Carbonation treatment to enhance mechanical and durability performances of RCA concrete

One way of reducing C&D waste, achieving environmental benefits and promoting a circular economy in the construction industry is by producing new concrete materials using recycled concrete aggregates (RCA). Unfortunately, RCA exhibit higher water absorption and lower density than natural aggregates. As a result, new concretes made from RCA also display poor mechanical and durability performances compared to ordinary ones. Recently, carbonation treatment has emerged as an efficient process for enhancing the quality and properties of RCA and allowing permanent carbon dioxide sequestration.

The main effect of a carbonation treatment is the precipitation of calcium carbonate that promotes RCA crack filling and the decrease of porosity. The improvement extent of the quality and performance of the carbonated RCA is highly influenced by treatment conditions (e.g. duration, CO₂ concentration, gas flow rate and temperature) and RCA characteristics (e.g. water content particle size and the strength of parent materials). Although concrete made from carbonated RCA exhibits higher compressive strength and durability performances than that made from untreated RCA, its performances are still lower than that of concretes made of natural aggregates. However, despite the increasing amount of research in this direction, the research on the durability properties of carbonated RCA-based concrete is still limited. In addition, proper optimization of the concrete mix design to gain environmental and performance advantages remains a big challenge.

Herein, Italian scientists, Dr. Nicoletta Russo and Professor Federica Lollini from Politecnico di Milano investigated the effects of carbonated RCA on the durability and mechanical properties of hardened concrete made from RCA. Durability and mechanical tests were performed on three types of concretes made with water/cement ratio of 0.5: first with fully natural aggregates, second with partial replacement with untreated RCA and third with carbonated RCA. RCA and carbonated RCA were obtained by crushing old alkaline and old carbonated concretes, respectively, with compressive strength in the range 40 – 90 MPa. A fraction of the carbonated RCA was also subjected to further carbonation treatment. Their work is published in the Journal of Building Engineering.

The authors observed a decrease in the water absorption in carbonated RCA than untreated RCA, partially attributed to the calcium carbonate precipitation in the pores. Further carbonation treatment of the RCA filled the crushing-induced cracks leading to a further reduction in water absorption and an improved quality of the aggregates. After 28 days of curing, the compressive strength of carbonated RCA concrete was higher than RCA concrete, gaining about 3%. However, both compressive strengths of RCA and carbonated RCA concretes were slightly lower than that of the reference concrete.

Considering durability performances, the water absorption of the RCA concrete was comparable to that of carbonated RCA concrete. When the untreated RCA aggregates were replaced with carbonated RCA, concrete showed similar water absorption, while sorptivity decreased by 10%, and the chloride diffusion coefficient decreased by 25 – 40%. However, the overall performances of carbonated RCA concrete were still lower than that of the reference concrete. It was worth noting that tailoring the mix design and prolonging the carbonation treatment duration could endow carbonated RCA concrete with mechanical and durability properties comparable to the reference concrete.

In summary, Dr. Russo and Professor Lollini evaluated the performance enhancement of concrete made of carbonated RCA to validate the potential of carbonation treatment. Overall, carbonated RCA concrete performed better than RCA concrete but lower than the reference concrete. In a statement to Advances in Engineering, Professor Federica Lollini said their findings contribute to designing high-performance carbonated RCA concrete for practical applications.