Over the past decades, ground granulated blast furnace slag (GGBS) has been extensively used as a replacement for cement in concrete production to mitigate cement carbon emissions. Hence, a substantial portion of the hydrated cement paste discarded at the end of a concrete’s lifespan could contain a certain quantity of GGBS. This study aims to investigate the feasibility of using hydrated cement paste containing 30% GGBS (BSCP) in cementitious binders via moisture carbonation treatment. The changes in the microstructure of BSCP before and after carbonation are analyzed using thermogravimetry-differential thermal analysis (TG-DTG), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR) techniques. The carbonated BSCP (CBSCP) is then used to substitute a part of cement (0%, 15%, and 30% by mass), and the flowability and compressive strength of the blended CBSCP-cement paste are examined at 3, 7, and 28 days. The results show that carbonation can significantly alter the microstructure of BSCP by forming calcium carbonate crystals and consuming a significant amount of calcium hydroxide. Consequently, the inclusion of CBSCP in new cement paste results in a reduction in flowability and enhancement of early-age strength. Overall, it can be concluded that carbonated BSCP can be utilized as an emerging binder up to 30% in cement-based materials, without significantly compromising the later-age strength.