Abstract: Germanium (Ge), a waste residue leaching from zinc (Zn) smelting process, has potential cementitious properties and could be recycled as a cement supplement activated by chemical reagents. In this work, a test was conducted to determine the hydration properties of Ge slag-cement-based composites with Ge slag (GS)/ordinary Portland cement (PC) contents of 0wt%, 5wt%, 10wt%, 15wt%, 20wt%, and 25wt% and water-to-binder ratio (w/b) of 0.4. The activators Ca(OH)₂, AlCl₃, NaAlO₂, and Na₂CO₃ were mixed under 1wt%, 2wt%, 3wt%, and 4wt% dosages of GS weight. The composition and microstructure of the hydration products were investigated by the combined approaches of X-ray diffraction (XRD), thermogravimetry-differential scanning calorimetry (TG-DSC), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). First, the GS cementitious property is attributed to the high content of CaSO₄·2H₂O. Second, the activators affected the acceleration performance in the following order:NaAlO₂, Na₂CO₃, AlCl₃, and Ca(OH)₂. More importantly, the 28-day unconfined compressive strength (UCS) is 45.34 MPa at the optimum formula of 0.6wt% NaAlO₂, 15wt% GS, and 85wt% PC, which is 9.16% higher than the control. Thus, NaAlO₂ is beneficial for the ettringite (AFt) generation, resulting in the C-S-H structure compaction. However, the Zn²⁺ residue inhibited the AFt formation, representing an important challenge to the strength growth with curing age. Consequently, the GS could be recycled as a supplement to the cement under the activator NaAlO₂.