Natural systems evidence for the effects of temperature and the activity of aqueous silica upon montmorillonite stability in clay barriers for the disposal of radioactive wastes

Abstract

Natural systems data can provide important constraints upon the performance and longevity of montmorillonite in clay engineered barriers in a geological disposal facility for radioactive wastes. Here, natural systems evidence for the effects and impacts of temperature and the activity of aqueous silica upon montmorillonite stability and behaviour in compacted clays has been evaluated. An assessment of the effect of temperature showed that montmorillonite may be stable to temperatures as high as 300 °C under appropriate chemical conditions. Although illitization is often the focus of smectite transformation considerations, reaction of montmorillonite may form beidellite, saponite and quartz, rather than illite, in the absence of excess K+.Thermodynamic modeling of clay stability using three different thermodynamic databases showed that stability fields for montmorillonite exist from 0 to 140 °C, but at low values of silica activity, a stability field for illite replaced that for montmorillonite. Pore fluid chemical and mineralogical data for sediments from a range of ODP sites from offshore Japan showed a trend from montmorillonite + amorphous silica stability at temperatures up to 60 °C to that for illite + quartz at higher temperatures. However, even over very long timescales (≫1 Ma), smectite did not transform appreciably to illite under thermodynamically-favourable conditions at temperatures <80 °C. Natural systems evidence showed that silica released from montmorillonite during clay mineral transformation reactions may cement clay grains as sheets of microcrystalline quartz, thereby increasing stiffness and brittle behaviour. This process occurred at temperatures in excess of 70 °C but may be significant only over million-year timescales.