Probing water properties and cationic exchange in calcium-silicate-hydrate : an atomistic modeling study

Abstract

Two problems related to the assessment of the performance of cementitious materials in the nuclear fuel cycle are investigated by means of atomistic simulations. The first is the structural and dynamic nature of water confined in the nano-pores of the highly disordered calcium-silicate-hydrate (C-S-H) which is the major binding phase of cement. The microscopic structure and dynamics of water confined in C-S-H have important implications on describing the cohesion and mechanical behavior of cement from its setting to its aging; nevertheless they have not been fully elucidated prior to this thesis. The second problem is the encapsulation of strontium-90, an important radionuclide, in C-S-H and its crystalline analogue tobermorite 9 A by means of cationic exchange with calcium. We showed that the nature of the interaction between the confined water and C-SH is hydrophilic. The interlayer calcium ions and the disorder in the silicate chains act synergistically to achieve this hydrophilic interaction. The water molecules ultra-confined in this hydrophilic and disordered interlayer space adopt a unique multi-range structure: at short range they are tetrahedrally coordinated but with distortions, at intermediate range they exhibit a structure similar to that of dense fluids and supercooled phases, and at ranges up to 10 A spatial correlations persist through dipole-dipole interactions that are enhanced by the directionality of the hydrogen bonds formed between the confined water and the defective silicate chains. This confined water exhibits a three-stage dynamics evidenced in the mean square displacement (MSD) results, with a clear cage stage characteristic of glassy dynamics similar to that of supercooled liquids and glass forming materials. The glassy dynamics is induced primarily because of the attractive interactions of water molecules with the calcium silicate walls, serving to constrain the motion of the water molecules at the interface, as if with an effective temperature lower than the actual simulation temperature. At intermediate time scales that correspond to the p-relaxation of glassy materials, the non-Gaussian parameter indicates a significant heterogeneity in the translational dynamics of the confined water, also consistent with the cage stage identified in the MSD of the water molecules. Strontium was shown to favor energetically substituting calcium in the interlayer sites in C-S-H and tobermorite 9 A with the trend more pronounced in the latter. The silicate chains in both cementitious waste forms were not affected by strontium substitution within our molecular dynamics simulation. Finally, we observed degradation in the mechanical properties in the strontium-containing cementitious waste form. The degradation increases with the increase of strontium concentration, but overall this degradation is not limiting for the use of C-S-H or tobermorite 9 A as candidates in immobilizing radioactive strontium.