Nowadays, the environmental aspect of industrial waste management has got urgent. The reusing of these residues in the civil building is increasing, since all industries have developed a growing concern for the emptying of natural resources. The ladle furnace slag (LDF-slag), a by-product of the secondary refining of steel, is mainly composed of calcium and calcium magnesium silicates. It has the potential to be a sustainable binder for mortars because the composition of LDF-slag is fairly similar to Portland cement (PC) clinker. Thus, this paper aims to improve the cementitious properties of LDF slag. The properties of cement mortars with LDF-slag were investigated including water demand of normal consistency, setting time and compressive strength. The experimental results show that the suitable specific surface area of LDF-slag is 1200 cm2/g (cement replacement amount is 20 wt%) and addition of LDF-slag with different fineness has retarding effect on cement.