Mineral admixtures are frequently utilized as cement substitution materials in high-performance concrete (HPC), and so many studies have explored the influence of mineral admixtures on the rheological behavior of HPC. Investigations were done to examine the impact of nanosilica less than 100 nm on HPC by substituting copper slag at a fixed substitution of forty percent for fine aggregate. Concrete samples were cast by substituting cement with nanosilica at (0.5, 1, 1.5, 2, 2.5, and 3) percentages. Examinations on mechanical properties and durability were done on specimens. The above tests demonstrated an increase in water demand because of the increase in the nanosilica substitution percentage. Mechanical and durability properties were improved at a larger rate with the incorporation of nanosilica. The outcomes indicated that colloidal nanosilica is an effective material that enhances the microstructure and acts as a catalyst for pozzolanic activity. The incorporation of nanosilica improves the strength up to two percentage substitution level.