Engineering Performance of Geopolymer Concrete with Coir Husk Ash and Coconut Cellulose Fibers

Geopolymer concrete offers a sustainable alternative for the construction industry by reducing carbon emissions compared with Portland cement–based materials. In this study, fly ash was used as the primary binder, and 50% of it was replaced with ground granulated blast furnace slag (GGBS) to enhance early-age reaction kinetics, shorten the setting time, and improve mechanical strength through the formation of calcium-rich C–(A)–S–H and hybrid N–A–S–H gels. Sustainability was further improved by replacing 5% of the fly ash with coir husk ash and incorporating natural cellulose coir fibers at 0.2–1.2% of the binder weight. The properties of the coconut cellulose fibers were characterised using Scanning Electron Microscopy, X-Ray Diffraction, Fourier Transform Infrared Spectroscopy, and Thermogravimetric/Differential Thermal Analysis. Fresh and hardened properties were evaluated through slump flow, compressive strength, split tensile strength, and flexural strength tests. Although increasing fiber content reduced workability, the hardened concrete exhibited a maximum compressive strength of 55 N/mm² at 28 days, corresponding to an improvement of about 34% compared to the control mix, while split tensile and flexural strengths increased by 35.7% and 33.3%, respectively. These results demonstrate that combining natural fibers with industrial by-products can produce geopolymer concrete with enhanced mechanical performance and improved sustainability.