Effect of Fine Aggregate Replacement Rate of non-calcined Cockle Shell Powder on Workability and Compression Strength of Mortar

Purpose: This study investigates the effect of replacing natural sand with non-calcined cockle shell powder (CSP) on the age-dependent compressive strength of mortar. Methods: Ordinary Portland cement (Type I), Jumunjin sand, and tap water were used. CSP (≥ 96% passing 325 mesh) was supplied by a local manufacturer and used without calcination. CSP replaced natural sand on a mass basis at 0, 10, 20, 30, and 40%. Mortar cubes (50 × 50 × 50 mm) were prepared and cured following KS L 5105, and compressive strength was measured at 7, 14, 21, and 28 days. In addition, raw-material characterization was conducted using X-ray photoelectron spectroscopy (XPS), and basic workability was recorded by flow testing. Results: Compressive strength increased with CSP replacement ratio at all ages. At 28 days, strength rose from 45.92 MPa (0%) to 62.24 MPa (40%), corresponding to a 35.5% increase. A pronounced improvement was observed at early age: the 7-day strength increased from 24.12 MPa (0%) to 40.61 MPa (40%) (+68.4%). Conclusion: Non-calcined CSP can partially replace natural sand (0-40% by mass) to enhance compressive strength of mortar, with the highest strengths observed at 40% replacement within the tested range. For practical application, further assessment of strength variability and workability is recommended.