The study demonstrated that increasing the concentration of the solvent enhances the efficiency of calcium extraction; however, it also causes the leaching of other elements, such as magnesium, which can impede the carbonation stage. The research also revealed that the extraction of calcium from the largest particles is constrained by mass transfer and the accessibility of calcium from the particle. A portable laboratory orbital shaking incubator was used for the leaching experiments. In addition, XRF and AAS were utilized for the sample characterization of solid residues and leach liquor, respectively. The authors hypothesize that the reaction products build up and hinder the reaction on the surface layer of the particles. On the other hand, with the smallest particle size, more calcium can be extracted before the surface is obstructed, and the maximum extraction efficiency has not been achieved yet. Based on the activation energy value of 70.51 kJ/mol, it is less likely that the leaching of blast furnace slag in ammonium nitrate is a product-layer-diffusion-controlled process, as the activation energy for this type of process is usually below 20 kJ/mol. Instead, the higher activation energy suggests that the leaching process may be controlled by surface chemical reactions or a mixed mechanism. However, more detailed analysis and experimental data would be required to confirm the reaction mechanism.