Banana fiber-reinforced diatomaceous earth slurry treatment for enhancing structural recycled aggregate concrete under elevated temperatures

Abstract The use of direct and treated recycled concrete aggregate (RCA) mitigates natural aggregate depletion, reduces construction and demolition (C&D) waste, and promotes sustainable construction. However, the fire resistance of treated recycled aggregate concrete (RAC) remains a critical concern. This study investigates the physical, mechanical, and microstructural performance of banana fiber-reinforced diatomaceous earth (DE) slurry-treated RAC at room temperature and after exposure to 200 °C, 400 °C, 600 °C, and 800 °C. Three treated coarse RCA types were produced by coating RCA with a slurry containing 5% DE, treated banana fiber (3%, 5%, and 7%), and ordinary Portland cement. The heating methodology followed the ISO 834 standard fire curve, and results were compared to those of concrete made with untreated RCA (UT-RAC) and natural aggregate (NAC). Concrete incorporating optimally treated RCA (5/5-RAC) retained 26–30% more compressive strength than UT-RAC and over 95% of NAC’s strength across all temperature exposures. The density improved by 15–20%, while peak strain decreased by 8.2–18.0%, indicating enhanced stiffness. The splitting tensile strength increased by 18–22%, while mass loss, capillary water absorption, and porosity were reduced by 20%, 25%, and 27.42%, respectively, at 800 °C. X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) confirmed improved retention of calcium silicate hydrate (C-S–H) phases and reduced microstructural degradation in treated RAC. The combined action of DE and banana fibers enhanced ITZ densification, C-S–H phase stability, and thermal crack resistance. These findings demonstrate that optimized DE-banana fiber slurry treatment can significantly improve the fire resistance and structural performance of RAC, contributing to sustainable construction.