Evaluation of CO2 captured in alkaline construction sludge associated with pH neutralization

Recently, the capture of carbon dioxide (CO2) using alkaline waste and byproducts has garnered considerable interest. Construction sludge may be categorized as alkaline waste, as it often exhibits high alkalinity during its generation or intermediate treatment. Hence, researchers have attempted to accelerate pH neutralization and recycle alkaline construction sludge by curing it under a high CO2 concentration. By exposing concentrated CO2 gas to an alkaline sludge, cement hydrates such as calcium hydroxide and calcium–silicate–hydrate gels form calcium carbonate (CaCO3). Subsequently, the generation of CaCO3 is expected to reduce the pH of the sludge. However, the amount of CO2 captured in sludge has not been investigated extensively, unlike those of other alkaline wastes. Therefore, the amount of CO2 captured in alkaline sludge that is associated with pH neutralization is evaluated in this study. It is demonstrated that accelerated carbonation tests using a CO2 incubator and carbonate content evaluation tests based on the gas pressure method successfully reveal the amount of CO2 captured in the alkaline sludge that is associated with pH neutralization. Additionally, the test results show that the amount of mCO2 (i.e., the amount of CO2 captured per 1 g of dry mass of alkaline sludge) increases with ΔpH (ΔpH is the difference between the initial pH and the pH after the alkaline sludge is neutralized). A maximum of 0.021 g of CO2 is captured per 1 g of dry mass of alkaline sludge when the addition ratio of quicklime AQL = 3% and 0.040 g when AQL = 6%. The CO2 capture ratio mCO2/mCO2max, which represents the ratio of CO2 captured in the sludge to the maximum capturing capacity, increases with ΔpH. CO2 capture ratios of up to 90.0% and 84.9% are recorded when AQL = 3% and AQL = 6%, respectively. It is discovered that a higher AQL results in a higher mCO2. Moreover, the test results indicate that a higher AQL causes a more significant change in the CO2 capture ratio, even when the pH decreases slightly.