Expandable clays

Normally, castables are 25 percent cements and 75 percent aggregates. The aggregate is the more chemically resistant of the two components. The ni est-strength materials have 30 percent cement, but too much cement results in too much shrinkage. The standard insulating refractory, 1 2 4 LHV castable, consists or 1 volume of cement, 2 volumes of expanded clay (Haydite), and 4 volumes of vermicuhte. 

Cations exchanged into the interlayers of expandable clays (smectites) are comparatively easy to study with NMR methods because the cations become major components of the phase and their concentrations are often several wt %. In addition to Cs Li, Na, K, and Cd have been studied by NMR. We have chosen to investigate Cs because it is a significant component of nuclear waste, because it provides an end-member case as the least electronegative cation, and because it has desirable nuclear properties (100% abundance, relatively high frequency, 65.5 MHz at H = 11.7 T, and small quadrupole moment)... 

Montmorillonite is a laminar and expandable clay with wet binding properties and widely available throughout the world. The layers have permanent negative charges due to isomorphic substitutions. The scientific interest of montmorillonite lies in its physical and chemical properties as well as its low price. Consequently, the industrial application of montmorillonite is an attractive process [1]. On the other hand, among numerous reports published so far, crystallization of zeolite Beta draws much attention because of its unique characteristics, in particular, acidity and acid catalysis. It is reasonable to conceive that a catalyst system based on Beta/montmorillonite composite with suitable composition should provide a good catalytic capacity. 

An understanding of much of aqueous geochemistry requires an accurate description of the water-mineral interface. Water molecules in contact with> or close to, the silicate surface are in a different environment than molecules in bulk water, and it is generally agreed that these adsorbed water molecules have different properties than bulk water. Because this interfacial contact is so important, the adsorbed water has been extensively studied. Specifically, two major questions have been examined 1) how do the properties of surface adsorbed water differ from bulk water, and 2) to what distance is water perturbed by the silicate surface These are difficult questions to answer because the interfacial region normally is a very small portion of the water-mineral system. To increase the proportion of surface to bulk, the expanding clay minerals, with their large specific surface areas, have proved to be useful experimental materials. 

Based on the study of expanding clay minerals, two models of water adsorbed on silicate surfaces have been proposed. One states that only a few layers (<5) of water are perturbed by the silicate surface, the other concludes that many layers (perhaps 10 times that number) are involved. The complexity of the interactions which occur between water molecules, surface adsorbed ions, and the atoms of the silicate mineral make it very difficult to unequivocally determine which is the correct view. Both models agree that the first few water layers are most perturbed, yet neither has presented a clear picture of the structure of the adsorbed water, nor is much known about the bonding of the water molecules to the silicate surface and to each other. 

Our approach has been to study a very simple clay-water system in which the majority of the water present is adsorbed on the clay surfaces. By appropriate chemical treatment, the clay mineral kao-linite will expand and incorporate water molecules between the layers, yielding an effective surface area of approximately 1000 m2 g . Synthetic kaolinite hydrates have several advantages compared to the expanding clays, the smectites and vermiculites they have very few impurity ions in their structure, few, if any, interlayer cations, the structure of the surfaces is reasonably well known, and the majority of the water present is directly adsorbed on the kaolinite surfaces. 

The concept zeolites conventionally served as the synonym for aluminosilicates with microporous host lattice structures. Upon removal of the guest water, zeolites demonstrate adsorptive property at the molecular level as a result they are also referred to as molecular sieves. Crystalline zeosils, AlPO s, SAPO s, MAPO s (M=metal), expanded clay minerals and Werner compounds are also able to adsorb molecules vitally on reproval of any of the guest species they occlude and play an Important role in fields such as separation and catalysis (ref. 1). Inclusion compounds are another kind of crystalline materials with open framework structures. The guest molecules in an inclusion compound are believed to be indispensable to sustaining the framework structure their removal from the host lattice usually results in collapse of the host into a more compact crystal structure or even into an amorphous structure. 

In zones of hydrothermal alteration it is apparent that the formation of dioctahedral montmorillonites is limited by temperature. They almost never occur in the innermost zone of alteration, typically that of sericitization (hydro-mica or illite), but are the most frequent phase in the argillic-prophylitic zones which succeed one another outward from the zone where the hydrothermal fluid is introduced in the rock. Typically, the fully expandable mineral is preceded by a mixed layered phase (Schoen and White, 1965 Lowell and Guilbert, 1970 Fournier, 1965 Tomita, et al., 1969 Sudo, 1963 Meyer and Hemley, 1959 Bundy and Murray, 1959 Bonorino, 1959). However, temperature is possibly not the only control of expandable clay mineral occurrence, the composition of the solutions and the rock upon which they act might also be important. It is possible that high magnesium concentrations could form chlorite, for example, instead of expandable minerals. 

Haque, N., Morrison, G., Cano-Aguilera, I. and Gardea-Torresdey, J.L. (2008) Iron-modified light expanded clay aggregates for the removal of arsenic(V) from groundwater. Microchemical Journal, 88(1), 7-13. 

Expanding clays clays that expand or swell on contact with water, e.g., mont-morillonite. 

The dioctahedral subgroup is by far the most abundant. The layer charge on the expanded clays ranges from 0.3 to 0.8 per Oi0 (OH)2 unit of structure. The low-charged (0.3-0.6), expanded minerals are called montmorillonite, montmorillonids, and smectites, among others. Subdivision of the expanded clay group is still a problem. 

Although they are rare in sediments, there is a wide spectrum of trioctahedral expanded clays. The most common in the low-charge range (0.3—0.5) are hectorite, which contains magnesium and lithium in the octahedral sheet, and saponite, which has considerable magnesium in the octahedral sheet and some aluminum substitution in the tetrahedral sheet. 

The material from the Hector area of California is believed to have formed by the action of hot spring waters containing Li and F on clinoptiolite. The Mg was obtained from the alkaline lake waters (Ames and Goldich, 1958). The material from Morocco is associated with marls and is believed to be authigenic. These two types of trioctahedral smectite appear to be the only ones with a relatively pure Si tetrahedral sheet. No analyses were found which indicated tetrahedral Al values between 0.02 and 0.30. Analyses of saponite indicate there is complete isomorphous substitution between the range Si3.70 Al0.3o and Si3.0s Al0.92 (Table XXXIX). Caillere and Henin (1951) reported an analysis of a fibrous expanded clay (diabantite) which had a tetrahedral composition of Si3.i7 Alo.49 Fe3+0.34. There is some question as to whether this should be classified as a smectite regardless, it indicates the possibility of Fe3+ substitution in the tetrahedral sheets of the trioctahedral 2 1 clays. 

Some of expanding clays in soils have the attributes of vermiculite, some of smectite, and some have features of both. The variation in properties is largely related to the layer charge. The charge is dependent on the original charge on the 2 1 layer of the parent mineral and the amount of ferrous iron in the octahedral sheet. The oxida-... 

Quite often Al, Fe, and Mg hydroxides partially fill the interlayer position of the derived vermiculites and decrease their exchange capacity and their ability to contract completely to 10 A when heated or when treated with a potassium solution. This material can usually be removed by treating the clay with a solution of sodium citrate (Tamura,1958). As the content of hydroxy interlayer material increases, the expandable clay tends to assume the character of a chlorite. Thus, in the weathering of a mica or illite it is not uncommon to form discrete vermiculite-like, beidellite-like, monf-morillonite-like and chlorite-like layers. These various layers can occur as discrete packets or interstratified in a wide variety of proportions. 

Much of the derived expanded clay, even that which resembles montmorillonite (holds two layers of ethylene glycol), will contract to 10 A when exposed to a potassium solution. Weaver (1958) has shown that these clays can obtain sufficient potassium from sea water and readily contract to 10 A. Vermiculite and mixed-layer biotite-vermiculites are rare in marine sedimentary rocks. Weaver (1958) was unable to find any expandable clays in marine sediments that would contract to 10 A when treated with potassium. A few continental shales contained expanded clays that would contract to 10A when saturated with potassium. Most vermiculites derived from micas and illites have high enough charge so that when deposited in sea water they extract potassium and eventually revert to micas and illites. Some layers may be weathered to such an extent that they do not have sufficient charge to afford contraction and mixed-layer illite-montmorillonites form. 

Diagenetic modification of expandable clay during burial is an important source of mixed-layer illite-montmorillonite. With increasing depth of burial and increasing temperature the proportion of contracted 10 A layers systematically increases. From about 50°C— 100°C the contracted layers are distributed randomly. At higher temperatures only a few additional layers are contracted but the interlayering becomes more ordered (Perry and Hower, 1970 Weaver and Beck, 1971a). The final product, 7 3 to 8 2, is relatively stable and persists until temperatures on the order of 200°C— 220° C are reached. 

The data for the expanded and contracted minerals plot as two separate linear relations with contracted clays having larger tetrahedral rotation values for given oct.Atet. values than the expanded clays. This is presumably due to the K which aids the tetrahedral rotation in the contracted clays. 

Weaver, C. E., 1958. The effects and geologic significance of potassium fixation" by expandable clay minerals derived from muscovite, biotite, chlorite, and volcanic material. Am. Mineralogist, 43 839-861. 

Another technique for removing spillage from a diked area is the use of an all-purpose sorbent. The effectiveness of using sorption as a removal technique is influenced by factors such as the physical and chemical attraction between the spilled material and the sorbent, the surface geometry and area of the sorbent, the contact time between the materials, and the density ratio of the spilled fluid and the sorbent (Bauer et al., 1975). Some effective sorbents include polymethacrylate foam resins, propylene fibers, molecular sieves, expanded clays, polyolefins, polymethylmethacrylates, and polystyrene sulfonates (Bauer et al., 1975). 

Clays in Water (Penetrating Liquid). The apparent densities of the expanding clays in water are higher than those of the same clays in hydrocarbon liquids. For example, the apparent density of sodium Wyoming bentonite in water is 2.793 grams per cc. (four determinations, standard deviation 0.008 gram per cc.). 

---