The use of waste is rapidly becoming a supra-disciplinary field in most parts of the world where the use of industrial wastes like fly ash, granulated steel slag, silica fume, and waste fibers in construction has become very popular since the last half of the 20th century. Other forms of industrial wastes are also re-used even for more sensitive applications on soils to upgrade soil texture. For example, waste from bauxite refining (red mud) is known to be extensively re-utilised. These concepts are yet to take tangible hold in Africa, despite the huge resources available. Electric-Arc Furnace Steel slag is a major waste product from the steel industry involving the melting of scrap to make steel in an electric arc furnace. Use of such waste materials in construction alleviates the huge environmental pollution problem which often exists in areas where they are produced and/or dumped. Currently, the material is mainly used in construction works as unbound aggregate for asphalt concrete pavements, or as road base in many countries. However, it consists predominantly of oxides and silicates of magnesium, calcium, aluminium, iron and thus can be used as substitute for cement. This paper compares the effect of utilising this type of Steel slag and Granulated Blast Furness Slag, as partial replacement for Portland cement. The influence of the physical and chemical characteristics of the two materials on the setting time, compressive strength, total porosity and pore-size distribution of cement pastes have been evaluated. For the experimental conditions studied, the result reveal adequate properties for high levels of replacement but suggests that superior qualities, compared with Portland cement concrete is possible only if replacement levels do not exceed about 10%.