Characterization of Mg-clays from the Neogene of the Madrid Basin and their potential as backfilling and sealing material in high level radioactive waste disposal

Abstract

Three types of sedimentary materials are studied, referred to as green, brown and pink clays. They are exploited in a number of 0.25-3 m thick layers and contain magnesium smectites. The smectite content in the green and brown clays averages over 80%, with illite and sepiolite and small quantities of quartz and feldspar. The pink clay composition corresponds to disordered interstratified kerolite-stevensite, with a varying stevensite content (20dash45%). The structural formula of the smectite in the <0.5 μm fraction of the the green clays corresponds to a saponite type (unit cell tetrahedral charge −0.65; octahedral charge: −0.24) and significant Fe 2+ presence. In the brown clays the saponite increases the charge in the octahedral layer to −0,44 and tends towards stevensite terms. Adsorption, external specific surface and swelling pressure are very similar in the saponitic clays, with, respectively, average values of: 73–75 meq/100 g ( CEC), : 160–170 m 2/g ( S BET) and 4.6-5.28 MPa ( Ps) (precompacted at dry density 1.4 g.cm −3). When there is more than 15–20% sepiolite in the green clays, however, the CEC drops, the external specific surface increases and with unvarying density the swelling pressure doubles. This increase in swelling pressure is related to an additional increase in osmotic pressure produced in the external surfaces. This reaches 15–16 MPa in the pink clays with a specific surface of 250–260 m 2/g. Maximum values of 19.4 MPa are reached in discrete levels of green clays containing 70% sepiolite and with a specific surface of 287 m 2/g. The genesis of these materials is related to weathering processes in the fluviolacustrine environment in the different zones (lake centre and edge, swamp soils and vadose zone), associated with the evolution of reducing conditions and an alkalinity increase in the water table. The kinetics of these processes would appear to be extremely slow, as long periods without sedimentation are required in order to form significant quantities of magnesium clays and especially saponite. Saponitic clays display qualities that were suitable for their possible use as backfilling and sealing material, although the green clays appear to provide better conditions regarding the rehydration properties associated with the exchange complex and their lower carbonate content.