Mathematical Prediction and Optimization of Engineered Properties of Recycled Concrete with Marble Fines using Central Composite Design

The reuse of wastes from demolished concrete is now seen as a promising solution for preserving natural resources and protecting the environment. This study aims to assess the influence of varying proportions: 0%, 5%, 10%, 15%, and 20% of marble fines on the behaviour of concrete made of recycled aggregates produced by demolition, using Central Composite Design (CCD) based on the response surface methodology (RSM). Multiple physical and mechanical parameters were examined to determine the optimal amount of fine marble for recycled aggregate concrete (RAC). The results suggested that recycled concrete with marble fines 20% (RC20) shows exceptional quality in terms of fineness modulus, resulting in an improved fresh density and a high drop in plastic during the fresh phase. In parallel, the introduction of 10% and 15% marble fines significantly improves the mechanical properties of hardened RAC. Specifically, RC10 (recycled concrete with 10% marble fines) increases compressive strength by 13.79% and reduces capillary absorption capacity by 46.75% compared to the control (RC0) after 60 days. The incorporation of fines 15% proves sufficient to significantly improve flexural strength at the same hydration time compared to other formulations. Furthermore, this study establishes polynomial correlations using an adjusted value of the coefficient of determination (R-squared) equal to or greater than 0.98, to relate various characteristics of fresh concrete (density and slump) and hardened concrete (porosity and water absorption by immersion) after the addition of marble fines. The proposed RSM model-based CCD validates a promising approach that allows optimal values of compressive strength (35.9 MPa) and flexural strength (4.92 MPa), with a desirability coefficient of approximately 93%.