Extensive drying of smectites can cause the interlayer space to break down (collapse). This can affect the properties of bentonites as geotechnical barriers of HLRW (highly radioactive waste) repositories. If and to what extent the collapse occurs depends strongly on the type of interlayer cation. In particular K is known to lead to ready dehydration, in contrast to Ca and Mg. In the present study, various bentonites and one illite/smectite clay were dried/heated at 90ºC for 1.5 a and in a different experiment at 120ºC for 4.5 a and investigated with respect to mineralogical and geochemical changes of the smectite.

Smectite alteration after extensive drying was restricted to changes of the exchangeable cations. The CEC decreased by 9% (90ºC test) and 14% (120ºC test). A slight decrease of exchangeable Na+ was observed following the 90ºC test. No significant further decrease was observed after the 120ºC test. In contrast, the larger cation exchange capacity (CEC) decrease after the 120ºC test could be explained by increased Ca/Mg fixation. A possible mechanism for the observations is presented.

This study describes geochemical barriers that have developed at three different localities in sulfiderich tailings in the Kemerovo region of Western Siberia, Russia. Iron oxyhydroxides, gypsum, malachite, chalcanthite, goslarite, bianchite, gunningite and copper and zinc chloridescrystallized in the sequence specified at an evaporative barrier around glassy slag produced by the Belovo zinc processing plant. A complex cemented barrier that has developed within the old Salair sulfide tailings contains two well defined layers: an upper layer containing Fe(III) mineralsand gypsum as cements in which Pb, As, Mo, Ni and Co have been deposited; and (2) a lower calcite- and gypsum-bearing layer, in which phases containing Zn, Cd and Cu have been deposited. A complex organic-mineral barrier below the Ursk sulfide tailings consists of peaty organic matter, clayminerals and iron oxyhydroxides cemented by gypsum. Elements that have leached from the tailings are present in this barrier in a variety of different forms: Ca and Mn are present as water-soluble species; Cu, Fe and Zn are present as species produced by interaction with organic matter viaion-exchange, metal humate formation and cation bridging in organic-mineral complexes; Pb and As are co-precipitated with and/or adsorbed onto iron oxyhydroxides; gold has been deposited as minute particles of native metal. The mechanisms for the formation of the different geochemical barriersare discussed.