Hybrid oil palm wood and rice husk biofillers for lightweight gypsum composites: Effect of biofiller content on thermal, mechanical, and physical properties

The development of lightweight and sustainable building materials with high thermal insulation performance requires a material design approach based not only on the selection of fillers, but also on their combined effect. Although hybrid approaches with multifillers have been reported for cement and gypsum composites, the combined use of palm oil and rice husk biofillers in a gypsum matrix and their simultaneous effects on density, water absorption, mechanical performance, and thermal insulation are still limited. This study aims to evaluate the combined impact of oil palm wood (OPW) and rice husk (RH) hybrid biofillers on the physical, mechanical, and thermal properties of gypsum-based composites as candidates for environmentally friendly non-structural building insulation materials. OPW/RH mass fractions of 10, 20, 30, 40, and 50%, with a fixed OPW to RH ratio, were applied to examine the effects of biofiller content on density, water absorption, mechanical strength, thermal conductivity, thermal stability, and microstructural characteristics. The results showed that OPW/RH incorporation modified the gypsum microstructure from a relatively dense matrix to a more porous biofiller containing structure, leading to lower density and reduced thermal conductivity. The lowest thermal conductivity of 0.0940 W/(m·K) was obtained at the highest biofiller content, indicating improved thermal insulation capability. At the same time, thermogravimetric analysis confirmed thermal stability up to 345°C. However, this improvement was accompanied by increased water absorption and decreased mechanical properties in most of the hybrid composite samples. The G20 composition showed the most balanced performance, with flexural and compressive strengths of 3.39 MPa and 7.27 MPa, respectively, while maintaining lower thermal conductivity than pure gypsum. These findings indicate that OPW/RH hybrid gypsum composites have potential as lightweight, environmentally friendly, and thermally insulating non-structural building materials, by optimizing the biofiller content to avoid excessive loss of mechanical strength.