Soluble silicates potassium and sodium

The primary reaction of any pozzolanic material is an attack on the SiOj or AljOj-SiOj framework by OH ions. It may be supposed that the OH ions attach themselves to silicon and other network-forming atoms, with consequent breaking of bonds between the latter and oxygen atoms. After this has occurred several times, the silicate or other oxy anion is detached from the framework. It may either remain in situ or pass into the solution. The charges of those that remain are balanced, partly by H, and partly by metal cations. Since a cement pore solution is essentially one of potassium and sodium hydroxides, the immediate product is likely to be an amorphous material with and Na as the dominant cations, but the more abundant supply of Ca and the lower solubility of C-S-H and hydrated calcium aluminate or silicoaluminate phases will ensure that this is only an intermediate product. Its presence is indicated by the relatively high potassium contents observed in or near to the reacting pfa particles. 

All carbonates, phosphates, chromates, and silicates are insoluble, except those of sodium, potassium, and ammonium. An exception is MgCr04 which is soluble. 

Silicate. Potassium silicate, K2Si03, colorless (when pure) glass, soluble, mp 976°C, formed by reaction of silicon oxide and potassium carbonate at high temperature, similar in properties and uses to the more common sodium silicate. 

SILICATES (Soluble). The most common and commercially used soluble silicates are those of sodium and potassium. Soluble silicates are systems containing varying proportions of an alkali metal or quaternary ammonium ion and silica. The soluble silicates can be produced over a wide range of stoichiometric and nonstoicluometric composition and are distinguished by the ratio of silica to alkali. This ratio is generally expressed as the weight percent ratio of silica to alkali-metal oxide (SiOj/MjO). Particularly with lithium and quaternary ammonium silicates, the molar ratio is used. 

The most important property of sodium and potassium silicate glasses and hydrated amorphous powders is their solubility in water. The dissolution of vitreous alkali is a two-stage process. In an ion-exchange process between the alkali-metal ions in the glass and the hydrogen ions in the aqueous phase, the aqueous phase becomes alkaline, due to the excess of hydroxyl ions produced while a protective layer of silanol groups is formed in the surface of the glass. In the second phase, a nucleophilic depolymerization similar to the base-catalyzed depolymerization of silicate micelles in water takes place. 

The weathering of surface rocks has had a critical role in the chemical evolution of the continental crust for most of the Earth s history. In the presence of air and water, mafic minerals tend to rapidly weather into iron (oxy)(hydr)oxides, clays, and other silicate minerals, and at least partially water-soluble salts of alkalis (sodium and potassium) and alkaline earths (calcium and magnesium). In contrast, quartz in felsic and intermediate igneous rocks is very stable in the presence of surface air and water, which explains why the mineral readily accumulates in sands and other sediments. 

The most common route for the s5mthesis of magnesium silicate is via a precipitation reaction between a soluble metal silicate (e.g., sodium orthosilicate, sodium metasilicate, or potassium silicate) and a soluble magnesium salt (e.g., magnesium sulfate, nitrate, or chloride). The aqueous suspension of the precipitate is filtered and the collected solid is washed and dried (Fig. 7.2) [6,7]. 

Sodium and potassium hexafluorosilicate are manufactured by reacting alkali salts (e.g. chlorides) with hexafluoro-silicic acid and subsequent separation of the poorly soluble alkali hexafluorosilicates. 

Boric acid is a relatively weak acid compared to other common acids, as illustrated by the acid equilibrium constants given in Table 4. Boric acid has a similar acid strength to silicic acid. Calculated pH values based on the boric acid equilibrium constant are significantly higher than those observed experimentally. This anomaly has been attributed to secondary equilibria between B(OH)3, B(OH)4 , and polyborate species. Interestingly, the aqueous solubility of boric acid can be increased by the addition of salts such as potassium chloride and sodium sulfate, but decreased by the addition of others salts, such as the chlorides of lithium and sodium. Basic anions and other nucleophiles such as fluorides and borates significantly increase boric acid solubility. 

Colloidal silica (Ludox HS-40) is a stable suspension of fine silica particles having a mean size of 150-200 A. The pH of the solution is around 9.5. These particles are obtained from a water-soluble glass and then purified to remove the major part of alkali ions. Shoup developed a process in which a solution of potassium or sodium silicate (80-90 wt%) is added to the colloidal silica (10 wt%). The potassium silicate solution contains mixtures of polysilicic anions, which deposit on colloidal particles if the pH of the solution is lowered [34,35]. 

Sodium silicate was the 45th largest volume chemical produced in the United States in 1980, according to the 1981 Chemical and Engineering News Survey ( ). Obviously, the analysis of this material as well as the other major soluble alkali silicate, potassium silicate, is very important commercially. This paper will briefly review the modern analytical instrumental methods that are used to determine the quality of commercial soluble silicates and instrumental... 

Sodium and potassium silicate are the soluble silicates of commercial importance. For potassium silicate, not nearly as extensive data from the laboratory or from human experience are available. The assumption of its similarity to sodium silicate in health and environmental effects appears to be valid, for an equivalent mole ratio of Si02 to alkali metal oxide. 

The only subsequent regulatory development thus far under TOSCA, directed specifically at soluble silicates, was a proposed rule (54) under Section 8(a) which would require manufacturers to keep certain records and report production and exposure related data on approximately 2300 chemicals to EPA. This information was held to be necessary to rank chemicals for investigation and to make preliminary risk assessments. Sodium silicate, potassium silicate, sodium metasilicate and sodium orthosilicate were included on the candidate list, presumably because reports to the initial inventory showed them to be manufactured in high tonnage volume. 

Table 10.2 summarizes some examples of zeolites based on their classification by chemical composition. Low-silica zeolites (Si/Al < 5) are synthesized in basic conditions (pH >13) using a silicon source, an aluminum source, and alkali hydroxides at moderate temperatures, typically less than 120°C. The identity of the alkaU species used is a determining factor in which phase is obtained from synthesis, as the relative rates of (alumino)silicate hydrolysis and condensation reactions are dependent on the identity of the alkali cation. It is also believed that hydrated alkali cations effectively direct the assembly of (alumino)silicate precursors into fuUy connected three-dimensional structures. Sodium and potassium hydroxide have been used most frequently in low-silica zeolite syntheses due to their low cost and high solubility in... 

There is a practical maximum concentration of amorphous silica that can be dispersed in the aqueous silicate solution. It is often desirable to incorporate as high a concentration of amorphous silica as possible, yet still have a workable fluid binder to apply to the sand. If the proportion of amorphous silica to soluble silicate is too low, than the shake-out will be adversely affected. On the other hand, if the ratio of amorphous silica to soluble silicate is too high, the mixture will be too viscous and must be thinned with water. Also, there will not be enough binder to fill the spaces between the amorphous silica particles in the bond, and it will be weak. In generally, the higher the content of amorphous silica relative to sodium or potassium silicate, the weaker the initial bond as set by carbon dioxide. Conversely, the more silicate in the binder, the higher will be the initial and retained strengths. 

The binder system should have a molar ratio of silica to alkali metal oxide which ranges from 3.5 to 10, preferably 3.5 to 7. This ratio is significant because the ratios of soluble potassium, lithium or sodium silicates commercially available as solutions lie within a relatively narrow range. Most of sodium silicates are within the range of Si02/Na20 of about 2 1 to 3.75 1. Thus, overall ratios of binder compositions obtained by admixing colloidal silica, such as ratios of 4 1, 5 1, 7 1 are mainly an indication of what proportions of colloidal silica and soluble silicates were mixed since the amount of amorphous silica in the soluble silicate at ratios of 2 1 to 3.75 1 are small. 

Continental aerosol particles contain a significant fraction of minerals. The insoluble fraction consists mainly of the major crustal elements silicon, aluminum and trivalent iron, which occur as alumino-silicates, quartz, and iron oxides. Elements that are eluted from minerals by water are sodium, potassium, calcium (inpart), and magnesium. The water-soluble inorganic salt ftaction is dominated by am-monimn sulfate. Again, sulfate arises from the oxidation of sulfur dioxide, both by gas-phase and by aqueous phase reactions. Whereas the mineral components are mainly found in the coarse particle size range, ammonium sulfate resides mainly in the accumulation mode. Nitrate occurs partly in association with ammoniirm in the accumulation mode, and partly together with sodiirm and other cations in the coarse particle mode. Thus, nitrate often shows a bimodal size distribution. 

It has been known since the seventeenth century that sand and sodium or potassium carbonate react at red heat to form a water-soluble glass called water glass. As noted by Vail (1), Johann Nepomuk von Fuchs was the first to investigate alkali silicates systematically and even before 1850 proposed their uses as adhesives, cements, and fireproof paints. By 1855 water glass was being made commercially, both in Europe and America. 

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