Development of Novel Composite Core Using Powdered Macadamia Nutshell and Its Sandwich Structures for Building and Other Engineering Applications.

Growing environmental concerns and the depletion of fossil-based resources have accelerated the demand for sustainable alternatives in engineering and construction materials. Among these, bio-based composites have gained attention for their use of renewable and eco-friendly resources. Macadamia nutshells, typically treated as agricultural waste, possess high strength, brittleness, heat resistance, and fracture toughness, making them attractive candidates for structural applications. Australia alone contributes nearly 40% of global macadamia production, generating significant shell by-products that could be repurposed into high-value composites. This study investigates the development of novel composite cores and sandwich structures using macadamia nutshell particles reinforced in an epoxy polymer matrix. Two weight ratios (10% and 15%) and two particle sizes (200-600 µm and 1-1.18 mm) were employed, combined with laminating epoxy resin and hardener to fabricate composite cores. These cores were further processed into sandwich specimens with carbon fabric skins. Flexural and short beam shear (SBS) tests were conducted to evaluate the mechanical behaviour of the composites. The results demonstrate that higher filler content with fine particles achieved up to 15% higher flexural strength and 18% higher stiffness compared to coarser particle composites. Sandwich structures exhibited markedly improved interlaminar shear strength (8-15 MPa), confirming superior load transfer and durability. The results demonstrate that higher filler content and finer particles provided the most favourable mechanical performance, showing higher flexural strength, stiffness, and shear resistance compared to coarser particle formulations. Sandwich structures significantly outperformed core-only composites due to improved load transfer and resistance to bending and shear stresses, with the 15% fine-particle configuration emerging as the optimal formulation. By transforming macadamia nutshells into value-added composites, this research highlights an innovative pathway for waste utilisation, reduced environmental impact, and sustainable material development. The findings suggest that such composites hold strong potential for structural applications in construction and related engineering fields, especially in regions with abundant macadamia production. This study reinforces the role of agricultural by-products as practical solutions for advancing green composites and contributing to circular economy principles.