Silicate gels

Syneresis of sodium silicate gels may occur under some conditions, eg, in pure gels or coarse formations. Cement grouting should then precede chemical grouting. Leaching that results from dissolution under water-saturated conditions may be eliminated by use of proper reagent proportions. 

Siliceous gel zeolite A synthetic, inorganic exchanger produced by the aqueous reaction of alkali with aluminum salts. 

The latter interpretation of data is more in accord with the recent Al and Si NMR findings of Ellison Warrens (1987), who found that the structure of an appreciable fraction of the glass changed under acid attack with some loss of aluminium including all in fivefold coordination (see Section 5.9.2). Thus, acid attack was not entirely confined to the surface layer of a glass particle. If this is so then silicic acid as well as ions must migrate from the body of the particle and it is reasonable to suppose that silicic acid deposits as siliceous gel at the particle-matrix interface. 

The picture of cement microstructure that now emerges is of particles of partially degraded glass embedded in a matrix of calcium and aluminium polyalkenoates and sheathed in a layer of siliceous gel probably formed just outside the particle boundary. This structure (shown in Figure 5.17) was first proposed by Wilson Prosser (1982, 1984) and has since been confirmed by recent electron microscopic studies by Swift Dogan (1990) and Hatton Brook (1992). The latter used transmission electron microscopy with high resolution to confirm this model without ambiguity. 

The form of silica in the matrix is at present unknown. In the freshly prepared cement there are appreciable amounts of silicic acid present which decline as the cement ages (Crisp, Lewis Wilson, 1976d). In the set cement silica could be present as a polymeric silicic acid, a siliceous gel or even a hydrated silicate gel, such as the tobermorite gel present in Portland cements (Taylor, 1966). 

The nature of the setting reaction was finally elucidated by Wilson et al. (1970a), who established that formation of an aluminium phosphate gel was responsible although siliceous gel was also formed it merely coated the partly reacted glass particles. 

In the late 1940s zeolites were synthesized according to the procedure shown in Fig. 3.24. First an amorphous alumino-silicate gel is formed. This process is completely analogous to the production of alumina and silica gels described before. Subsequently this gel is crystallized into zeolite. The preparation of zeolites has drawn tremendous attention of the scientific and industrial community. A wide variety of zeolites have been synthesized, and reproducible synthesis procedures have been reported (often in the patent literature). Natural zeolites also exist massive deposits have been discovered in many places in the world. 

Silicate gel enhances the sweep efficiency of a waterflood, gasflood, or steamflood operation by reducing the permeability of the high-permeability zones. Weak acids may be added to control gel generation rate [377]. 

Role of alkali and NH cations in the crystallization of ZSM-5 Introduced in an aqueous (alumino) silicate gel (sol), the bare alkali cations will behave in various ways firstly, they will interact with water dipoles and increase the (super) saturation of the sol. Secondly, once hydrated, they will interact with the aluminosilicate anions with, as a result, the precipitation of the so formed gel (salting-out effect). Thirdly, if sufficiently small, they also can order the structural subunits precursors to nucleation species of various zeolites (template function-fulfilled by hydrated Na+ in the case of ZSM-5 (11,48)). ... 

Other nanocomposite CNT electrodes have been reported by mixing CNTs with granular Teflon [48], chistosan [110], polystyrene [111], polysulfone [112] and epoxy [103, 113] or by incorporating them into a silicate gel matrix [13, 114]. 

Similarly, the iminodithiocarbonate 72 serves as a thio nitrile ylide synthon (38). When treated with fluoride ion in the presence of aldehydes and ketones, it gave the alcohols 73 as isolable products that were converted into 74 on treatment with silical gel. Best results were obtained with aromatic aldehydes containing electron-withdrawing groups yields were poorer when electron-donating groups were present and for ketones. 

Del Nero, M., Advocat, T., Jollivet, P. Bontems, G. 1999b. Sorption of neptunium (V) on an alteration gel of alumino-borosilicate glasses and on synthetic silicate gels. Proceedings 2nd International Symposium on Nuclear Fuel Cycle Safety Engineering Research Facility NUCEF 98, Hitachinaka, Ibaraki, Japan, 584-595. 

A solution of 1.0 g of this product and 0.025 mL of 25% methanolic NaOMe (10 mL) was stirred at room temperature until the reaction was completed. The mixture was cautiously neutralized with Amberlite IR-120 (H+) ion-exchange resin. After filtration and concentration, the material was passed through a small bed of silical gel pretreated with 2% triethylamine in hexane. Elution with chloroform-methanol (8 1) containing 1% of tri-ethylamine gave the pure title compound as a pale yellow solid (0.61 g, 90%) mp 93°C, [a]D +4.0° (c 0.9, MeOH). 

Octadecyl-bonded phases Copper silicate gel Cyclodextrin-bonded phases Hydroxyapatite-covered silica Graphitized carbon... 

DeKimpc, C.R., 1969. Crystallization of kaolinite at low temperature from an alumino- silicic gel. Clays Clay Miner., 17 37-38. 

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