AbstractFlue gas desulphurization (FGD) gypsum is a kind of industrial solid waste containing impurities (i.e., heavy metal ions Zn2+ and Ni2+), which may lead to threats to the ecological environment and public health. To safely use the FGD gypsum, this study investigated the mechanical strength, releasing property of SO42- and leachability of heavy metal ions of a series of FGD gypsum (40%) based mortar with various cement to slag ratios. The hydration process, pore structure, and morphology of the sample were then characterized and analyzed by isothermal calorimetry, H1 low-field nuclear magnetic resonance (NMR), and scanning electron microscope with energy dispersive X-Ray analysis (SEM-EDx). It was found that the highest mechanical strength could be obtained when the C/S ratio was 1:5, and the decrease of C/S ratios from 4:2 to 1:5 leads to less SO42- releasing from the mortar. The heavy metal leachability of the mortars is lower than that of the limitation of solid waste stipulated by the specification of GB5085.3-2007, and less of the heavy metal are leached from the sample as the C/S ratios decrease. The heat flow for the paste sample with 40% of FGD gypsum was delayed as the cement to slag ratio decreased from 5:1 to 2:4. In addition, the released amount of Zn2+ ions linearly decreases with the pore size of gel pores and capillary pores but increases with the pore size of macropores, and the ettringite in the paste exhibits the transformation from the needle to the clubbed shape when smaller cement to slag ratio was used. This work helps to understand how to reduce the heavy metals leachability of FGD gypsum from cementitious material and provides baseline for safely using the FGD gypsum for the preparation of building materials.