Abstract

Objective This study investigates the property changes in recycled concrete aggregates （RCA） before and after carbonation treatment， focusing on the reinforcement mechanisms that carbonation exerts on both macro-properties and microstructural characteristics through multi-scale analysis. The correlation between carbonation processes and property enhancement in RCAs was established for the first time， aiming to enhance the resource utilization efficiency of construction waste while contributing to carbon sequestration and emission reduction. The findings provide valuable theoretical support for advancing low-carbon recycling technologies in the construction sector， aligning with the "carbon peak and carbon neutrality" goals.

Methods In this study， RCA were crushed and sieved into three categories of particle size （5~10 mm， 10~20 mm，and 20~26 mm）. The aggregates underwent pre-wetting treatments of 0， 1， 12， and 24 hours before being subjected to carbonation under controlled conditions （20% CO₂ concentration and 0.4 MPa pressure） for 6 hours. The effect of prewetting duration on carbonation efficiency was analyzed， and further investigations were conducted based on the identified optimal pre-wetting duration （1 hour） to examine the variations in mass growth rate for the three particle sizes after carbonation durations of 6， 12， and 24 hours. A comparative analysis was performed to evaluate the effects of different carbonation durations on key physical properties， including apparent density， water absorption， crushing value， and microstructural characteristics of the aggregates.

Results and Discussion The carbonation treatment showed optimal properties after a 1-hour pre-wetting， with the 5~10 mm fraction demonstrating enhanced reinforcement due to its higher specific surface area. Under the controlled carbonation conditions （20% CO₂ concentration and 0.4 MPa pressure）， the 5~10 mm aggregates exhibited a 3.34% increase in apparent density （2.430~2.512 g·cm^-3）， a 28.42% reduction in water absorption （4.12%~2.95%）， and a 13.36% decrease in crushing value （18.7%~16.2%） after 6 hours of carbonation. The CaCO₃mass loss was 18.16%， which was attributed to the crystalline phase transformation detected through XRD analysis and corresponded to a carbon sequestration efficiency of 41.27%， approximately 2.29 times higher than the non-carbonated control group （12.57%）. In contrast， the large aggregates （20~26 mm） displayed a carbon sequestration efficiency of only 32.02%， which was 22.41% lower than the smaller aggregates. This size-dependent difference originated from variations in CO₂ diffusion kinetics， where the smaller aggregates， with their shorter gas transport paths， allowed for more complete carbonation reactions.

Conclusion Carbonation treatment results in the formation of dense calcium carbonate crystals on the surface of RCA，filling the pores within the aggregates， thereby increasing apparent density and reducing both water absorption rate and crushing value. Factors such as pre-wetting duration， carbonation time， and particle size significantly affect the carbonation efficiency. A shorter pre-wetting duration and longer carbonation time lead to optimal properties in smaller particle size aggregates.

Keywords：carbonation； recycled concrete aggregate； water absorption rate； crushing value； calcium carbonate