Use of copper slag (CS) as a replacement for fine aggregate (FA) in RC slender columns was experimentally investigated in this study. Twenty columns measuring 150 mm x150 mm x 2500 mm were tested for monotonic axial compression load until failure. The concrete mixture included ordinary Portland cement (OPC) cement, fine aggregate, 10 mm coarse aggregate, and CS. The cpercentage of cement, water and coarse aggregate were kept constant within the mixture, while the percentage of CS as a replacement for fine aggregate varied from 0 to 100%. Four 8 mm diameter high yield steel and 6 mm mild steel bars were used as longitudinal and transverse reinforcement, respectively. Five cubes measuring 100 mm x100 mm x100 mm, eight cylinders measuring 150 mm x 300 mm and five prisms measuring 100 mm x 100 mm x 500 mm were cast and tested for each mixture to determine the compressive and tensile strengths of the concrete. The results showed that the replacement of up to 40% of the fine aggregate with CS caused no major changes in concrete strength, column failure load, or measured flexural stiffness (EI). Further increasing the percentage reduced the concrete strength, column failure load, and flexural stiffness (EI), and increased concrete slump and lateral and vertical deflections of the column. The maximum difference in concrete strength between the mixes of 0% CS and 100% CS was 29%, with the difference between the measured/ control failure loads between the columns with 0 and 100% CS was 20% the maximum difference in the measured EI between the columns with 0 and 100% CS was 25%. The measured to calculated failure loads of all specimen varied between 91 and -100.02%. The measured steel strains were proportional to the failure loads. It was noted that columns with high percentages of CS (≥ 60%) experienced buckling at earlier stages of loading than those with lower percentages of CS.