In this study, cement additives are developed by substituting granulated blast furnace slag and steel slag ( SS) with the mixture of clinker-plaster. Mortar specimens of cement are prepared with these additives. Measurable properties of the series of cement additives such as the fineness moduli, Blaine values and specific gravities are computed before the casting. The Vicat setting times of specimens are determined according to the terms stated in TS EN 196-3 and the changes in their expansion levels are also detected. Furthermore, the cements produced and the mortar specimens casted are undergone thermal processes at the temperatures of 20, 100, 200, 300, 400, 500, 600, 700 and 800 degrees C, and mechanical and physical experiments are carried out for each series after the thermal processes. In all the series, sudden losses in strength are observed during the thermal process beyond 600 degrees C. Consequently, M3a series specimens that are produced with 80% clinker and gypsum in 12% blast furnace slag and 8% SS yield the best results in terms of compressive strength, elasticity modulus and decomposition.