Relationship between petrographic texture and swelling property in cemented bentonite ores

Ishiwata, Tobimaru*; Kikuchi, Ryosuke*; Otake, Tsubasa*; Sato, Tsutomu*; Kawakita, Ryohei  ; Takayama, Yusuke  ; Mitsui, Seiichiro  

It is known that the cementation of bentontie may cause the reduction of swelling pressure. However, fundamental mechanism of this reducing swelling pressure has not been well understood. To understand the mechanism, it is essential to identify the cementitious materials and observe the changes in microstructure associated with cementation. In this study, the microstructural observation and swelling tests were conducted on several bentonite ores collected from Tsukinuno Mine in Yamagata Prefecture, Japan, as natural analog of cementation, and the relationship between petrographic texture and swelling pressure was discussed. Thin sections of bentonite ores were prepared by dry polishing technique to observe flat surfaces in micro scales. Scanning electron microscopy (SEM) demonstrated the widespread distribution of authigenic, a few micron silica in the matrix of montmorillonite. Transmission electron microscopy (TEM) revealed that the micro silica minerals (commonly quartz and less commonly amorphous silica) adhered to the edge of montmorillonite grains. These observations suggest that the adherence of these cementitious materials to montmorillonite prevents water access to the interlayer of montmorillonite and swelling, resulting in a decrease in swelling pressure. While these microstructures were observed commonly in all samples collected from several bentonite beds, the relative abundance of montmorillonite and micro silica minerals varied among different beds. It is possible that the swelling pressure in the cemented bentonite is affected by the relative abundance of montmorillonite and cementitious materials.