Fig. 6. Computed tomography (CT) scans for quartz columns collected at the middle of the column (40 mm from the inlet). Digitized images (c and d) are shown with different colors (blue, red, and green for black, gray, and white spots, respectively) in a limited square field area (212.1 mm²). (a) Dry column; (b) initial water saturated column; (c) after 8 d of reaction; (d) after 22 d of reaction.