Abstract: The effect of heavy metals on the properties and hydration of blast furnace slag–cement composites (BFS-CC) remain unclear. In this study, two BFS-CC (denoted as DBFS-CC and WBFS-CC) were prepared by dry and wet grinding of BFS, respectively. The effect of Cu(II) on BFS-CC’s properties and hydration was investigated by adding representative copper contaminants (CuO, CuCl₂, and CuS) to the composites. Adding 1.0wt% CuO and 0.5wt% CuS increased the 3-d compressive strength of DBFS-CC by 14.9% and 5.7%, respectively, but suppressed the 3-d strength of WBFS-CC. This trend reversed at 28-d curing, where adding 1.5wt% CuO, 2.0wt% CuCl₂, and 1.5wt% CuS enhanced the compressive strength of WBFS-CC by 23.4%, 6.2%, and 13.6%, respectively, but adversely affected the strength of DBFS-CC. For 28-d hydration, adding CuCl₂ decreased the hydration degree of DBFS-CC but enhanced that of WBFS-CC. Adding CuO promoted the hydration degree of both composites, while adding CuS exhibited inhibitory effects. DBFS-CC immobilized CuCl₂ better due to a higher hydration degree, while WBFS-CC immobilized CuO and CuS better due to having finer unhydrated BFS particles and a denser matrix. This study not only focuses on the Cu(II) immobilization effect but also reveals the differential effects of Cu(II) species on the hydration process, providing novel insights into heavy metal interactions in BFS-CC systems and their safe disposal.