Characterization of Bentonites from the In Situ ABM5 Heater Experiment at Äspö Hard Rock Laboratory, Sweden

The Alternative Buffer Material ABM5 experiment is an in situ medium-scale performed at Äspö Hard Rock Laboratory (HRL) conducted by SKB Sweden with the aim of analysing long-term stability bentonites used as engineering barrier for a high-level radioactive waste repository (HLWR). In this work, four different ring-shaped Ca- and Na-bentonite blocks, which were piled around carbon steel cylindrical heater, subjected to maximum temperature 250 °C hydrated saline Na-Ca-Cl groundwater (0.91 ionic strength), characterized after dismantling. This work allowed us identify main geochemical processes involved, well modifications physico-chemical properties pore water composition 4.4 years treatment. No significant mineralogy observed samples close heater contact, except increase Fe content due C-steel corrosion, carbonate dissolution/precipitation (mainly calcite siderite) Mg increase. magnetite low amount Fe(II) inside clay mineral structure detected. smectite structure, slight total tetrahedral charge. A decrease external surface area cation exchange capacity (CEC) was found all samples, lower values being detected contact. As consequence diffusion infiltrating groundwater, modification sites occurred. Ca-bentonites increased their Na sites, whereas Ca content. Exchangeable decreased bentonites, MX-80 located bottom part package. salinity gradient through bentonite blocks from granite contact anions are controlled anion exclusion. chemistry evolved function transport chemical equilibrium cations processes. These reactions turn dependent on vapor fluxes.