Abstract Pidgeon magnesium slag (PMS) is produced during the production of metallic magnesium by the Pidgeon reduction process, and the disposal of PMS causes serious environmental problems. In this study, the physicochemical characteristics and solubility of PMS were comprehensively investigated. Use of Fourier transform infrared to characterize PMS and the synthesis of a slow-release K fertilizers using PMS as raw material were reported for the first time. Major chemical composition of PMS was determined to be 52.57% CaO, 26.31% SiO2, 6.89% MgO, 1.44% Al2O3, and 3.32% Fe2O3. Major constituent phases were 23.6% β-Ca2SiO4, 53.7% γ-Ca2SiO4, 6.6% periclase, 4.5% free-CaO, and 11.6% the rest phase; minor phases were Ca(OH)2, Mg(OH)2, CaCO3, and MgCO3. PMS was bound within a stable crystalline phase, as occurs in natural ore deposits; heavy metal content and the average values of IRa and Ir of PMS were lower than the limit values of organic−inorganic compound fertilizer (GB 18877-2009) and national standard l...