Influence of Accelerated Carbonation Conditions on the Physical Properties Improvement of Recycled Coarse Aggregate

The preparation of new-generation concrete from recycled coarse aggregate (RA) is an effective way to realize the resource utilization of construction waste. However, loose and porous attached mortar leads to RA showing low-density, high-water absorption, and high crushing value. However, carbonation modification treatment can effectively improve the performance of RA. This paper studied the effects of carbon dioxide (CO₂) concentration, gas pressure, and moisture content on the RA physical properties (apparent density, water absorption, crushing value, and soundness) of waste concrete. The results showed that, when the (CO₂) concentration increased from 20% to 60%, the apparent density of RA after carbonation increased by 0.23-0.31%, the water absorption decreased by 0.57-0.93%, the crushing value decreased by 0.36-0.61%, and the soundness decreased by 0.47-0.85%. When the (CO₂) concentration was further increased from 60% to 80%, the apparent density of RA after carbonation was increased by 0.04-0.05%, the water absorption was improved by 0.15-0.31%, the crushing value was reduced by 0.06-0.07%, and the soundness was reduced by 0.09-0.11%. During the carbonation modification process, the performance of RA was significantly enhanced when the moisture content was 3.4% and the dissolution of hydration products was accelerated. The diffusion rate of CO₂ and the carbonation reaction rate decreased with the high moisture content of RA. As gas pressure increases to 0.01 MPa, the physical properties of RA change significantly, because gas pressure promotes the carbonation reaction between hydration products and CO₂ in attached mortar. As the gas pressure increased to 0.5 MPa, RA's apparent density gradually increased, while its water absorption, crushing value, and stability gradually decreased. The result improved RA's performance. SEM images show that carbonation modification of RA under different gas pressures increases CaCO₃ in attached mortar, filling the Interfacial Transition Zone (ITZ), and decreasing crack width and number. Gas pressure accelerates CO₂ diffusion and reaction with hydration products, resulting in narrower ITZ and dense mortar.