Compacted bentonite is a promising material as an engineering barrier to enclose nuclear waste. The migration of nuclides occurs in the water of bentonite, where the major mineral is sodium montmorillonite. To determine the thermodynamic properties of wafer in compacted sodium montmorillonite, the equilibrium vapor pressure of the water in the montmorillonite was measured as a function of water content and temperature, without external pressure. The thermodynamic properties depend on water content but not on the dry density of unsaturated specimens. In montmorillonite, single-layer adsorption may proceed from 0 to 16 wt water content, two-layer adsorption from 16 to 27 wt, and three-layer adsorption above 27 wt ; pore water appears only in the last region. It is probable that 30 wt of the total water included in saturated montmorillonite is not in the interlayer between platelets at 45.0 wt water content and 0.80 x 10(3) kg/m(3) dry density. There is a very slight amount of water, which is not bound between platelets at dry densities of 1.20 and 1.76 x 10(3) kg/m(3). This water is not a dilute electrolytic solution but has higher ionic strength, like typical seawater of salinity 23 parts per thousand and saturated NaCl. References: 21
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