The goal of the completed PhD project (Dolder, 2015, Dolder, 2014) was to characterize and quantify the cement/bentonite skin effects spatially and temporally in laboratory experiments. A newly developed mobile X-ray transparent core infiltration device was used, which allows performing X-ray computed tomography (CT) periodically without interrupting a running experiment. CT scans allowed tracking the reaction plume and the core volume/diameter in the running experiments. The experiments attested an effective buffering capacity for bentonite and sand/bentonite, a progressing coupled hydraulic-chemical sealing process and also the preservation of the physical integrity of the interface region in this setup with a total pressure boundary condition on the core sample. No complete pore-clogging was observed but the hydraulic conductivity got rather strongly reduced in 3 experiments, explained by clogging of the intergranular porosity (macroporosity). Such a drop in hydraulic conductivity may retard the saturation time of the buffer in a nuclear waste repository.
The Mt. Terri CI (cement clay interaction) experiment contains bentonite in contact with different cements. Intact interfaces have been sampled by RWI, and are currently in the focus of mainly Japanese research (CRIEPI).
Extensive investigations on concrete-Opalinus Clay (OPA) interfaces recovered after 2.2 y of interaction from the Cement-Clay Interaction (CI) experiment at the Mont Terri Underground Laboratory (St. Ursanne, Switzerland) revealed significant alteration in the different cements within a couple of millimetres, but minor changes in OPA (Jenni et al., 2014).
Owing to the especially developed drilling technique for coring undisturbed interfaces (Jenni et al., 2014), a larger number of high-quality samples could be retrieved from the same interfaces after 4.85 y of interaction, allowing for more extensive investigations. These include
The thin interaction layers in both cement and OPA, the multi-mineral concrete aggregates that disturb investigations on the cement in between, and the presence of poorly crystalline and/or solid-solution phases, necessitate this comprehensive suite of spatially resolved expensive techniques.
RWI studies concentrate on three concrete-OPA interfaces, whose concretes contain 1) Portland cement (OPC), 2) ESDRED cement especially designed for repository applications (40% of cement substituted with silica fume), and 3) low alkali cement (LAC, containing slag and nanosilica).
In general, the chemical zonation and the thicknesses of the zones did not change from 2.2 y to 4.85 y of interaction, but strongly depend on the type of cement. The striking Mg enrichment close to OPA occurs only in the low-pH cements LAC and ESDRED, where the OPA is also enriched close to the interface. Only a fraction of this Mg enrichment can be attributed to the observed increase of Mg on the clay exchanger (at the expense of Ca and Na), but µXRD indicates the presence of a poorly crystalline magnesium silicate hydrate (MSH). The same phase is present in the Mg enriched cement, as shown by µXRD (Figure) and XANES, both spectra are compared with measurements on pure synthesised MSH. Structure and thermodynamic property investigations are ongoing, first results indicate similarities to talc. Further characteristics of the interaction zone are a decalcification in all three cements close to OPA, and a sulphur depletion, followed by a layer enriched in S further away. Thicknesses of observed carbonation layers mainly depend on the water/binder ratio of the cement. Decalcification decreases, carbonation increases porosity in the cements.
Data from 2.2 y and 4.85 y samples both suggest diffusion of CO2/HCO3-, SO42-, and Mg species, from OPA into cement. Possible pH decrease in the cement next to the interface leads to instability of sulphur-bearing cement hydrates, the S in solution diffuses towards higher pH (away from the interface), where ettringite can form and incorporate S. In turn, the cement releases Ca, which is partially found in short calcite veins extending into the OPA.
Reactive transport modelling helps to understand mechanisms, which interact and leave behind complex traces in interface samples. Modelling cement – clay rock interaction is extremely demanding:
Two multicomponent reactive transport codes meet these demands: CrunchFlowMC (Carl Steefel, clay-specific extensions developed in collaboration with RWI, Alt-Epping et al, 2014), and an in-house developed extension of FLOWTRAN. The set-up of the Mt. Terri CI experiment, a concrete – Opalinus Clay interface, was simulated from right after casting the concrete into a drill hole. The model results reproduce chemical zonations measured by SEM EDX after 3 and 5 y, and meet mineralogical gradients observed by various methods (Jenni et al., 2014). The numerical results allow for isolating the major interacting mechanisms:
All these mechanisms influence porosity at different locations. The model outcome can give an estimation of porosity evolution with time, and emphasise the change in transport properties.