Experimental Study on Early Mechanical Properties of Recycled Aggregate Concrete after NaHCO₃ Carbonation

In order to improve the carbon fixation efficiency and mechanical properties of recycled aggregate concrete (RAC), sodium bicarbonate (NaHCO₃) solution was used for soaking curing and accelerated carbonation of RAC. Cube and prism compression tests and four point bending tests were conducted to study the effects of three carbonation environments (natural carbonation, carbonation box carbonation, NaHCO₃ solution carbonation) and carbonation age (3-28 d) on the early mechanical properties of RAC. The results showed that compared to the natural environment, the carbonation environment of NaHCO₃ solution can increase the early compressive strength and flexural strength of RAC by up to 8.4% and 12.4%, respectively, but it can also increase brittleness. Before 21 days, the early compressive strength and strength development of RAC after carbonation of NaHCO₃ solution were slightly lower than those in the carbonation environment of the carbonation box, but significantly improved at 28 days; The carbonation age has little effect on the flexural strength of RAC, but at the same age, the flexural strength of NaHCO₃ solution after carbonation is the highest. Through thermogravimetric analysis, it was found that the calcium carbonate content in RAC after carbonation of NaHCO₃ solution was 10.3% higher than that after carbonation in the carbonation box, and 16.5% higher than that of natural carbonation. Finally, a calculation method and constitutive equation for the early mechanical properties of RAC after carbonation of NaHCO₃ solution were proposed.