Effect of Partial Replacement of Fine Aggregate with Recycled Copper Slag and Fly Ash as Supplementary Cementitious Material on Mechanical and Durability Properties of M30 Grade Concrete

per year in India from Hindalco, Sterlite, and Hindustan Copper smelters, presents a significant industrial waste management challenge while simultaneously offering a dense, angular, highly abrasion-resistant aggregate substitute for fine aggregate (river sand) in concrete — addressing both the ecological cost of copper metallurgy and the growing deficit in natural river sand supply resulting from regulatory restrictions on riverbed mining. This experimental study investigates the combined use of copper slag as fine aggregate replacement (at 20%, 40%, 60%, and 80% replacement levels by weight of river sand) and Class F fly ash as supplementary cementitious material (20% cement replacement) in M30 grade concrete, evaluating fresh properties (slump, compacting factor), mechanical properties (compressive strength at 7, 28, and 56 days; split tensile strength; flexural strength at 28 days), and durability properties (water absorption, sorptivity, chloride ion penetration by RCPT, and sulphate resistance at 90 days) across nine mix designs. The optimal formulation — M30 with 40% copper slag fine aggregate replacement and 20% fly ash (Mix M5) — achieves 28-day compressive strength of 38.6 MPa (12.9% above control), split tensile strength of 3.18 MPa, flexural strength of 4.82 MPa, and RCPT value of 1,148 Coulombs (classified 'Low' per ASTM C1202). Scanning electron microscopy confirms a denser interfacial transition zone in the optimal mix relative to the control, with copper slag particles exhibiting strong mechanical interlocking with the cement paste matrix. Beyond 60% copper slag replacement, workability deteriorates significantly (slump below 20 mm) and strength gains reverse, establishing 40% as the practical upper limit for structural concrete applications.