Carbon Absorption and Carbon Absorption Enhancement of Recycled Concrete Aggregate Masses

Carbonation of concrete is a well-known phenomenon as it lowers the pH-level within the concrete structure, compromising the integrity of steel reinforcements. This reaction with atmospheric carbon dioxide (CO₂) binds the carbon to the concrete permanently. When concrete structures are demolished and recycled, fresh surface area exposed to air contact and thus carbonation increases. This enhances the carbon absorption. Carbonation conditions within a mass of recycled concrete aggregate (RCA) in the field, however, remain somewhat unknown. The carbon dioxide content, relative humidity, and temperature were measured within four types of RCA piles. Two of the piles were sheltered from rain to effect water infiltration to the concrete piles. Two of the piles were made of concrete, sifted for removal of the finer particles (<20 mm in diameter) to enable air circulation (CO₂ contact) inside the mass of RCA and reducing its capacity for water retention. CO₂ levels inside normal masses of RCA that contained the finer particles ranged from 20 ppm to 100 ppm, depending on depth. CO₂ levels inside the sifted mass were constant around 400 ppm. With removal of the finer particles from the mass, more particles are in full air contact, which enables carbonation to take place throughout the mass. In masses including finer particles, CO₂ concentration is lower but still in the range of 20 ppm to 100 ppm, so particles still carbonate but more slowly. Using this particle size manipulation in applications or storage of RCA, the carbon sinking capacity of concrete can be enhanced.