Saturation compacted bentonite

The three-layered clay mineral montmorillonite (bentonite) is characterised by a low-hydraulic conductivity and a capacity to bind water molecules and positively charged ions (cations). As such, water-saturated compacted bentonite powder is used as a hydrological barrier in areas such as waste disposal, for example around land-fill sites where the desire is to prevent leakage of contaminants from the land-... 

Bourg, I. C., G. Sposito, and A. C. M. Bourg. 2008. Modeling the diffusion of Na+ in compacted water-saturated Na-bentonite as a function of pore water ionic strength. Appl. Geochem. 23 3635-3641. 

PusCH, R. 1980. Water uptake, migration and swelling characteristics of saturated and unsaturated, highly compacted bentonite. Swedish Nuclear Fuel and Waste Management Co. (SKB), KBS TR 80-11, Stockholm, Sweden. 

Bradbury, M. H. Baeyens, B. 2003. Porewater chemistry in compacted re-saturated MX-80 bentonite. Journal of Contaminant Hydrology, 61, 329-338. 

In these experiments a commercially available bentonite, marketed under the name Colclay A90 (Ankerpoort, Geertruidenberg, The Netherlands) was used. It is a sodium-montmorillonite with a third of the exchange complex occupied by calcium. 5.0 g of the air-dried powdered bentonite was weighed into a stainless steel mould with an ID of 50 mm between two porous stones of the same diameter. Then the clay was subjected to a compaction pressure of 20.3 MPa for 30 minutes. After compaction, the mould was placed in a bowl of NaCl-solution for five days in which the clay became saturated and swollen. Thus samples were obtained with thickness of 3.8 and 2.8 mm respectively and a diameter of 50 mm. 

The FEBEX in situ test has been conducted as a multi-national project coordinated by the Spanish organization ENRESA. In essence, the experiment involves the installation of two cylindrical heaters (4.54 m long and 0.97 m in diameter) centered in a tunnel (diameter 2.27 m) and surrounded by a barrier (thickness 0.64 m) made of highly compacted unsaturated blocks of bentonite (Figure 1). The tunnel is located in a mountainous area of massive saturated granite about 420 m below ground surface. 

Figure 5 shows the volume change measured in the samples loaded at different temperatures keeping their water content constant. As it has been observed in saturated materials, the temperature increases the compressibility of the bentonite. A certain reduction in the size of the elastic domain with the temperature can be observed (Hueckel Borsetto 1990), despite the fact that the vertical stresses applied are smaller than the compaction load. 

The experiment, as for the Buffer/Container Experiment, was based on a 1.24 m diameter 5 m deep borehole at the 240 m Level of AECL s Underground Research Laboratory. A 50 50 bentonite sand buffer was compacted to a depth of 2m into the base of the borehole, (Chandler, 2(XX), Dixon et al., 2002). A 1.25 m thick concrete plug was then installed above the buffer to act as a vertical resistance to any swelling that might occur. The granite rock around the experiment could be considered to be un-fractured and nearly saturated, (Chandler, 2000). The layout of the experiment is shown in Figure 1. 

Many of the lining materials commonly used for liquid storage cannot be used in salt gradient solar ponds. Compacted soils, native clays, soil additives or soil cement, swelling clays such as Bentonite, are not Impermeable to high temperature saturated salt brine solutions. Only the flexible membrane lining materials offer the potential for impermeability (zero leakage)... 

---