We have studied the effect of chromium ions and lead ions on the chemical stability of hardened slag paste with toxic wastes during the stabilization/solidification process. The influences of Cr and Pb ions on the hydration of slag were also investigated. Sodium silicate (Na2SiO3), 5 wt.% of slag, was used as an alkali activator for slag hydration. The physical stability of hardened paste containing partial replacement of slag with fry ash and gypsum was also examined. When gypsum was added to slag, the compressive strength of hardened slag paste developed, accompanying the activation of alumino-ferrite-tricalciumsulfate (Al2O3-Fe2O3-3CaSO(4), AFt) and alumino-ferrite-monocalciumsulfate (Al2O3-Fe2O3-CaSO4, AFm) phase generation. Those phases caused densification of the microstructure. Concurrently, the leaching amount of heavy metal ions was decreased. When fly ash was added to slag, the compressive strength increased and the leaching amount decreased with both active formation of aluminate hydrates and ion substitution. Lead ions were mostly stabilized through physical encapsulation by the hardened slag paste's hydrate matrix. In the case of chromium ions, we observed that it was mainly solidified through the formation of a substitutional solid solution with aluminum atoms in the structure of aluminate hydrates.