Characterization of the Porous Structure of Cob Materials Using X-Ray Microtomography

This article analyzes the porous microstructure of cob materials, which are composites of earth and plant fibre, using X-ray tomography (micro-CT) and Dragonfly software. The objective is to understand how the addition of plant fibre (3% and 6% by mass) impacts the microstructure and functional properties of these natural materials. The fibre, primarily composed of cellulose, lignin, and hemicelluloses, are porous and hygroscopic, thus influencing the overall porosity of the composite. Tomography allows for the acquisition of high-resolution 3D images, where different phases (dense clay, fibre, pores, cracks) are distinguished by their grayscale levels. The fibre appears as dark gray voxels, while the clay is represented in light gray, and the small pores in dark gray. The study shows that the addition of fibre increases the total porosity and alters the distribution of pores depending on the height of the sample. In particular, the proportion of isolated pores decreases with an increase in fibre content. Isolated porosity values obtained via micro-CT are 4.76% for fibre-free samples, 23.94% for the 3% fibre samples, and 34.98% for the 6% fibre samples. These are lower than the experimental open porosities, which are 20.6% for fibre-free samples, 37.0%, and 45.4% for fibre contents of 3%, and 6%, respectively. This research highlights the importance of the porous microstructure in the performance of earth-fibre composites and emphasizes the key role of microtomography in optimizing these low-carbon geosourced materials for use in ecological and sustainable construction.