Analyzing the Mechanical and Long-Term Properties of Concrete Containing Waste Foundry Sand and Fly Ash

The growing demand for concrete is increasing the consumption of cement and river sand, while industrial by-products such as waste foundry sand (WFS) and fly ash (FA) accumulate in landfills. This study investigates the strength and durability of M40-grade concrete in which river sand was partially replaced with WFS and cement was partially replaced with Class F FA. Mixes were proportioned according to IS 10262:2019 at a constant water–cement ratio of 0.4, with WFS at 0, 25, 50 and 100% (and 75% for slump and chloride tests) and FA at 0 and 25%. Workability was assessed by the slump test, compressive and split-tensile strengths at 7 and 28 days, and durability by water absorption and the rapid chloride penetration test (RCPT) at 90 and 180 days. Slump fell from 140 mm to 10 mm at 100% WFS, and FA improved slump at every level but did not offset this loss. Strength peaked at about 25% WFS, where the 28-day compressive strength rose by about 4% to 46.7 MPa; it remained acceptable up to 50% and dropped by about 25% at 100%. FA lowered early-age strength, but the gap narrowed from about 20% at 7 days to 15% at 28 days. Water absorption was largely unchanged up to 50% WFS (2.70–2.80% at 90 days) but rose to 3.75% at 100% WFS. Chloride penetration decreased steadily with WFS, from 2,010 C to 590 C at 90 days (about 71%). About 25% WFS gives the best mechanical performance and up to 50% is acceptable, whereas complete replacement is not recommended.