Flue-gas desulfurization (FGD) gypsum has occasionally been used as an additive to cement. Consequently, appropriate facilities are required to ensure the environmentally safe processing of FGD gypsum and the resulting cement material properties. Such facilities are yet to be developed because the amount of FGD gypsum used is still small when compared with the vast amounts of FGD gypsum generated. In this study, we analyze the effect of FGD gypsum addition on the physical properties, stabilization, and radon count of steam-cured mortar and compare its performance with air-cured mortar. Our results show that the steam-cured pozzolanic hydration products of ettringite and C-S-H promote the densification of the mortar structure, thereby resulting in nanopore size reduction and increased strength of FGD gypsum mortar subsequent to the steam-cured hydration process. Further, our environmental test results indicate that steam-cured pozzolanic materials composed of FGD gypsum are environmentally safer than air-cured cementitious materials.