Adsorption and Diffusion of Na+, Cs+ and Ca+2 Ions in C-S-H and C-a-S-H Nanopores

Cementitious materials act as a diffusion barrier, immobilizing liquid and solid<br>radioactive waste and preventing their release into the biosphere. The retention capability of hydrated<br>cement paste and its main hydration product, C-S-H gel, has been extensively explored experimentally<br>for many alkali and alkaline earth cations. Nevertheless, the retention mechanisms of these cations at<br>the molecular scale are still unclear. In this paper, we have employed molecular dynamics simulations<br>to study the capacity of C-S-H to retain Cs, Ca and Na, analyzing the number of high-affinity sites on<br>the surface, the type of sorption for each cation and the diffusivity of these ions. We have also explored<br>the impact of aluminum incorporation in C-S-H at a constant concentration of the ions in the gel pore.<br>We found strong competition for surface sorption sites, with notable differences in the retention of the<br>cations under study and a remarkable enhance of the adsorption in C-A-S-H with respect to C-S-H

Cs Retention and Diffusion in C-S-H at Different Ca/Si Ratios

<div>Cement and concrete have been widely used as a barrier to isolate many types of contaminants, including radioactive waste, in repository sites. Nevertheless, the intrusion of groundwater in those nuclear repositories may release those contaminants by leaching mechanisms. Because of this, the retention and diffusion processes in cement matrix require to be analyzed in depth. The adsorption in cement and C‐S-H gel, its main hydration product, is influenced by factors as the pH, the composition or the alkali and alkaline earth content. In this work, molecular dynamics simulations were employed to study the role of Ca/Si ratio of the C‐S‐H in the capacity to retain Cs and diffusivity of these ions in gel pores. For that purpose, we built four different C‐S‐H models with Ca/Si ratios from 1.1 to 2.0. The results indicate better cationic retention at low Ca/Si ratios due to the interaction of the cations with the bridging silicate tetrahedrons. However, the average diffusion coefficients of the cations decrease at higher Ca/Si ratios because the high ionic constraint in the nanopore that induces a longrange ordering of the water molecules.</div

Molecular Forces Governing Shear and Tensile Failure in Clay-Dye Hybrid Materials

Hybrid materials based on photoactive molecules confined into nanostructured substrates are very promising for technological applications. However, little is known about the impact of organic dyes on the mechanical properties of the substrate, a key aspect for their practical implementation. In this work, we use atomistic simulation methods to investigate the mechanical properties of two hybrid systems consisting on a clay matrix (laponite) loaded with two different cationic dyes (LDS-722 and pyronin Y). We applied tensile and shear deformations to the layered hybrid materials and characterize the key mechanism triggering their failure. It has been observed that the water and dye molecules located in the interlaminar spaces are those involved in the deformation processes, while the structure of the laponite layers does not change. Furthermore, it has been also found that the incorporation of dye molecules modifies the hydrogen-bonding network of water in the interlaminar space, worsening the mechanical properties of the hybrids with respect to the clay. The information obtained by molecular simulation help us to assess the mechanical behavior of these materials, and to design materials with tailored strength