Microstructure of saturated bentonites characterized by X-ray CT observations

Bentonite is a very attractive raw material as a compacted clay liner at general waste disposal sites and also as a buffer material at radioactive waste disposal sites. It has been considered that the permeability of bentonite, the most important index for evaluating barrier materials, is closely related to its microstructure. Recent studies in geological material science show that X-ray computerized tomography (X-ray CT) is a very powerful tool for the study on microstructure and hydro-osmotic phenomena (e.g. Wong and Wibowo [Wong, R., Wibowo, R., 2000. Tomographic evaluation of air and water flow patterns in soil column. Geotech. Test. J. 23, 413–422]). Permeability tests and micro X-ray CT observations of Wyoming bentonite were performed to describe the relationship between microstructure and permeability of the bentonite used as a barrier material. Two types of samples, compacted bentonite–quartz sand mixtures and raw bentonite ores, were used in this study.The X-ray CT observations of the bentonite–quartz sand mixtures show that ‘vacant pores’ and ‘bentonite–water complexes’ of the bentonite samples after water permeation are distinguishable in X-ray CT images, and that the micro-structural differences are closely relating to the sample permeability, and depend on the mixing and saturation conditions. Permeability tests and X-ray CT observations of the bentonite ore samples show that the permeability and the microstructure are independent to the sedimentary texture developed within the ore samples. In addition, it is characteristic that the bentonite ore samples with micro-cracks show low hydraulic conductivity, comparable to the compacted powder bentonite, implying that cracks in the sample are filled with ‘bentonite–water complexes’ formed after permeation.