The objective of this work is to understand the roles of carbonation induced CaCO3 (from basic oxygen furnace slag, BOFS) on the hydration, mechanical properties and microstructural change in the aluminum-rich GGBS cementitious system, and study comparatively with fresh BOFS and conventional limestone. The 1 day compressive strength results show that the composite paste containing 45 wt% GGBS mixed with 15 wt% carbonated BOFS attained only 38 % of the corresponding OPC paste, however, in comparison the presence of nano-sized rhombohedral and cauliflower calcite on the surface of carbonated BOFS (rougher than CaCO3 of limestone) is better to enhance the early hydration of the composite. More importantly, the 28 days compressive strength of carbonated BOFS-GGBS composite paste can reach 97 % of that of OPC paste, attributed to the carbonated BOFS actively grows together with the hydrated cement matrix by consuming the nano-CaCO3 shell and generating in-situ carboaluminates. The formed carboaluminates not only can enhance the bonding behavior of the cementitious matrix but also refine the pore structure as well as the mechanical properties of the composite paste.