Soluble Silicates and Aluminates

Silica and silicates are soluble in aqueous alkali. The silicate anions produced in solution may be regarded as derived from silicic acid [H4Si04 or Si(OH)4], which has a first pAa of 9.47 at 0.6 mol ionic strength  

however, is an oversimplification, since these monomeric anions are in rapid equilibrium with oligomers (small polymers) such as the dimer (cf. Fig. 7.2b), cyclic trimer (7.2c), and other small silicate oligomers, 22 of which have been identified by silicon-29 magnetic resonance methods and many of which have no counterparts in solid silicate minerals  

The aqueous chemistry of aluminum(III) above pH 6 differs from that of silicates in that the only important species, other than solid Al(OH)3 at pH 5-8, is A1(0H)4, which, although isoelectronic with Si(OH)4, shows no tendency to catenate. On the other hand, below pH 5 Al unlike the poorly soluble Si(OH)4, is freely soluble as AP (aq) [actually Al(OH2)6 , Section 13.2], while at intermediate pH hydrolytic A1 species, including the ion A1i304(OH)24(OH2)i2 referred to above, predominate in solution. However, A1(0H)4 units can readily insert themselves into silicate anion species in solution. The result is usually the prompt precipitation of an aluminosilicate gel (a typical zeolite precursor), although over some limited Al, Si, and OH concentration ranges quite high concentrations of dissolved aluminosilicates can be maintained over many months.  

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Starting from soluble silicates and aluminates, the crystallization fields of 14 zeolites were explored in the Na O-AhOs-SiOr-H2O, K20-A Ws-Si02-H20 and K2O—NCL2O—A l20s—Si02—... 

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. 

Because calcium oxide is a fairly reactive powder, it forms calcium hydroxide when in contact with water. This reaction is exothermic and hence heats water during formation of the hydroxide. Because of this excess heat, it cannot directly be used to form phosphate ceramics by reacting it with an acid phosphate solution and must be used in a less soluble form as sparsely soluble silicate or hydrophosphate. In spite of this difficulty, because human bones contain calcium phosphate, there have been sufficient efforts in developing methods of forming biocompatible CBPCs of calcium phosphate by using partially soluble phosphates of calcium rather than using oxide itself. A similar approach may also be taken if one uses partially soluble silicate or aluminate of calcium. These routes are discussed in Chapter 13. 

With the procedure given above, one may also calculate the pH dependence of (Ca (aq)) from dissolution of calcium aluminates. The results of such calculations are summarized in Table 13.1. Figure 13.2 shows the solubility diagram for (Ca (aq)) as a function of pH for both silicates and aluminates drawn from the equations given in Table 13.1. 

Note in Fig. 13.2 that the solubility of monocalcium silicate is higher than that of the corresponding aluminate at any pH > 3. In the acidic region that is of interest for forming CBPCs, both may be considered as sparsely soluble, and if they are reacted with a phosphate salt, ceramics may be formed. Thus, monocalcium silicate and aluminate are starter minerals to form calcium phosphate ceramics. 

Solubility dependence of calcium silicate and aluminate as a function of pH. 

Some soluble aluminosilicates (probably structural units participating in the crystallization of zeolites) have been detected in dilute solutions containing silicate and aluminate anions.448... 

Zhdanov and colleagues for the first time discussed in detail the solution-mediated transport mechanism.[27] They believed that 1) nucleation happened in the solution or at the interface of the solution and solid gel 2) the further growth of zeolite nuclei consumed the silicate and aluminate ions in solution 3) the solution supplied the soluble structural units for the growth of zeolite crystal and 4) the consumption of the liquid component during the crystallization process resulted in the continuous dissolution of solid gel. 

The use of organic cations has opened a new field in the synthesis of new zeolites (28-30). These cations are thought to increase the solubility of the silicate and aluminate ions and to arrange the water molecules in a template-like fashion. Using tetramethylammonium cations and Y type zeolites were... 

If solutions of sodium silicate and aluminate, having the same pH and resistance, are mixed, and if the concentrations are so low that gel formation does not occur, then a considerable increase in conductivity is noted. This must be due to formation of soluble aluminosilicate complexes."""... 

Correlations and concentrations of components in the liquid phase of gels appear to be quite different from those in the solid phase and initial mixtures. It is essential that in all cases both the silicate and aluminate ions are present in the liquid phase of gels, and the product of their concentrations is comparatively constant at significant changes in the absolute values for ion concentrations of each type. This can be an indication of the existence of an equilibrium similar to that of dissociation or solubility. 

These data show that increases in concentrations of all components in the liquid phase of gels take place on heating. This may be connected with the growth of solubility of the aluminosilicate skeleton with increasing temperature. Consequently, increases in concentration of the silicate and aluminate ions in the liquid phase precede the beginning of gel crystallization. 

In contrast to more or less well defined kinetics of the crystal growth (5,6,12-16), various nucleation mechanisms have been proposed as zeolite particles forming processes. Most authors explained the formation of primary zeolite particles by nucleation in the liquid phase supersaturated with soluble silicate, aluminate and/or aluminosilicate species (1,3,5,7,16-22), with homogeneous nucleation (1,5,7,17,22), heterogeneous nucleation (5,2 1), cell walls nucleation (16) and secondary nucleation (5) as dominant processes of zeolite particles formation, but the concepts dealing with the nucleation in the gel phase are also presented in the literature (2,6,11,12,1 1,23-25). 

An example of the first approach (matrix assimilation) would be to match the acid content in the standards with the acid content in the samples. Matrix assimilation is only effective provided that the interference is not severe and the sample matrix is relatively simple. For more marked interferences, a second cation can act as a release agent. As an example, lanthanum [as La(N03)3] can be added to solutions in which Ca is to be determined in the presence of P04 , silicate or aluminate in an air/C2H2 flame. An example of the third approach would be to add a strong complexing agent (such as EDTA) to both samples and standards. Many metals have an appreciable tendency to hydrolyse in aqueous media moreover the hydroxides can be sparingly soluble yet precipitates can be difficidt to detect visually in dilute solutions. To limit this process, samples are customarily prepared in acidic media. 

Barium is reported in the kiln spill in three different forms water-soluble, eg, BaS, BaO acid-soluble, eg, BaC03, aluminate, silicate, ferrate and insoluble, eg, unreduced barite. 

The cooled product is ground and extracted with water. The soluble sodium aluminate is removed as a filtrate from the dicalcium silicate and insoluble matter. Injection of carbon dioxide into the filtrate produces a precipitate of aluminium hydroxide and sodium carbonate liquor (11.10), which is recycled to the initial stage. The aluminium hydroxide is heated to above 300 °C to produce pure alumina which is dissolved in fused cryolite (NaAIp4) at about 900 °C and electrolysed between carbon electrodes to produce molten aluminium. 

The manufacture of molecular sieve adsorbents has been reviewed by Breck > and more recently by Roberts. The steps in the process are shown schematically in Figure 1.12. A variety of different starting materials may be used. In the hydrogel process the reagents are added in soluble form as sodium silicate and sodium aluminate, whereas in the clay conversion process the alumina is added as a clay mineral, usually metakaolin. Formation of the desired zeolite depends on maintenance of the correct conditions of pH, temperature, and concentration. Seeding may be used to promote crystalliza-... 

Other substances, including the iron oxides, remain undissolved and are filtered off. As the hot solution of sodium aluminate cools, aluminum hydroxide precipitates, leaving the soluble silicates in solution. In practice, the solution is seeded with aluminum hydroxide to start the precipitation. 

Many substances are known to act as accelerators for concrete. These include soluble inorganic chlorides, bromides, fluorides, carbonates, thiocyanates, nitrites, nitrates, thiosulfates, silicates, aliuninates, alkali hydroxides, and soluble organic compounds such as triethanolamine, calcium formate, calcium acetate, calcium propionate, and calcium butyrate. Some of them are used in combination with water reducers. Quick setting admixture s used in shotcrete applications and which promote setting in a few minutes may contain sodium silicate, sodium aluminate, aluminum chloride, sodium fluoride, strong alkalis, and calcium chloride. Others are solid admixtures such as calcium aluminate, seeds of finely divided Portland cement, silicate minerals, finely divided magnesium carbonate, and calcium carbonate. Of these, calcium chloride has been the most widely used because of its ready availability, low cost, predictable performance characteristics, and successful application over several decades.In some countries the use of calcium chloride is prohibited, in some others, such as Canada and the USA, the use of calcium chloride is permitted provided certain precautions are taken. Attempts have continued to find an effective alternative to calcium chloride because of some of the problems associated with its use. 

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