The effect of pressure on the dehydration reaction of interlayer water in Na-montmorillonite (SWy-1)

The temperature of the dehydration of interlayer water of Na-montmorillonite (SWyl, American Clay Mineral Society Source Clay) is determined at pressures to I kbar, using differential thermal analysis (DTA). Two dehydration reactions occur, about 40 and 100C above the boiling curve of water. Above the critical point of water the dehydration reactions show only a modest increase of temperature with pressure. No significant differences in temperature were found using different heating rates and different size fractions, as defined by their hydraulic diameter. The presence of two dehydration reactions suggests that not all interlayer water is bonded equally; a distinction may be made between weakly bonded and strongly bonded water. It is concluded that hydrated Namontmorillonite is stable under normal pressure and temperature distributions in sedimentary basins; in these basins dehydration must involve more complex chemical interactions with pore fluids. The large increase in the stability of a hydrated montmorillonite with a modest increase in pressure may have important bearing in its use in nuclear waste disposal.