Hydrated clay minerals
Many countries are planning to use engineered compacted or natural clay barriers in their final/deep repository for high-level radioactive waste. Upon water saturation from the surrounding ground-water, smectite-rich clays expand their clay smectite layers through different swelling/hydration processes, leading to an overall swelling pressure and self-healing capacity of the clay barrier. Smectite is a natural alumino-silicate mineral that consists of very thin sheets (layers) of an octahedral aluminium oxide sheet sandwiched between two tetrahedral silicate sheets. The smectite layers are ~0.93 nm thick and approx. 50 to 500 nm size in the lateral directions. During the expected lifetime of these types of repositories (>10^5y), the brine-like pore-water composition will change over time and may in a worst-case scenario be replaced by low-ionic strength glacial melt water, in case of fracture formation in the host rock, with the risk of clay colloid generation and clay barrier dissolution and erosion. In order to perform a sound safety assessment of the clay barriers integrity and functionality during >10^5y, a thorough understanding of the clay particles behavior under varied T, water-loading and ionic strength is needed. This study aims to investigate electric double layer properties over physically realistic clay smectite surfaces using atomistic molecular dynamics simulations, along the lines of our previous MD work using HPC resources at NERSC (Hopper, Edison, Cori), see: Holmboe and Bourg, JPC C 2014,118,1001-1013, Tinnacher and Holmboe, 2016 Geo. et Cosm. Acta 177 (2016) 130–149, Tournassat, 2016, Clays and Clay Min. pre-publication doi: 10.1346/CCMN.2016.0640403, to compare and complement previous previous work using classical DFT methods, Gumoin Yang and Liu, 2015, J. Chem. Phys. 142, 194110. Topics we intend to address is the swelling/hydration processes of the negatively charged smectites clay layers having Na+ and Ca2+ as counter-ions, and the evolution of smectite layer expansion from the discrete crystalline regime (<4 water mono-layers) to the osmotic free swelling regime (corresponding >8 water layers), using umbrella sampling free energy methods and/or consecutive "desorption" runs, by sequential removal of water molecules between different runs, see Holmboe and Bourg 2014. We will also inspect the diffuse double layers (DDL's) and their sensitivity to total charge and charge location in the negatively charged smectite surfaces, since it is well known that depending on the location of the charge defects in the clay lattices, a varying ratio of inner-sphere and outer-sphere complexation of the counter-cations can occur. By also incorporating clay lattice edges as in our latest study (Tournassat, 2016, doi: 10.1346/CCMN.2016.0640403), open clay stacks can also be simulated, giving us the possibility to probe Na+/Ca2+ selectivity coefficients and possibly also clay layer swelling behavior, i.e. actual translation of the clay layers.