Development of sustainable natural waste-reinforced epoxy composites for potential building applications

This study aimed to develop high-performance, sustainable epoxy composites for construction applications, using natural waste materials such as walnut shells (NR) and pine bark (PR) as reinforcing agents for epoxy resin (EP). The study analyzed the effects of particle size (150 and 250 µm) and addition ratios (2-10%) on mechanical, thermal, acoustic, and morphological properties. The NR/EP and PR/EP samples were produced by the hand lay-up technique using a 3 : 1 epoxy resin to hardener mixing ratio. Particle distribution and absorption were guaranteed, and the samples were then dried under controlled conditions. The results showed significant performance improvements. NR/EP10 exhibited the highest tensile strength (28.5 MPa), PR/EP1 achieved the best flexural strength (28 MPa), NR/EP6 reached the highest flexural modulus (1.038 GPa), and NR/EP9 demonstrated the highest impact strength (795 kJ m^-2), with a maximum hardness of 87.4 shore D at high addition ratios. Thermally, NR/EP1 exhibited the lowest thermal conductivity (0.8834 W m^-1 K^-1), while NR/EP10 showed the best heat transfer coefficient (0.09 W m^-2 K^-1), and PR/EP10 achieved the highest sound insulation (96.05 dB). Scanning electron microscopy (SEM) results supported these findings, revealing that the fracture surfaces of NR/EP composites (particularly NR/EP5 and NR/EP10) exhibited high homogeneity and strong matrix-fiber bonding with reduced gaps, compared to the pure EP matrix, which displayed smooth and brittle fracture surfaces, and PR/EP composites, which exhibited some agglomeration and irregularity at high addition ratios (PR/EP10). These results confirm that improved microstructure is directly related to enhanced mechanical and thermal performance, thus enhancing the suitability of these composites for use in multifunctional, sustainable building materials.