Modelling the Long-Term Chemical Evolution of Cement-Groundwater Systems

The importance of the large volumes of cement and concrete in the planned Swiss L/ILW repository in influencing the chemical environment of the repository and the sorption behaviour of the radionuclides has lead to an interest the long-term evolution of cement-groundwater systems for safety assessment purposes. A model is presented which simulates the leaching of a column hydrated cement/concrete with fresh groundwater, using a sequential mixing tank approach. The chemical model includes the incongruent dissolution of the CSH-gel phase and reaction of three other cement phases which make up over 90 wt. % of the hydrated cement paste. The model takes account of porosity changes due to dissolution and precipitation. Different concrete compositions can be included in the column in order to represent, for example, a section through a repository cavern. The model can be run as a 1-D flow-through system or as a quasi-1-D system in which flow in any segment is proportional to porosity. Although the model does not include the sophisticated transport models of currently available coupled codes, it does allow the possibility of assessing the duration of high pH conditions within the repository and the influence of different concrete/cement types or different waste type/loading as a first stage to determining long-term transport of radionuclides from the near field.