Aluminate Gels

The aluminosilicate compound mullite, Al6Si20i3, is one of the most commercially important materials to be conveniently prepared by thermally decomposing gel precursors. The reactivity of the precursor gel depends on the preparation method, which 

Knowledge of the aluminium species present under alkaline conditions is of critical importance to industrial chemical operations such as the Bayer process in which the aluminium present in bauxite ores is dissolved in concentrated sodium hydroxide solution. Al NMR has been used to investigate the A1 coordination states in the resulting sodium aluminates after freeze drying (Bradley and Hanna 1994). At an OH Al ratio 4.4, the predominant species is Na[Al(OH)4] (Q ) with a Al resonance at 86.6 ppm. A broader resonance at 71.3 ppm is probably composed of a variety of other poly-oxoanionic species such as [Al(OH2)(OAl)2] (Q ), [Al20(0H)6] (Q ) and 

The thermal development of amorphous sialon fibres from polyaluminocarbosilane gels at 1000°C and their crystallisation to p-sialon fibres at 1400-1500°C has been studied by Al and Si NMR (Soraru etal. 1993). The gel precursor shows the typical 3-peak Al spectrum, with resonances at 3.2, 32 and 57.4 ppm but the amorphous fibres formed at 1000°C contain only tetrahedral Al. On crystallisation of p-sialon, the tetrahedral Al resonance broadens due to the occurrence of mixed AIO4 - xNx sites (Soraru eta/. 1993). 

The spectra of a series of framework aluminosilicate glasses (Oestrike et al. 1987) indicate the presence of only tetrahedral Al, corresponding to the structures of the corresponding crystalline aluminosilicates, but with broadened Al resonances. As with the crystalline framework aluminosilicates, the Al becomes progressively deshielded as the compositional parameter Si/(Si(IV) 3- Al(IV)) decreases, leading to a linear relationship between this parameter and the chemical shift 8 

A series of lanthanum aluminosilicate glasses has been shown by Al MAS NMR to contain principally Al(IV) which therefore acts as a network-former, with small amounts of A1(V) and Al(VI) appearing in glasses of higher AI2O3 content (Clayden et al. 1999). 

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Many cements used today are composites of Portland cement and industrial waste materials that can enter into the hydration reactions and contribute to the strength of the hardened product. These substances include pulverized fuel ash (PFA) from burning of pulverized coal in thermal power stations, crushed blast-furnace slag (Section 17.7), and natural or artificial pozzolanas—that is, volcanic ash and similar finely particulate siliceous or aluminosilicate materials that can react with the Ca(OH)2 in Portland cement to form hydrated calcium silicates and aluminates. As noted earlier, the solubility of Ca(OH)2 is such that the pH of pore water in Portland cements will be about 12.7, at which the Si-O-Si or Si-O-Al links in the solid pozzolanas will be attacked slowly by OH- to form discrete silicate and aluminate ions and thence hydrated calcium silicate or aluminate gels. 

Figure 5.21. Stacked and contour representations of the A1 MQMAS NMR spectra of lanthanum aluminate gels of (A) low-lanthanum content, in which three resonances assigned to Al, Al and Al can be distinguished, and (B) high-lanthanum content gel, in which only the octahedral and tetrahedral A1 resonances are observed. The solid line indicates the isotropic chemical shift, the dashed line shows the direction of the anisotropic quadrupolar line broadening and the dotted-dashed line indicates the direction of the quadrupolar-induced shift. From luga et al. (1999), by permission of the American Chemical Society.

Similarly influence the soluble fluorides for example the F ions released from NaF to the solution form the insoluble CaFj which covers the cement grains. The complex fluorosilicates Na2SiFg and NajAlFg can additionally result in the precipitation of siUcate or aluminate gel on the surface of cement grain poor in calcium. Phosphogypsum, soluble calcium phosphates and sodium fluoroaluminates containing by-products exhibits the retarding effect too [96]. 

Figure 3.9 Solubility diagram of an ainorpliou.s aluminic gel (data obtained from [29bJ)...

Figure 3.10 Po.ssible reaclioii niechanisnis for the formation of Al(OH)i (gibbsite, bayerite) during evolution of the aluminic gel at pH < 5 and pH > 8. XRD of the solids precipitated at pH 10 and aged (a) at pH 4.5 for ) month (gibbsite) and (b) at pH 10 for 1 week (bayerite) (J. Hernandez, and J.P. Jolivet. unpublished results)...

The s)mthesis of zeolites is traditionally performed by crystallisation from a sol-gel mixture comprising reagents such as silica, sodium aluminate, sodium hydroxide and water. Another key component of the sol-gel mixture is a base whose main role is to regulate the pH of the mixture. If an organic base is used then a templating effect may also be observed... 

Fig. 6. Adsorption capacity of various dessicants vs years of service in dehydrating high pressure natural gas (39). a, Alumin a H-151, gas 27° C and 123 kPa, from oil and water separators b, siUca gel, gas 38° C and 145 kPa, from oil absorption plant c, sorbead, 136-kPa gas from absorption plant ...

Typical gels are prepared from aqueous solutions of reactants such as sodium aluminate, NaOH, and sodium siUcate other reactants include alumina trihydrate (AI2O2 3H2O), coUoidal siUca, and siUcic acid. Some synthetic 2eohtes prepared from sodium aluminosihcate gels are given in Table 3. 

In gel-forming processes, the reactive aluminosibcate gel is first formed into a pellet which reacts with sodium aluminate solution and caustic solution. The 2eobte crysta11i2es in situ within an essentiaby self-bonded pellet, or as a component in an unconverted amorphous matrix. 

Gel-Based Activated Aluminas. Alumina gels can be formed by wet chemical reaction of soluble aluminum compounds. An example is rapid mixing of aluminum sulfate [17927-65-0], Al2(S0 2 XH20, and sodium aluminate [1302-42-7], NaA102, solutions to form pseudoboehmite and a... 

Multicomponent sol—gel fibers have been successfuUy developed (1,52). The early stages of sol formation and gelation are cmcial for controlling the fiber microstmcture. Aluminosilicates, zirconates, and aluminates (1,18,52) can be prepared by sol—gel methods. MuUite [55964-99-3] 3AI2 O3 -2Si02, fibers... 

Other reactions taking place throughout the hardening period are substitution and addition reactions (29). Ferrite and sulfoferrite analogues of calcium monosulfoaluminate and ettringite form soHd solutions in which iron oxide substitutes continuously for the alumina. Reactions with the calcium sihcate hydrate result in the formation of additional substituted C—S—H gel at the expense of the crystalline aluminate, sulfate, and ferrite hydrate phases. 

The important compounds in Portland cement are dicalcium silicate (CazSi04) 26%, tricalcium silicate (CasSiOj) 51%. tricalcium aluminate (Ca3Al206) 11% and the tetracalcium species Ca4Al2Fe2 Oio (1%). The principal constituent of moistened cement paste is a tobermorite gel which can be represented schematically by the following idealized equations ... 

In such complex system as aluminosilicate precursor gels, various lifetime components can appear reflecting material structure. The gel structure is a result of direct interaction of cations with silicate, aluminate and aluminosilicate anions, redistribution of charges and electron density over the system of aluminosiloxane bonds with effect on the formation of different structural units [24],... 

The synthesis of ECS-2 was carried out following the procedure reported by Carati et al. [5], ECS-2 was obtained using l,4-bis-(triethoxysilyl)-benzene (BTEB) as silica source while sodium aluminate (NaAlC ) and NaOH were used as aluminum and alkaline sources, respectively. The resulting gel was charged into a stainless steel autoclave and heated at a temperature of 100°C for 28 days under autogeneous pressure. Once cooled... 

The synthesis of SSZ-26 (24) involved combining 76 g of a 0.45 M solution of 2 with 158 g of NaOH. Then, 0.9 g of sodium aluminate were added followed by 9.1 g of fumed silica. The gel was heated at 175°C for 6 days with stirring at 60 rpm. A Y type zeolite has also been successfully used as the source for aluminum in the recipe. Note that the methyl groups in SSZ-26 template 2 could point up or down relative to the cyclohexyl ring but as it turns out both isomers make this phase. 

Calcium alginate gels, 4 728 Calcium alloys, 4 530 Calcium aluminate, 2 345t Calcium aluminate cement, 2 415-416 5 500t, 502... 

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