This study is aimed at investigating the effect of aggressive media on the strength properties of mortar made with ordinary Portland cement (OPC) used in construction in Nigeria. Mortar specimens were prepared using ordinary Portland cement and subjected to Sea water, tannery waste effluent, and prepared NPK and Urea solutions. The samples were immersed in the media at immersion ages of 3, 7, 28 and 56 days. The results obtained also revealed the aggressiveness of the solutions of Sea Water, Tannery waste effluent, NPK and Urea solutions as they led to decrease in rates of strength development of mortar specimens. Corrosion rate is highest in Sea Water at early age and this is followed by Tannery effluent, NPK and Urea respectively. It is also found that the entire aggressive medium led to decrease in strength of mortar specimens after two months of immersion. The aggressivity followed the same pattern with both flexural and compressive strength with its effect most in flexural strength.