The use of cocoa pod shells as a bio-based concrete retarder

This study investigated the potential of cocoa pod husk extract (CPHE) as a natural Type B retarding admixture for cementitious materials, aiming to promote sustainable and bio-based alternatives to traditional petrochemical retarders. The CPHE effect at dosages ranging from 1 to 5% on initial and final setting times, compressive and flexural strength, and drying shrinkage of mortar and concrete was evaluated. In addition, total carbon analysis, thermogravimetric analysis, and scanning electron microscopy were used to elucidate the retardation mechanism. The retardation mechanism was attributed to the adsorption of carboxylic acids and phenolic compounds on C 3 S and C 3 A surfaces, weak complexation with Ca 2+ ions, and increased pore solution viscosity induced by polysaccharides, which collectively slow cement hydration. The results indicated that a 3% CPHE dosage optimally delayed initial and final setting times by 30% and 50%, respectively. Early-age compressive strength (3–7 days) was slightly reduced but recovered by 14–28 days, while flexural strength remained 98–99% of the control group. Moreover, on-site mixing simulation results demonstrated that CPHE improved concrete workability and effectively reduced slump loss, with performance comparable to citric acid-based retarders at the same total organic carbon concentration. Overall, the CPHE indicated significant potential as a sustainable, eco-friendly alternative to petrochemical retarders.