Silicate: anhydrous crystalline

Common (dry) cement consists of anhydrous crystalline calcium silicates (the major ones being tricalcium silicate, CasSiOs, and (3-dicalcium silicate, Ca2Si04), lime (CaO, 60%), and alumina (a complex aluminum silicate, 5%). While cement is widely used and has been studied in good detail, its structure and the process whereby it is formed are not completely known. This is due to at least two factors. First, its three-dimensional arrangement of various... 

The crystallization of amorphous, water-containing sodium silicate to anhydrous crystalline layered sodium disilicate is crucial. It is effected in a further step. The formation of delta-crystal phase is an exothermic process at 685°C [89],... 

Certain anhydrous crystalline silicates known to contain SiO ions, such as NaH2SiO,-8HjO (NajSi0,-9H20), Ca2SiO<. LuSiO, and MgKjSiO all gave the... 

Certain anhydrous crystalline silicates yield layerlike crystalline hydrated silicas with the approximate formula (HjSiiO ),. These are cited to illustrate the complexity of such apparently simple compounds. 

When anhydrous cement mix is added to water, the silicates react, forming hydrates and calcium hydroxide. Hardened Portland cement contains about 70% cross-linked calcium silicate hydrate and 20% crystalline calcium hydroxide. 

Both the anhydrous, porous IDPs and Tempel 1 appear to have a higher crystalline to amorphous silicate ratio than is inferred from infrared spectroscopy of interstellar or most circumstellar grains. This could simply be a result of grain alteration in interstellar space, with radiation processing converting crystalline silicates to amorphous silicates. 

There have been some very thorough structural studies conducted on MO, (78,86-88) MT, (78,82) and DT (79) systems using Si NMR. Harris and Newman (87) have looked at the trimethylsilylation products of wollastenite, a form of anhydrous calcium metasilicate, and pseudo-wollastenite, a crystalline modification of that mineral. The chemical shifts of the four model compounds listed in Table XI were compared in order to determine the degree of spectral dispersion that would occur in the many polymeric structures formed from trimethylsilylation of the decomposed calcium silicate. 

Crystalline layered sodium disUicates are the most recent development in the field of detergent silicates. In contrast to hydrous sodium polysilicates and metasilicates, they display a three-dimensional crystal lattice characterized by corrugated silicate layers, which are separated by sodium ions. The chemical formula is NajSijO,. This reflects that the material is anhydrous and has a formal molar ratio of 2. Several crystal phases differ in the kind of corrugation of the silicate layers [3,21,66,86-88]. 

Reference also should be made here to the unique, highly siliceous crystalline zeolite-like insoluble quaternary ammonium silicates from which the organic ions can be removed, leaving anhydrous, hydrophobic, microcrystallinc SiOj of an entirely new type discovered and described by Flanigen and Grose (see Chapter 1, refs. 71c, 7Id). 

Monomeric crystalline silicates dissolve and are neutralized to liberate monosilicic acid at about pH 2. Kraut (23) prepared monosilicic acid by dissolving sodium meta-silicate hexahydrate in various acidic solutions at low temperature. He reported that monosilicic acid is most stable at around pH 2-3. Weitz, Franck, and Schuchard (20) demonstrated that when Na2Si03-9Hj0 was reacted with acetic acid it liberated Si(OH)4. Also olivine (magnesium orthosilicate, Mg-SiOO dissolved in 1.0 N HCl to give a practically 100% yield of monosilicic acid, the solution containing 0.04% SiO,. Thus monosilicic acid may be liberated from silicates which contain SiO ions separated by cations, such as are present in anhydrous orthosilicates. 

Another form of Si02 is called silica gel. When an aqueous solution of sodium silicate is acidified and then roasted or dried to remove most of the excess water, a white, amorphous (or noncrystalline), high-porosity powder called silica gel is obtained. Because of its anhydrous, highly porous nature, sifica gel finds a variety of uses as a desiccant, catalyst, and chromatographic support. For similar reasons it is used in the marketplace as an anticaking agent in various finely powdered or crystalline food products such as cocoa and powdered fruit juices. 

The most commonly used silicates in detergency are the sodium silicates. The sodium silicates used as builders are in the form of powders or solid granules, which may be crystalline or amorphous, hydrated or anhydrous. Among these solid forms are found metasilicates with Rm = 1, and also some of higher ratio, where Rm = 2 to 3.5. These higher-ratio silicates are commercially available as concentrated aqueous solutions as well. 

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