This paper aims to investigate the effectiveness of the mechanical activation of local water-cooled slag (WCS), to protect mortars exposed to aggressive environments. OPC mortar samples made with different contents of WCS (0, 20, 35 and 50%, by weight of OPC). WCS with various finenesses (0.5, 2.0 and 3.3 m2/g) were prepared, and mortar samples were exposed to different environments of sulphates, chlorides and combined solution of sulphates and chlorides. The examined mortar properties included the compressive strength, expansion, weight gain, and determination of chloride profile and penetration depth. 50 mm cubical specimens were used for determining the compressive strength at various ages of 8, 16, 26 and 52 weeks, while 25.4x25.4x285 mm prisms were utilized for measuring both the weight and length change regularly every two weeks through the period of one year. Two 75x150 mm cylinders were prepared from each mortar mix to establish chloride profile and depth of chloride penetration into the cover zone. The results showed that mechanical activation of the local slag had led to appreciable reduction in the degradation of OPC mortar. The expansion, weight gain and chloride penetration were significantly reduced due to the use of WCS in mortar. The environment containing sulphate ions only was the harshest on mortar samples regarding length and weight change.