This study investigated the strength of mortars made with cements incorporating Ground Granulated Blast-Furnace Slag (GGBFS) and Ground Steel Slag (GSS) as partial replacement of Portland cement clinker in different ratios of replacement and fineness. Combination of 70% GGBFS and 30% GSS was partially replaced from 20% to 80% of Portland cement clinker. Tests were performed for sulphate resistance, chloride attack, and high temperature resistance tests. Test results showed that as fineness and replacement ratio of slag was affected volume expansions. Fine ground slag combination has increased sulphate attack durability. Increases in compressive strength were observed as replacement ratio was smaller than 40%. Behaviour of mortar specimens of high temperature resistance test are similar to that of the mortars made with ordinary Portland cement. It was concluded that 40% replacement is suitable for 3800 gr/cm2 Blaine fineness of slag combination. The replacement ratio can be raised to 80% if the Blaine fineness of the combination increased up to 4600 gr/cm2.