Effect of cement slurry impregnation-carbonation strengthening on the properties of low-quality recycled concrete aggregate

Recycled concrete aggregate (RCA) presents several challenges such as irregular shape, high water absorption, high crushing value, and low apparent density, which restrict its application in concrete. Although carbonation technology offers a more effective solution, its effect on inferior RCA remains unclear. Compared to its common counterpart, inferior RCA is characterized by longer aging time, lower crushing value, and reduced carbonizable substances. This study focuses on low-quality RCA, employing a method of impregnation and carbonation with cement slurry to strengthen the properties of RCA. The investigation studied the carbonation system (hydration time + carbonation time), water-cement ratio of the cement paste, and carbonation effects and mechanisms via mercury injection, nanoindentation tests, X-ray diffraction, and scanning electron microscopy. The findings indicate that the optimal carbonation system involves hydration for 1 d and carbonation for 14 d (H1d + C14d). In this system, the water-cement ratio exhibits minimal influence on carbonation depth. After the impregnation-carbonation treatment, RCA displayed the best performance improvement at a water-cement ratio of 0.8 (0.8-RCA). This condition resulted in a 15.41% reduction in crushing value and an 18.00% decrease in porosity alongside a 21.42% increase in apparent density. Moreover, the interfacial transition zone (ITZ) between aggregate and mortar and the elastic modulus of adhesive mortar are improved after strengthening. Compared with the primitive RCA, the distribution of Ca2+ in the adhesive mortar of the strengthened RCA was more uniform and denser. The moderately fluid cement slurry penetrated the RCA pores easily, enabling CaCO3 produced by carbonation to efficiently fill defects.