start at the surface, then slowly move deeper and deeper into the concrete. The rate of carbonatation is dependent on the relative humidity of the concrete - a 50% relative humidity being optimal. If the object is cracked, the carbon dioxide in the air will be better able to penetrate into the concrete.

Eventually this may lead to corrosion of the rebar and structural damage or failure.

The carbonatation process is used in the production of sugar from sugar beets. It involves the introduction of limewater (milk of lime - calcium hydroxide suspension) and carbon dioxide enriched gas into the "raw juice" (the sugar rich liquid prepared from the diffusion stage of the process) to form calcium carbonate and precipitate impurities that are then removed. The whole process takes place in "carbonatation tanks" and processing time varies from 20 minutes to an hour.

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