Scanning Electron Microscopy and clay geomaterials: From sample preparation to fabric orientation quantification

Scanning Electron Microscopy (SEM) has been widely used to investigate the microstructure of clays, and several studies have examined the influence of the particles orientation on the mechanical behaviour of the soil. However, as the effect of the sample preparation that precedes the imaging on the orientation of the soil particles has not been systematically studied yet, the extent to which the clay microstructure is disrupted is not quantified. This work investigated samples having different mineralogy, stiffness, particles orientation and aggregation, and stress history imaging them at different magnifications in their Freeze, Oven and Air dried state using SEM, and wet state using the Environmental SEM (ESEM) and CryoSEM. The particles orientation imaged in the different micrographs was quantified with MiCA (Microstructural Clay Analyser), a Computer Vision based tool. The results shows the greatest effect for the dispersed soft samples, where the detected order increases with decreasing drying speed, regardless of mineralogy. However, this seems to affect the degree of the particles orientation when the CryoSEM technique is employed. Moreover, soft flocculated samples generated at acidic sedimentation environment overall experience a lower degree of reorientation. Finally, the fabric of stiff undisturbed samples is not significantly influenced by the applied method.