Determination of bentonite content in soil-bentonite liners by X-ray diffraction

The full-scale hydraulic performance of a soil-bentonite liner depends on the homogeneity of bentonite content in the mix; any prediction must consider the distribution function of the local bentonite content, mainly that of below average values. The degree of homogeneity in the field depends on the mixing method and the sand characteristics. Several techniques, such as double hydrometer tests, or methylene blue tests, can evaluate the bentonite content in a sample. They are not reliable when the sand varies in gradation and mineralogy. This led to the investigation of whether X-ray diffraction could determine the bentonite content in a soil-bentonite sample and a method was developed. First, the mix must be thermally treated to eliminate the variation in diffraction intensities resulting from the variable hydration of the montmorillonite. Then, an internal standard must be used to control the X-ray absorption. This paper presents the preliminary calibration work, the different steps to follow for quantitative determination of the bentonite content, its accuracy, and its application to a special project where the usual methods were not reliable due to a large variability in sand properties. The parameters needed to calculate the bentonite content in a mix depend on the type of bentonite but not on the type of sand. The accuracy depends on the variability in mineralogical quality of the bentonite powder. For one project, it is shown that the in situ mixing technique produced highly variable local bentonite contents, and that the average content was higher in the lower half of the lift than in its upper half.