The nanoscale structural response of a natural kaolinitic clayey soil subjected to uniaxial compression

In recent years, clay materials have been increasingly used as natural or engineered barriers for containing hazardous waste. An understanding of the clay structure at a molecular level is needed for accurate assessment of the long-term engineering performance of the barriers. This is the aim of the present study. The soil sample used in this study has been taken from a natural kaolinitic soil that has been used for containing industrial liquid and sludge waste at a state government operated waste management centre in New South Wales, Australia. Reconstituted water-saturated samples were prepared from a slurry-like soil-water mixture. The nanoscale structural responses of the samples to the uniaxial compression at two different stresses, 400 kPa and 800 kPa, were determined by the small and ultra-small angle neutron scattering methods in the scattering vector (q) range from 5.00 x 10(-5) Angstrom(-1) to 2.20 x 10(-1) Angstrom(-1). The scattered patterns were anisotropic in q-space, indicating that randomly oriented clay particles were realigned to form an asymmetric layered structure in response to the applied compression. A fractal model was used to analyse the patterns in terms of the major and minor axes and to estimate parameters describing the hydrated clay structure. The average ellipticity values (e) determined for the samples were 0.87+/-0.03 and 0.86+/-0.04 for 400 kPa and 800 kPa, respectively. These similar values indicate that stress increase from 400 kPa to 800 kPa resulted in only minor particle realignment and the major realignment occurred at a much lower stress. Assuming minimum scale length values between 10(-10) m and 10(-9) m, the total surface areas were estimated to be 13<S<250 m(2)/g for the sample at 400 kPa but 6<S<80 m(2)/g at 800 kPa. These values are consistent with the value of 30 m2/g obtained for air-dried fractions sieved through a 0.2-cm mesh, as determined by the Brunauer-Emmett Teller (BET) method. This suggests that the small-angle neutron scattering (SANS) studies provide a valuable method for measuring the surface area of fluid-saturated porous media. (C) 2004 Elsevier B.V. All rights reserved.