CHEMO-MECHANICAL EFFECTS AND MOISTURE TRANSPORT DURING CLAY DEHYDRATION

Swelling clay hydration/dehydration has been extensively investigated. However, experimental studies using X-ray diffraction (XRD) usually probe equilibrium hydration states in an averaged manner and thus cannot capture the fast moisture transport and structural changes in interlayers during hydration/dehydration. Using molecular dynamics simulations, we demonstrate that clay dehydration is a two-stage process. In the first stage, the dehydration process is controlled by the evaporation at the particle edges and the mass loss varies linearly with time. Water molecules near hydrophobic sites (e.g., siloxane groups) and the first few water molecules of the hydration shell of cations move fast to the edges for evaporation. The second stage is dominated by the slow water desorption and transport in the interlayers. Water transport and interlayer collapse are strongly coupled with layer charge distribution and interlayer ion type. Our molecular dynamics simulation results are supported by our results from thermogravimetric analysis, differential scanning calorimetry and in situ XRD experiments. Our mechanistic interpretation of clay dehydration can be key to the coupled chemo-mechanical behavior in natural/engineered barriers.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. The views expressed in this article do not necessarily represent the views of the U.S. Department of Energy or the United States Government. This work was supported by the DOE Spent Fuel Waste Science & Technology (SFWST) Program.