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Smectite-rich clays are to be used in nuclear repositories for sealing in the radioactive waste. As the radioactive decay produces heat it may affect the chemical, physical and hydromechanical properties of the clay components in the repository. An ‘alternative buffer material’ (ABM) experiment is a Svensk Kärnbränslehantering AB (SKB)-led in situ heating test placed in boreholes in the Äspö tunnel (Sweden). The 2nd ABM package was dismantled in April 2013, after 6.5 y of equilibration with Äspö groundwater and 5 y of heating. The objective was to investigate the long-term effects of the iron heater and Äspö groundwater on four of 31 compacted blocks made of MX-80, Deponit CaN and Friedland clays.Compared to the starting materials, major changes in the exchangeable cation populations were observed. Within horizontal profiles, water-soluble sulfate, Ca, K and Mg increased; poorly crystalline Fe oxide contents decreased; total Mg, Ca and S increased; and a decrease in the amounts of total Na and K away from the host rock towards the heater was observed. At the boundary with the heater, an increase in the total Fe content, decreases in total Si and Al contents, precipitation of gypsum and anhydrite, dissolution of cristobalite and feldspars, and indications of the formation of trioctahedral clay minerals were observed. A decrease in swelling pressure for the Friedland clay (in drill-cored samples) was recorded which was recovered after grinding and recompaction. No effects of hydraulic conductivity were found, after 6.5 y of reaction time, in the subsurface of any of materials studied.

The Swedish and Finnish nuclear waste repository design, KBS-3H, foresees horizontal emplacement of copper canisters-bentonite modules surrounded by a titanium shell. The interaction of titanium with bentonite was studied here using a combination of wet chemistry and a spectroscopic approach to evaluate the potential impact of Ti corrosion on the clay. For natural analogue clays with high Ti contents, spectroscopic investigations showed that titanium occurs as crystalline TiO2. In contrast, the Ti in the MX-80 bentonite occurs in the clay structure, presumably in the octahedral sheet. Hydrothermal tests conducted at 200°C using synthetic montmorillonite showed little if any change in the montmorillonite structure at near-neutral and acidic conditions. Under alkaline conditions, limited alteration was observed, including the formation of trioctahedral clay minerals and zeolite. These changes, however, occurred independently of the addition of Ti. In the batch tests conducted at 80°C, Ti did not occur as separate TiO2particles. The comparison of experimental data with spectroscopic simulations provides sound evidence that Ti was incorporated in a neoformed phyllosilicate structure.

The interaction of two different bulk bentonites (Na- and Ca-types) with three types of simulated cement waters (pH 9.7, 11.3 and 12.0) and one saline groundwater simulate (pH 8.3) as a reference, was studied in batch reactors at 25°C. The solution pH was monitored in order to keep the pH as steady as possible by replacing the leaching solution with fresh one when needed. After 554 days, one set of parallel samples was removed from the experiment in order to investigate the possible changes in the bentonite materials.The buffering capacity of bentonite was clearly observed, especially at the beginning of the high-pH experiments, as the pH of the leaching solutions decreased quite dramatically due to interaction with bentonite. The solution chemistry results showed a decrease of Ca content in all leachate samples, but especially in pH 12.0 experiments. Small amounts of silica were released throughout the experiment. Both bentonites in pH 12.0 experiments also released detectable amounts of Al, while in the lower pH experiments the levels were below detection limit. These observations were also supported by chemical analyses of the bentonite materials. Only minor changes were detected in the mineralogy, and they were mainly concentrated on experiments at pH 11.3 and pH 12.0. The measured swelling pressure showed an increase in pH 12.0 experiments. The results obtained in this research may facilitate modelling of bentonite interaction with high-pH solutions.

Bentonites are candidate materials for encapsulation of radioactive waste. The cation exchange capacity (CEC) has proven to be one of the most sensitive parameters for detecting changes in mineral properties in bentonite-alteration experiments. An interlaboratory study of CECs and exchangeable cations for three reference bentonite buffer materials that were used in the Alternative Buffer Material test (ABM) project in Aspo, Sweden, was conducted to create a suitable database. The present study focused on CEC accuracy and compared CEC methods where care was taken to prevent dissolution of soluble minerals such as calcite and gypsum. The overall quality of the CEC and exchangeable cation values measured using non-Cu-trien CEC methods were good, with CECs of 74–91±0.5–3.3 meq/100 g and exchangeable cation values of 22–61±1.2–3.9 meq/100 g Na+, 7–23±0.8–1.5 meq/100 g Mg2+, and 19–39±0.8–1.6 meq/ 100 g Ca2+. The precision was comparable to the standard Cu-trien method even for exchangeable Ca2+, although the laboratories used different solution/solid ratios and reaction-time parameters for Cu-trien which affect carbonate dissolution. The MX80 and Dep.CAN bentonite exchangeable Ca2+ values were more accurate than standard-Cu-trien values. Using the optimized methods of this study, MX80 and Dep.CAN exchangeable Ca2+ values averaged 20.2±1.6 and 38.8±1.4 meq/100 g which amounts to ~70% of the inflated Cu-trien values. A more complex analysis of the CEC data using different methods, anion analyses, and mineralogical analysis is necessary to obtain plausible and accurate CEC values. Even with a more complicated analytical procedure, the CEC and exchangeable cation values were still not accurate enough because of excess anions. Chloride, sulfate, and dolomite might have increased the exchangeable Na+, Mg2+, and Ca2+ values.

Bentonites are candidate materials for encapsulation of radioactive waste. The cation exchange capacity (CEC) has proved to be one of the most sensitive parameters for detecting changes of mineral properties such as swelling capacity and illitization in alteration experiments. Whether measured differences in CEC values of bentonite buffer samples before and after an experiment are (1) actual differences caused by clay structural changes such as illitization or (2) simply data scatter due to the different methods used by international research teams is an open question. The aim of this study was to measure the CEC of clay samples in five different laboratories using the same method and to evaluate the precision of the values measured. The Cu-trien method and four reference materials of the Alternative Buffer Material (ABM) test project in Aspo, Sweden, were chosen for this interlaboratory study. The precision of the Cu-trien method, which uses visible spectroscopy, was very good with a standard deviation of ±0.7–2.1 meq/100 g for CECs that ranged from 11 to 87 meq/100 g. For the same CEC range, analysis of Cu-trien index cations using inductively coupled plasma (mass spectrometry) and atomic absorption spectroscopy were less precise with a standard deviation of ±2.8–3.9 meq/100 g. Based on the measured precision, greater measured differences in Cu-trien CEC and exchangeable cation values of bentonite buffer samples, before and after an experiment, might be actual differences. Great care must be taken when interpreting measured CEC differences, and analytical characterization of any structural changes may be needed. Compared with results from the ‘International Soil-Analytical Exchange’ (iSE) program for soils, most absolute concentrations were much larger for the clays studied; however, for the two parameters exchangeable Ca2+ and CEC the range was similar to the iSE ring test and, most importantly, the precision was comparable. Future studies should discuss the accuracy of CEC and exchangeable cation values and compare them to alternative CEC methods in which care is taken to prevent dissolution of soluble minerals, such as calcite and gypsum.