MINERALOGY AND CHEMISTRY OF MEXICO CITY CLAY AND THEIR CONTRIBUTION TO THE UNUSUAL GEOTECHNICAL INDEX PROPERTIES OF THE CLAY
2012 年 16 巻 3 号 p. 95-104
詳細
2012 年 16 巻 3 号 p. 95-104
The mineralogical, chemical, and physical properties of Mexico City clay sediments from depths of 5 to 19 m are presented, and an attempt is made to specify the mineralogy and chemistry responsible for the unusually high liquid limit of the clay. An X-ray analysis of < 2 μm clay fractions indicated the presence of smectite, as a principal clay mineral, and amorphous material. The contents of poorly crystalline Si, Al, and Fe extracted with oxalate were found to be in the ranges of 32-57, 23-54, and 133-273 g/kg, respectively, and the ferrihydrite content was as high as 226-464 g/kg. The high observed pH of 9.1 to 10.1 was due to the presence of large amounts of carbonate. Pore water salinity ranged from 51 to 84 g/L and was 1.4 to 2.4 times higher than seawater salinity. Clay and silt were the dominant grain-size fractions, and the clay fraction (< 2 μm) varied from 19% to 51%. The specific surface area varied widely from 26 to 436 m^2/g. Because of the high pore water salinity, the original water contents of the clay needed to be modified by taking into account the weight of the salt present in the oven-dried samples. The modified natural water contents, liquid limits, and plastic limits were in the range of 42% to 641%, 45% to 798%, and 33% to 76%, respectively. The activity of Mexico City clay ranged from 5.6 to 13.9, which was much higher than that for reference samples of bentonite and smectitic marine clays. Despite smaller clay fractions in Mexico City clay than in bentonite, Mexico City clay exhibited liquid limits as high as 194% to 798%, which were equivalent to those of bentonite. This is attributed to the predominance of smectite, ferrihydrite, and amorphous Si and Al in the clay and also to the formation of aggregates due to the presence of ferrihydrite. A linear correlation was found between the liquid limit and specific surface area for the combined data of Mexico City clay, bentonite, and marine clay.
