Multi-response optimization of mechanical, chemical durability, and microstructural performance of sustainable hybrid fiber-reinforced high-strength concrete incorporating pyrolyzed coffee grounds using RSM

The use of agricultural waste materials and hybrid fiber reinforcement in concrete offers a sustainable approach for producing high-performance construction materials with improved mechanical properties and durability. This study investigated the mechanical, chemical durability, thermal, and microstructural performance of high-strength concrete incorporating 15% pyrolyzed coffee grounds (PCG) produced at 350 °C as a partial fine-aggregate replacement. Hooked-end steel fibers and alkali-treated banana fibers were used as hybrid reinforcements, while Response Surface Methodology (RSM) based on Central Composite Design (CCD) was employed to optimize the effects of fiber dosage and steel–banana hybridization ratio. Mechanical performance was evaluated through compressive, splitting tensile, and flexural strength tests, while durability was assessed under 10% NaCl, 5% HCl, and 5% HNO 3 exposure. Microstructural characterization was conducted using SEM/EDX, XRD, and TGA/DTG analyses. The results showed that the combined incorporation of PCG and hybrid fibers enhanced concrete performance through improved crack-bridging, matrix densification, and pore refinement. The optimum mixture, containing 1.25%–1.50% total fiber dosage and a steel-to-banana fiber ratio of 80:20, achieved a compressive strength of 69.6 MPa, splitting tensile strength of 9.0 MPa, and flexural strength of 14.0 MPa. The same mixture exhibited superior chemical durability, with minimum mass losses of 2.45%, 3.90%, and 4.40% under NaCl, HCl, and HNO 3 exposure, respectively. Microstructural analyses confirmed a denser matrix, stronger fiber–matrix bonding, reduced pore connectivity, and enhanced hydration-product formation, while TGA/DTG results indicated improved thermal stability. Validation experiments closely matched model predictions, confirming the reliability of the CCD-RSM models. Overall, the synergistic use of pyrolyzed coffee grounds and hybrid steel–banana fibers produced a durable, high-strength, and environmentally sustainable concrete suitable for structural applications in aggressive environments while promoting the valorization of coffee-processing waste within a circular economy framework.