The production of cement results in emission of many greenhouse gases in atmosphere, which are responsible for global warming. Hence, this study motivated of using the waste material having cementing properties, which can be added to the concrete as partial replacement of cement, without reducing its strength, which will result in reduction of cement manufacturing thus decreasing in emission in greenhouse gases.  The granulated slag (GS) is a waste product from the steel industry, which may be used as partial replacement of cement in concrete. This is an experimental research of strength of concrete prepared with Portland cement, partially substituted by GS in different quantities varying from 0% to 50%. It is noted from the study that the strength of concrete is inversely proportional to the percentage of replacement of cement with GS. It is concluded that the 10% replacement of cement is possible without reducing the strength with 42 days curing.