Response surface optimization of palm oil fuel ash concrete reinforced with kenaf fibers

Amid growing environmental concerns, including global warming, natural fibre concrete (NFC) development has garnered increasing attention. These materials present distinct advantages, such as reduced environmental impact and cost-effectiveness, making them a compelling alternative for sustainable construction. Hence, this study investigates the combined application of palm oil fuel ash and kenaf fibres in high-strength concrete, utilising response surface methods to enhance its mechanical and sustainability attributes gradually. Cement was partially substituted with 20% POFA, while kenaf fibre (KF) was introduced at varying dosages of 0.5%, 1%, 1.5%, and 2% and lengths of 10,20,30 mm fibres by volume. Twelve concrete mixes were formulated and evaluated for key mechanical properties, including uniaxial compressive strength, splitting tensile strength, flexural strength, and impact resistance. The optimal blend, reinforced with 20 mm kenaf fibre, underwent detailed microstructural characterisation using Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and Fourier-Transform Infrared Spectroscopy (FTIR). These analyses provided insight into fibre–matrix interactions, phase composition, and the overall microstructure, helping to correlate improved mechanical performance with internal structural features. The results showed that the incorporation of kenaf fibres resulted in a slight increase of 5% in compressive strength; however, significant improvements were noted in split tensile strength (30%), flexural strength 31%, and impact resistance 76.4% compared to the control concrete. Incorporating 20 mm kenaf fibre into the concrete yielded superior performance compared to 10 mm and 30 mm length fibres, which exhibited optimal results. SEM revealed strong fibre–matrix bonding and fewer microcracks, XRD confirmed well-formed hydration products, and FTIR indicated strong chemical interactions. These results highlight the fibre’s effectiveness in boosting concrete’s structural integrity. Concrete with 20 mm length kenaf fibre showed a denser, more refined microstructure. Notably, a 1% KF dosage demonstrated the most pronounced improvement after 28 days of curing at all length fibres. This enhancement is attributed to kenaf fibre's intrinsic elasticity and reinforcing ability, which improves strength and durability in normal-strength concrete. Predictive modelling using Response Surface Methodology (RSM) enhances mix design optimisation for sustainable construction. A strong correlation was observed between the experimental results and the developed model, with the independent variables accounting for approximately 98.04% of the variation in the response for compressive strength of concrete. • A novel and eco-conscious concrete composite was engineered using 20% Palm Oil Ash (POA) as a supplementary cementitious material and kenaf fibres (KF) as a natural reinforcement. • The synergy between 1% kenaf fibre (20 mm length) and POA led to optimal mechanical performance, with marked enhancements in compressive (4.75%) , tensile (22.9%) , flexural (23.72%) , and impact strength (55.45%) compared to the control. • Microstructural investigations (SEM, XRD, FTIR, EDS) confirmed a refined, denser matrix , improved fibre–matrix interfacial bonding, and intensified pozzolanic activity , contributing to the improved durability and structural integrity of the concrete. • Response Surface Methodology (RSM) effectively modeled and optimized mix parameters, yielding a strong correlation (R² = 0.9804) between predicted and experimental compressive strengths, validating the robustness of the statistical approach. • The incorporation of renewable natural fibres and agro-industrial waste presents a sustainable pathway toward the development of high-performance, low-carbon concrete , aligning with global construction and environmental priorities.