Exploring the diffusive interaction between Opalinus Clay and concrete using reactive transport modelling.

Abstract

The diffusive interaction between concrete and host rock of a repository for nuclear wastes will take place over timescales much greater than what can be investigated with experiments. Reactive transport modelling is a powerful tool in exploring such systems: Boundary conditions that are not known a priori, e.g. the reaction products, can be varied and thereby their importance for the overall outcome of the modelling be studied. Our work is based on earlier modelling and experiments under advective-diffusive transport conditions [1] that used drill cores of Opalinus Clay, which is a potential host rock in Switzerland, and simplified cement pore solution. Because the latter is not a constant over long time-scales, the present model includes simplified concrete consisting of aggregate, portlandite, CSH, ettringite, hydrogarnet and hydrotalcite. In the Opalinus Clay and on a time-scale of 50 000 years, the modelling predicts a zone of less than 23 cm where pH increases from initially 7.2 to values larger than 9. The altered zone is characterised by consumption of primary dolomite, kaolinite, quartz as well as illite in some scenarios, and the formation of zeolites, hydrotalcite, sepiolite as well as in some cases neoformation of illite. The degradation of the concrete mainly consists of consumption of portlandite and transformation of the CSH towards lower Ca/Si-ratios. Most primary concrete constituents are consumed within thousands of years, except the aggregate. First results suggest that on time scales of ten thousands of years, concrete degradation is mainly driven by internal degradation processes, i.e., reactions between aggregate and pore fluid. The mineral reactions result in zones of elevated and of reduced porosity. Precipitation of zeolites in the Opalinus Clay is an important sink of mass, which is mainly derived from the concrete. If zeolite formation is excluded, mass is transferred from the Opalinus Clay to the concrete where CSH phases precipitate. On a 50 000 years time-scale and if porosity is set constant for the transport calculations, the zones with important porosity variations in concrete and Opalinus Clay are less than 30 cm thick. Model runs accounting for the feedback of mineral reactions on transport predict clogging of porosity within tens to hundreds of years.