Compressive Strength Optimization of Concrete Containing Calcined Shell Powder as a Partial Cement Replacement

Abstract

Ordinary Portland cement (OPC) production contributes substantially to global anthropogenic CO₂ emissions, motivating research into low-carbon supplementary cementitious materials. Shell waste from coastal seafood processing, abundant in Indonesia including West Sumatra, is rich in calcium carbonate and may improve concrete performance when used at optimal replacement levels. This study experimentally investigated the compressive strength development of concrete incorporating calcined and ground shell powder as a partial cement replacement at five levels (0%, 5%, 10%, 15%, and 20% by binder weight). Concrete cubes were tested at four curing ages (7, 14, 28, and 56 days), with three replicate specimens per mix-age combination, totaling 60 specimens. One-way ANOVA of 28-day compressive strength indicated a significant effect of replacement level (F = 15.997, p < 0.001). Tukey HSD comparisons showed that the 10% and 15% replacement mixes achieved the highest strengths, 40.01 MPa and 41.50 MPa, respectively, significantly exceeding the control strength of 34.91 MPa. The 20% mix reached 38.21 MPa and showed a significant decline compared with the 15% mix (p = 0.044). Strength increased with curing age across all mixtures, with 56-day strengths 10–24% higher than 28-day values. Overall, shell powder demonstrated potential as a partial cement substitute, with an optimum replacement range of approximately 10–15%.