In this study, thermal treatment of phosphogypsum(PG) at different temperatures 200,400,600 and 800 oC were attempted to purify PG and improve its quality to make it fit for the manufacture of Portland slag cement (PSC). PG is a waste of phosphoric acid manufacturing by dehydrate process in Egypt. It is a fine powder with high calcium sulphate content. This waste causes various environmental problems when discharged directly to the environment. The thermally treated PG was found to have lesser amount of impurities of phosphates, fluorides and organic matter than the impure material. In this investigation, preparation of Portland slag cement (PSC) in laboratory was carried out by mixing Portland cement clinker (PCC) and Blast-furnace slag (BFS) with thermally treated PG at different temperatures instead of raw gypsum (RG). The characteristics of prepared mortars were investigated by determination of compressive strength, bulk density and total porosity. The hydration kinetics of cement mortars was evaluated by determination free lime and chemically combined water contents. IR spectroscopic analysis was used to investigate the change in structure of mortars after curing... The change in morphology and microstructure of some hardened pastes were investigated by scanning electron microscopy (SEM) tests. The results show that, Portland slag cement produced using 6% thermally treated PG at 800oC improves its hydraulic properties.