A key question concerning the application of recycled concrete aggregates (RCA) into pavement is the assessment of self-cementing properties of RCA, which increase the strength and stiffness of unbound base and subbase layers. In this paper, the assessment of self-cementing properties of RCA, the hydration of unhydrated binder and their influence on the microstructure and mechanical behaviour were studied by subjecting two different RCA (NRCA and ORCA, showing significantly different self-cementing properties), after curing for 0, 360 and 720 days, to the thermogravimetric analysis (TGA), scanning electron microscopic analysis (SEM) and monotonic triaxial tests. The results show that the unhydrated binder content of NRCA is low (between 6.2% and 7.9% in fines), and most of the unhydrated binder was hydrated after curing for 360 days. Besides, the sulfate attack products, ettringite, were also observed from both TGA and SEM, which could also lead to the self-cementing properties. The hydration products of unhydrated cement and sulfate attack products not only form bonds between particles, but also fill the small pores (width less than 3 μm), turning the open type of an unbound structure to a rigid dense skeleton structure.