Assessment long-term geochemical evolution of disposal cell – NaNO3-CaSO4 interactions with cement and host rock from Eurobitum bituminized waste

In a geological disposal facility, the geochemical evolution of a component in the near field (waste form, backfills, structural components and the host rock) is, amongst other factors, influenced by the geochemical properties of the other factors. One approach to assess the evolution of a so-called disposal cell is by coupled reactive transport modeling. Under the assumptions made in the model, this approach allows to evaluate the long-term interactions of the different components of the disposal cell, providing valuable insights into dominating processes across different time-scales, and critical parameters. The present study considers a specific disposal cell configuration with a bituminized waste form, a cementitious engineered barrier system and a clayey host rock. After resaturation of the disposal facility, the hygroscopic salts present in bituminized waste will drive the uptake of water by the waste form. NaNO₃ and CaSO₄ salts in the waste form will dissolve and diffuse into the cementitious materials. Eventually, these salts may reach the host rock, thereby altering properties such as for example the occupancy on the cation exchange complex. At the same time, there is the alkaline plume effect on the host rock, and the leaching of cement materials. The objectives of this study are (i) to implement a two-dimensional coupled reactive transport model at the disposal cell scale (engineered barriers and near host rock), (ii) to evaluate the geochemical evolution in the cementitious materials and the host rock as a consequence of leaching of soluble salts from the bituminized waste over a time-scale of a few thousands of years, and (iii) to identify the most critical parameters and processes. The results show that NaNO₃ will alter the cement by transforming AFm phases into mononitroaluminate, while CaSO₄ will first convert to ettringite and later also gypsum. Specifically, the latter will influence porosity and tortuosity of the cement materials. For the assumptions made in the model and considered time scales, the host rock geochemistry is only limitedly perturbed by these salts. In fact, geochemical changes in the host rock close to the interface with the engineered barrier system are mainly caused by cement leaching.