Concrete suffers from low tensile capacity, high brittleness, and increasing environmental impact due to cement production. This study presents a green synthesis route for extracting cellulose nanofibers from Phragmites australis, an abundant invasive plant, and evaluates their effectiveness as a sustainable reinforcement for concrete. The novelty of the work lies in converting an environmental waste material into nano-cellulose fibers through an eco-friendly alkali-based extraction process, followed by mechanical milling to achieve nano-scale sizes (80–100 nm). The nanofibers exhibited increased crystallinity, enhanced purity, and improved surface morphology as confirmed by XRD, FTIR, SEM, XRF, and particle size analysis. Concrete mixes incorporating 5%, 10%, and 15% nanofibers were tested at 7, 28, and 90 days. The optimum performance occurred at 10% nanofiber content, which improved flexural strength by 36% (from 4.09 to 5.57 MPa) and maintained compressive strength at 29.4 MPa compared with the control (28.2 MPa). Physical properties also improved, with reductions in porosity and water absorption at the optimum dosage. However, 15% fiber addition caused strength and workability reductions due to fiber agglomeration and increased voids. These findings demonstrate that Phragmites australis nanofibers can serve as a low-cost and sustainable reinforcement to enhance concrete performance while valorizing an invasive plant. The study supports the broader trend of integrating bio-based nanomaterials into construction materials to reduce environmental impact and improve mechanical efficiency.
