X-ray CT observations and geochemical modeling of bentonite alteration by hyperalkaline fluids

Nakabayashi, Ryo*; Elakneswaran, Y.*; Sato, Tsutomu*; Oda, Chie  ; Yoneda, Tetsuro*; Kaneko, Katsuhiko*

The objective of this study is to develop a microstructural analysis method by X-ray CT to track the alteration process arising from bentonite and hyperalkaline fluid interactions as a function of time and to use these data to underpin a geochemical model of the alteration process. Advective alteration experiment of bentonite was performed at 80 C (pH = 13.5 at 25 C) for 180 days and in-situ analysis by X-ray CT during the experiment was conducted. The geochemical reactive transport code PHREEQC was used to simulate the results of the experiments. Consequently, the formation of secondary minerals (analcime) in the bentonite could be confirmed by X-ray CT at different times and quantified by X-ray CT images analysis. The predicted time-dependency of volume of analcime by a geochemical reactive transport model became consistent with the results obtained by X-ray CT analysis, and the concentration of dissolved Si and Al in the output solution could be replicated.