Soluble Silicates Potassium and Sodium Silicate

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. 

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 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. 

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. 

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. 

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. 

The sodium and potassium silicates are available as two-component systems filler and binder with the setting agent in the filler. Sodium and potassium silicates are referred to as soluble silicates because of their solubility in water. This prevents their use in many dilute acid services while they are not affected by strong concentrated acids. This disadvantage becomes an advantage for formulating single component powder systems. All that is required is the addition of water at the time of use. The fillers of these materials are pure silica. 

Adsorption by Clays. — Owing to the possibility of chemical reactions between the clay and the adsorbed substances, the phenomena here are much more complicated than is ordinarily the case with many colloidal systems. According to Sullivan changes between the radicals are often involved. For instance when acid or neutral salts are adsorbed, sodium, potassium, and magnesium from the clay may be released or dissolved, while an equivalent amoimt of the adsorbed basic radical remains with the clay. The addition of alkaline solution is still more complicated. Not only may there be free alkali but basic solutions may be formed because of the hydrolysis of salts of a strong base and a weak acid, e.g., carbonates and phosphates. Three different reactions are now possible. First, the free alkali may react with the colloidal silica. Second, the silicate radical from the clay may form insoluble salts with the adsorbed base. Third, the sodium, potassium, or magnesium displaced from the clay may form soluble carbonates and phosphates, and these salts in turn be adsorbed by the clay constituents. These reactions are of great importance in the study of the fertilization of the soil. It has been claimed that the addition of lime not only neutralizes the undesirable acids, but also renders the potassium of the clay available for the plant. 

About one decade ago Bass et al. [13,14] proposed first that such approach could help in exploring the structure of water dissolved silicates. Following this initiative, recently we critically evaluated how the published FTIR and Raman assignments could be adopted for differentiating between the molecular structures of some commercially available sodium silicate solutions [7-9,15], In this paper we present comparative structural studies on aqueous lithium and potassium silicate solutions as well. According to some NMR studies, the nature of A+ alkaline ion and the A+/Si ratio barely affects the structural composition of dissolved silicate molecules [5], In contrast, various empirical observations like the tendency of K-silicate solutions to be less tacky and more viscous than their Na-silicate counterparts, the low solubility of silica films obtained from Li-silicate solutions compared to those made from other alkaline silicate solutions, or the dependence of some zeolite structures on the nature of A+ ions in the synthesis mixture hint on likely structural differences [16,17]. It will be shown that vibrational spectroscopy can indeed detect such differences. 

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