FOR PURE CLAY MATERIALS USING PELDRI II

-A new chemical, Peldri II, is evaluated as a compound for drying soft clay materials. Peldri II, a fluorocarbon (1,1-difluorotetrachloroethane), is a solid at room temperature and is a liquid above 25~ Clay gels are embedded in Peldri II by immersing them in the liquid and allowing it to solidify. Once solidified, Peldri II will sublime, with or without vacuum, to a dry specimen, probably without introducing surface tension. Wyoming montmorillonite saturated in 10 3 M NaC1 and 1 M CaCl2 solutions has been examined to compare preservation of its initial structure after Peldri II, critical point, and freeze drying techniques. Transmission electron microscopy ofultrathin sections, scanning electron microscopy, and mercury intrusion porosimetry techniques were used. No differences were detected between Peldri II and critical point drying methods. Peldri II appears to be a significant improvement as an alternative drying agent for clay materials in studies of their structural properties. It is also very convenient for drying large numbers of samples. Key Words--Critical point drying, Freeze drying, Mercury porosimetry, Montmorillonite, Peldri II, SEM, TEM. I N T R O D U C T I O N One of the basic problems that needs to be solved in the preservation of clay structure during the drying process is how to remove water from the sample while minimizing structural modifications. Many methods have been described in the literature for the drying of clay specimens to preserve the initial organization of clay particles. Most of these methods are based on removal of meniscus at the solid-liquid-gas interface without allowing the development of capillary forces. These forces play a major role in the reorganization of the dehydrated clays (Tessier, 1987). The two techniques for moisture removal that have been most commonly employed and are regarded as the most promising are freeze drying (FD) and critical point drying (CPD). Several workers have confirmed that structural changes have taken place in freeze-dried clays and clay gels (Norrish and Rausell-Colom, 1962; Tovey, 1971; Greene-Kelly, 1973). Norr ish and Rausel l -Colom (1962), working with montmoril lonite and nontronite, found that during freezing there was a rapid redistribution of pore water. X-ray analysis showed a decrease in basal spacing of the clay minerals corresponding to the withdrawal of interlamellar water. Particle rearrangement involving larger structural units was found by Tovey (1971) who freeze-dried clay soils and observed samples during the sublimation stage with a scanning electron microscope. Greene-Kelly (1973) and Lawrance et al. (1979) also conducted freeze drying experiments on clay soils and concluded that the technique was not satisfactory. Although FD of swollen Copyright 9 1992, The Clay Minerals Society clay materials often leads to only small losses of the total porosity, some doubts have been raised concerning the validity of the resultant pore size distribution. These problems are summarized in a review article by Lawrance (1977). Some workers (Thompson et al., 1985; Jongerius and Heintzberger, 1975) found that ice crystals altered the porosity, particularly in the center of frozen specimens. FD may also cause artifacts in the form of fissures due to water migration (Murray and Quirk, 1980) or the formation of cracks due to thermal gradients (Gillott, 1980). Others have claimed that FD does give satisfactory results very similar to those obtained by the CPD technique (Diamond, 1970; Mitchell, 1956). The CPD of fine-textured soil has so far been restricted to samples containing either water or liquid carbon dioxide. Diamond (1970) and Sridharan et al. (1971) have both conducted CPD of wet clay samples and found little or no sample shrinkage. Lawrance et aI. (1979) concluded that FD and CPD of clay soils leads to some shrinkage. As with FD techniques the CPD method also has inherent limitations. Possible causes of fabric damage include thermal and pressure stresses in the sample, and possibly also the effects of fluid movements (Gillott, 1980). Nevertheless, it appears that the CPD method modifies the initial structure less than the FD method (Bruand and Tessier, 1987). Therefore, there is a need to explore the possibilities of some alternative, faster and comparable drying techniques to preserve the initial microstructure of clay materials. The present report describes the application of a new