Visualization of microstructure and measurement of mass transport parameters for granulated bentonite mixtures

Bentonite is often considered as one of the key components of the Engineered Barrier System (EBS) for the radioactive waste disposal; mechanical, thermal, and hydraulic behaviors of bentonite should be thoroughly studied to ensure the emplaced the quality and to control the performance of EBS. This study examined the effects of dry density and relative humidity (RH) on microstructure and mass transport characteristics of Granulated Bentonite Mixtures (GBM). The tested GBM samples were prepared by sieving and grading Japanese bentonite (trade name: OK Bentonite, Kunimine Industries, Japan). For dry density-controlled conditions, air-dried GBM samples (RH 60%, water content of 6.38% by weight) were packed at six different dry densities ranging from 1.25 to 1.75 g/cm3. The samples for humidity-controlled conditions were prepared at the dry density of 1.45 g/cm3 and stored at RH 90% (0 to 12 weeks). A microfocus X-ray computerized tomography (MFXCT) scanning apparatus was used to visualize the microstructures of packed tested samples. For each tested sample, the mass transport parameters of gas diffusivity (Dp/Do), air permeability (ka), thermal conductivity (λ), and volumetric heat capacity (C) were measured. The results indicated that dry density greatly affected the CT brightness values of GBM; the histogram of CT brightness well captured the internal distribution of density and the CT brightness homogenized more with increasing in dry density. The measured Dp/Do and ka. were mainly controlled by the dry density. On the other hand, the thermal properties such as λ and C were governed both dry density and RH.