5:15 PM - 6:45 PM
[HCG20-P08] Quantitative investigation of factors contributing to swelling pressure decrease of bentonite by cementation based on image processing
Keywords:Bentonite, Cementation, Swelling, Image processing
Bentonite clay is planned to be used as a buffer material in the geological disposal of high-level radioactive wastes. The swelling property of bentonite is expected to keep low permeability and control the migration of radionuclides thereby ensuring the safety of geological disposal. In the long-term, however, cementation of bentonite may degrade the swelling property where secondary minerals precipitate and adhere to the montmorillonite component of bentonite. Difficulties in replicating the process of cementation in short-term laboratory experiments have resulted in poor understanding of the mechanism by which cementation decreases swelling pressure. In the present study, natural bentonite ores were investigated as analogues to cementation of bentonite in geological disposal.
Six bentonite ores were collected from Tsukinuno bentonite deposit in Yamagata prefecture, Japan, and assumed to have undergone cementation through diagenesis. A few microns of silica distributed widely in the matrix of montmorillonite could be observed by scanning electron microscopy (SEM). Additionally, some samples also contained carbonates and few microns of K-feldspar as accessary minerals in the matrix of montmorillonite. To quantitatively compare the characteristics of petrographic texture, a new image processing method was developed, combining back scattered electron (BSE) image obtained by SEM and elemental distribution image obtained by electron probe micro analyzer (EPMA). These two types of image were integrated into “mineral map", which could visually illustrate the distributions of individual mineral grains including fine cementitious minerals such as micro silica and K-feldspar. Quantification by a mineral map provides some parameters related to petrographic texture such as grain sizes, perimeters, distance between centroid of grains, etc. Finally, multi regression analysis (MRA) was used to investigate correlation between petrographic parameters and swelling pressure. MRA showed swelling pressure of bentonite increases with the content of montmorillonite, and decreases with the total perimeter of accessary minerals, mainly micro silica and to lesser extent carbonates and K-feldspar. This suggests that the presence of secondary minerals can inhibit the swelling of montmorillonite and the area of contact between montmorillonite and secondary minerals might be a key parameter requiring more through investigation.
This study was partly founded by the Ministry of Economy, Trade and Industry of Japan (METI) through the program “The project for validating near-field system assessment methodology in geological disposal” (2021 - 2023 FY, Grant Number:JPJ007597).
Six bentonite ores were collected from Tsukinuno bentonite deposit in Yamagata prefecture, Japan, and assumed to have undergone cementation through diagenesis. A few microns of silica distributed widely in the matrix of montmorillonite could be observed by scanning electron microscopy (SEM). Additionally, some samples also contained carbonates and few microns of K-feldspar as accessary minerals in the matrix of montmorillonite. To quantitatively compare the characteristics of petrographic texture, a new image processing method was developed, combining back scattered electron (BSE) image obtained by SEM and elemental distribution image obtained by electron probe micro analyzer (EPMA). These two types of image were integrated into “mineral map", which could visually illustrate the distributions of individual mineral grains including fine cementitious minerals such as micro silica and K-feldspar. Quantification by a mineral map provides some parameters related to petrographic texture such as grain sizes, perimeters, distance between centroid of grains, etc. Finally, multi regression analysis (MRA) was used to investigate correlation between petrographic parameters and swelling pressure. MRA showed swelling pressure of bentonite increases with the content of montmorillonite, and decreases with the total perimeter of accessary minerals, mainly micro silica and to lesser extent carbonates and K-feldspar. This suggests that the presence of secondary minerals can inhibit the swelling of montmorillonite and the area of contact between montmorillonite and secondary minerals might be a key parameter requiring more through investigation.
This study was partly founded by the Ministry of Economy, Trade and Industry of Japan (METI) through the program “The project for validating near-field system assessment methodology in geological disposal” (2021 - 2023 FY, Grant Number:JPJ007597).