Alkali silicate binder

Much effort has therefore been expended to develop water-based shop primers with the advantages but without the disadvantages of organic solvent-based primers. In one approach, alkali silicate binders, e.g. aqueous solution of 3.3 ratio sodium silicate, were used. However, such binders contain relative large amounts of alkali metal cations, which remain in the coating after it has dried and after subsequent coatings have been applied to the steel surface. Exposure to water, for instance seawater, may cause blistering, that is, local delamination, due to the presence of too much alkali metal ions in the primer. 

Other fast-setting cements not mentioned so far include alkali-activated slag cement (see section 8.5), alkali-activated fly ash binder (see section 9.1), geopolymeric cement (see section 15.7.1), zinc phosphate cement (see section 12.4), magnesium oxychloride and oxysulfate cement (see section 15.1), and alkali silicate binder(see section 15.3). 

In alkali silicate binders the setting and hardening is based on a reaction between a water glass (a solution of an alkali silicate in water) and alkali fluosilicate (see also section 15.3). The reaction product is an amorphous form of Si02 with embedded ciystals of alkah fluoride. The amorphous Si02 remains preserved until about 550°C and converts gradually to quartz and tridimite at higher temperatures. At 750°C crystalline sodium disilicate starts to be formed. 

Zinc dust coatings are utilized in large quantities for the protection of structural steel, also including underwater steel construction and ship building [5.56]. Zinc dust is used in organic and inorganic binder systems (alkali silicates, ethyl silicates) [5.56, 5.105]. 

Water-soluble silicate, containing an organic alkaline cation, can be obtained by interaction of quaternary organic ammonium base salts and amorphous silica. The relationship of Si02 to the organic base is 1.65 1 for DBUS and 2.0 1 for DBNS. It should be noted that the soluble organic alkali silicates, for example, tetrabutylam-monium silicate (TBAS) is also used as a component of self-extinction binder. 

Among test specimens that had been heated to 900°C some exhibited no visible changes, some were moderately bent, and one even melted, depending on the aggregate employed. The strength was generally increased by exposure to elevated temperature. There are several fields of applicatiorr, where the use of an alkali silicate based binder may be advantageous ... 

The alkaline activators include sodium or potassium hydroxide, silicate, and carbonate. The rate of setting and hardening of alkali-activated binders and the quality and quantity of the resulting reaction products depend on a variety of factors, including the quality of the starting aluminosilicate, the quality and amount of alkaline activator, and the temperature of reaction. 

The alkali silicates is one of the raw materials classically used in the formulation of new materials like geopolymers. Geopolymers are amorphous three-dimensional aluminosilicate binder materials which are synthesized at ambient temperature by the alkaline activation of silica solution and aluminosilicates derived from natural minerals, calcined clay or industrial byproducts. Previous study focused on the aluminosilicate sources have shown that the presence of impurities and the reactivity of the metakaolin (aluminosilicate source) can lead to the formation of one or several networks in geopolymers materials [1]. Indeed, the different sources of metakaolins conduct to the presences of various siliceous species in the solution which react with alumina. These multiple combinations lead to the formation of different networks (then various properties of geopolymers). To understand the formation of these various networks, studies relative to the neutralization of siliceous species in solution have been done. Parmentier [2] showed that the ammonium molybdate could react with silica to create silicomolybdic compounds. More recent studies demonstrated that ammonium molybdate could also react with these species in an alkaline environment [3]. These analyses showed that ammonium molybdate could react not only with monomers and dimers, but also with larger molecules. According to this, molybdate can permit to complex siliceous species and to modify polymerization reactions. The aim of the study is to study the influence of the ammonium molybdate addition on the kinetics of the polycondensation reaction as well as on the formation of several networks. 

Inorganic zinc-rich primers consist of zinc and a reactive binder such as an alkali metal silicate (sodium, potassium, lithium or quaternary ammonium silicate) or hydrolyzed ethyl silicate as binder. On mixing hard and cohesive films of silicate are formed. The structure of the inorganic zinc-rich silicate may appear as shown in Figure 1.66. 

This new nanostructured binder was obtained by laminar mixing of the liquid glass containing alkali metal cations as Na, TFS, and a water-soluble silicate, containing an organic alkaline cation (WSS) such as l,8-diazabicycloundecene-7 (DBUS, Structure 7.1) or l,5-diazabicyclononene-5 (DBNS, Structure 7.2). 

New fibers are being developed which dissolve in the slightly alkaline pH of the lung. These fibers may eventually replace the more common alumino-silicate fibers especially in use where human contact with breathable fibers is likely. Alumino-silicate fibers do not have the same small size as asbestos fibers, but the mere fact that they are fibers, could become airborne, and the body has no rejection mechanism, has prompted some countries to view them with suspicion. Having fibers which provide the same thermal properties but which would dissolved in the lung would preempt the possible problem. However, fibers which dissolve in alkaline conditions cannot be used with alkali stabilized colloidal silicas. They need a binder which is stable but acidic in pH as described above. Such products were first developed at DuPont and are now available through W.R. Grace who acquired the Ludox colloidal silica business from DuPont... 

To prepare a binder having Si02/M20 ratio of 3.5 to 3.8 it is therefore not necessary to add any colloidal silica if an alkali metal silicate solution is used already in the ratio range. On the other hand, if an alkali metal silica solution with ratio lower than 3.5 is used, it is necessary to add at least some colloidal silica in the form of a sol to prepare out binder. 

In summary, binder compositions of our invention comprise (1) aqueous solutions of alkali metal oxide silicates with or without amorphous silica present therein and (2) amorphous colloidal siUca, if the silicate does not have any amorphous silica present therein or if the level of amorphous silica in the silicate is not sufficient... 

Preferred for use in the compositions of the invention are binder wherein the alkali metal silicate is sodium... 

The compositions of the invention contain 3-15 parts, per 100 parts of sand binder mixture by weight, of a binder system comprising a water soluble alkali metal silicate, amorphous colloidal silica and water. The key is to have very finely divided amorphous silica particles of colloidal size dispersed within the alkali metal silicate bond. It is inherent in the nature of water soluble alkali metal silicates having a molar ratio Si02/alkali metal oxide (M2O) above about 2.5, that colloidal silica is present. In the case of silicates having a ratio higher than 3.5, the colloidal silica content is such that they may be employed without adding more colloidal silica, but in the case of alkali metal silicates of lower sUica/alkali metal oxide ratio there is little or no amorphous colloidal silica present so that amorphous colloidal silica must be added in order to produce the cores and molds of the present invention. 

I have discovered ways of using high ratio alkali metal silicates as foimdry sand binders without introducing excessive amounts of water into the sand and without employing unstable commodities. 

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. 

In a series of patent applications [28-31] Davies et al. show that the problems caused by high amounts of alkali metal ions could be overcome by using silica sols as the water-based binder instead of solutions of alkah silicates. The preferred sols had a particle size between 5 and 10 nm and a Si02/Na20 mole ratio of about 50 1. They also found that they could increase the pot life of the shop primer by using aluminum-modified silica sols. 

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