Thermal conductivity and microstructure of a novel clay-sorghum fiber biocomposite: Analytical homogenization models and X-ray computed tomography

The building sector accounts for approximately 40% of global energy consumption, creating an urgent need for sustainable, low-carbon construction materials. In hot and arid climates such as those prevailing in the Sahelian region of Burkina Faso, unfired earth bricks incorporating natural fibers represent a promising solution for improving the thermal performance of buildings while using locally available and renewable resources. This study presents a comprehensive investigation of the thermal conductivity and the three-dimensional microstructure of a novel biocomposite consisting of Boudry clay reinforced with sorghum fibers at mass fractions ranging from 0 to 10%. The microstructure of the composites was characterized using X-ray computed tomography at three spatial resolutions (4, 10, and 22 μm /voxel), enabling a quantitative assessment of porosity, fiber volume fractions, and spatial fiber distribution. Effective thermal conductivities were measured experimentally and interpreted through a hierarchical three-step analytical homogenization approach combining image analysis and two-phase analytical models. The results demonstrate that increasing the sorghum fiber content from 0 to 10 wt% leads to a significant 42,7% reduction in thermal conductivity (from 0.886 to 0.508 W . m − 1 . K − 1 ), accompanied by a decrease in dry density from 1924 to 1069 k g · m − 3 . The intrinsic thermal conductivity of sorghum fibers was estimated at 0.407 W . m − 1 . K − 1 using the inverse Glicksman model. Excellent agreement was obtained between experimental data and analytical predictions, particularly with the Glicksman model. These findings confirm the strong potential of sorghum fibers as a bio-based reinforcement for enhancing the thermal insulation properties of unfired earth construction materials.