Hydration of bentonite in natural waters: Application of “confined volume” wet-cell X-ray diffractometry

The hydration behavior of compacted bentonites (Na- and Ca-montmorillonite varieties) in natural ground and sea water is studied using in situ wet-cell X-ray diffraction monitoring techniques. This approach allows us to determine the mechanism and rate of solution uptake in a confined volume, flow-through reactor and serves as an experimental analogue for predicting the performance of repository clay sealants. The pressed bentonite powders (densities of 0.94–1.14 g/cm3) show continuous and strongly partitioned water uptake into montmorillonite interlayers, onto clay particle surfaces and within open pore spaces. During the hydration of compacted Na-bentonite in both ground and sea water, roughly equal quantities of both interlayer and non-interlayer water enter the material. In contrast, the Ca-bentonite was dominated by the intake of more loosely bound, surface and pore water, which amounted to roughly three times more than that incorporated into interlayer sites. Our experiments demonstrate how a confined reaction volume and the strength of the ionic solution both inhibit the interlayer expansion process. Based on the weakly compacted Na-bentonite analogue, a 1 m thick clay sealant is predicted to saturate within 7 years when infiltrated by typical continental ground water, and within 3 years in the case of a sea water breach. As significant volumes of solution are incorporated as loosely bound, non-interlayer water, quantification of the mechanism and rate of water storage is a necessary requirement for improved modeling of elemental transport in a hydrating bentonite medium.