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: Compacted bentonite is currently being considered as a suitable backfill material for sealing
underground repositories for radioactive waste as part of a multi-barrier concept. Although showing
favorable properties for this purpose (swelling capability, low permeability, and high adsorption
capacity), the best choice of material remains unclear. The goal of this study was to examine and
compare the hydration behavior of a Milos (Greek) Ca-bentonite sample (SD80) in two types of
simulated ground water: (i) Opalinus clay pore water, and (ii) a diluted saline cap rock brine using
a confined volume, flow-through reaction cell adapted for in situ monitoring by X-ray diffraction.
Based on wet-cell X-ray diffractometry (XRD) and calculations with the software CALCMIX of the
smectite d(001) reflection, it was possible to quantify the abundance of water layers (WL) in the
interlayer spaces and the amount of non-interlayer water uptake during hydration using the two
types of solutions. This was done by varying WL distributions to fit the CALCMIX-simulated XRD
model to the observed data. Hydrating SD80 bentonite with Opalinus clay pore water resulted
in the formation of a dominant mixture of 3- and 4-WLs. The preservation of ca. 10% 1-WLs and
the apparent disappearance of 2-WLs in this hydrated sample are attributed to small quantities of
interlayer K (ca. 8% of exchangeable cations). The SD80 bentonite of equivalent packing density
that was hydrated in diluted cap rock brine also contained ca. 15% 1-WLs, associated with a slightly
higher concentration of interlayer K. However, this sample showed notable suppression of WL
thickness with 2- and 3-WLs dominating in the steady-state condition. This effect is to be expected for
the higher salt content of the brine but the observed generation of CO2 gas in this experiment, derived
from enhanced dissolution of calcite, may have contributed to the suppression of WL thickness. Based
on a comparison with all published wet-cell bentonite hydration experiments, the ratio of packing
density to the total layer charge of smectite is suggested as a useful proxy for predicting the relative
amounts of interlayer and non-interlayer water incorporated during hydration. Such information is
important for assessing the subsequent rates of chemical transport through the bentonite barrier.