Interlayer / Micropore Exchange of Water and Ions in Clays : a Molecular Dynamics Study

Cement bentonite interactions need to be studied since they will occur in deep geological repositories.They are specifically relevant in the clay host rock reference concept. The reactivity of a Mg-homoionicFEBEX bentonite was studied at 60 oC in contact with a young cement water characterized by the leachingof alkaline hydroxides (K/Na 4/1 –OH, pH = 13.5 at 25 oC) and with an evolved cement water controlledby the portlandite dissolution (Ca(OH)2, pH = 12.5 at 25 oC). The experimental approach was to rundiffusion experiments carried out in a cylindrical compacted bentonite sample, 2.1 cm long with a diameterof 7.0 cm, which is exposed on one circular face to a solution of cementitious water. A second reservoirof fluid is located at the opposite face of the bentonite sample; this contained MgCl2 solution which wasused during the homoionization process. The bentonite sample is maintained at a constant temperature of60 oC throughout the experiments run for 6 and 12 months. The key processes investigated were to identify and confirm, considering previous studies (Sánchez et al.,200; Savage et al., 2007), the nature of the newformed mineral phases (i.e., zeolites, CASH and Mg-silicatephases) as a result high pH reactivity of bentonite. This was complemented by addressing the spatialextension affected by mineralogical and geochemical modifications in compacted bentonite, including theextension of cationic exchange. The diffusion of the hyperalkaline plume (OPC K/NaOH solution) through compacted bentonite (1.6 g/cm3dry density) produces a mineralogical alteration front characterized by a critically cemented rim ofapproximately 2-3 mm (Figure 1). The cemented material is characterized by a drastic reduction on itsexternal specific surface (from 80 to 20 m2/g) as well as the CEC (100 to 50 m2/g). The thickness of therim did not evolve with time, then, diffusion becomes very slow due to the reduction of porosity. The selfsealing of the high pH concrete-bentonite interface has been predicted in some models and this processshould be taken into account as a potential self-stopped reactivity scenario. The mineralogical composition of the rim is a mixture of poorly ordered, Mg-rich, clay materials, mainlybrucite, hydrotalcite and tri-octahedral Mg-smectite. Montmorillonite is partially dissolved and a part of itremained trapped within the newformed cements. The alteration rim has been accurately measured bymeans of EDX-chemical profiles in flat polished sections examined under SEM microscopy (Figure 2)Alkali-zeolites have not formed at all as far as no pore-space is available for these lower density silicates.Then, alkaline cations (mainly K+) have diffused beyond the altered rim, affecting the whole 2.1 cm lengthof the compacted bentonite disc. The K+ exchange in the montmorillonite is homogeneous in the bentoniteprobe but it did not saturate completely the exchangeable positions. This can be another indication of thestopped reactivity process. The same studies are being performed at 90 oC in order to compare the extentof the diffusion and reaction processes. At pH 12.5 and 60 oC there was not detected any significant mineralogical alteration. The main outcomeof these experiments evidence the very limited thickness of mineralogical alteration affecting a highlycompacted bentonite exposed to the effect of hyperalkaline solutions.