Hydrated silicate polymers

Silicates in Ceramics and Glass Hydrated silicate polymers are important in the ceramics industry. A colloidal dispersion of particles in a liquid is called a sol. The sol can be poured into a mold and, following removal of some of the... 

Clays are layered, hydrated silicates of aluminum, iron, and magnesium.225 Most are crystalline inorganic polymers, but a few are amorphous. They consist of tetrahedral and octahedral layers as shown in Fig. 6.2. 

Silicate anions are bases when acidified, they produce silicic acids, which are unstable and decompose to silica. The silica obtained, however, is not a crystalline solid or powder. Depending on the acidity of the solution, the silica is obtained as a colloidal dispersion, a gelatinous precipitate, or a solid-like gel in which all the water is entrapped. These hydrated silicates are polymers of silica formed by the elimination of water molecules between neighboring molecules of silicic acid. The process begins with the following reaction ... 

Talc is a hydrated magnesium silicate that is composed of thin platelets primarily white in color. Talc is useful for lowering the cost of the formulation with minimal effect on physical properties. Because of its platy structure and aspect ratio, these extenders are considered reinforcement. Polymers filled with platy talc exhibit higher stiffness, tensile strength, and creep resistance, at ambient as well as elevated temperatures, than do polymers filled with particulate fillers. Talc is inert to most chemical reagents and acids. The actual chemical composition for commercial talc varies and is highly dependent on the location of its mining site. 

Silicate anion structures in Portland cement pastes have been studied by the methods described in Section 5.3.2 for calcium silicate pastes. Trimethylsily- i lation (TMS) studies (L20,T12,S69,T36,L31,M43,M44) show that, as with C,S. the proportion of the silicon present as monomer decreases with age and that the hydration products contain dimer, which is later accompanied and eventually partly replaced by polymer (>5Si). Some results have i indicated that fully hydrated pastes of cement differ from those of CjS in that substantial proportions of the silicate occur as monomer (S69,L31), but the results of a study in which pastes of CjS, P-CjS and cement were compared (M44) suggest that the differences between the anion structures of cement and CjS pastes are probably within the considerable experimental errors inherent in the method. The recovery of monomer from unhydrated P-CjS was only 66% and results for cement pastes can only be considered semiquantitative. 

Fine crystalline quartz or amorphous substances (gel, quartz glass, etc.) are used as one of initial components for the synthesis [19]. Depending on temperature, pressure, pH of the medium and the presence of salts, silica can exist in solution both as simple ions or molecules, and as more complicated polymer particles. Under normal conditions silica passes into solution in monomer form, as silicic acid Si(OH)4 at large pH, silicate ions SiOj - are formed. Monosilicic acid is a very weak acid however, at increased temperature its dissociation constant increases substantially. The amount of monomer form also increases with temperature. The dissolution of Si02 is due to hydration as well as to depolymerization. 

There was some criticism of our work — contradictory results reported by another investigator. I felt satisfied with what we had done, and did not go to the trouble of carrying out more studies, except that I got a senior student, Frank Dickey, to carry out the study with hydrated silica—precipitated silicic acid polymer. [22]... 

The term amorphous silica encompasses an almost infinite variety of structural forms, from ordered opaline aggregates to extended gel-like materials. In all these structures, the mineral exists as a hydrated, covalent inorganic polymer of general formula [SiO /2 (OH)4- ]m 4). This formula, where n = 0 to 4 and m is a large number, indicates the variation in residual functional groups within the condensed structure. There is similar variation in the extent of hydration. This flexibility in composition and reactivity indicates that biogenic silica is not a stoichiometric mineral (in the way that CaCOs is, for example) and that the nature (density, hardness, solubility, viscosity) and composition of siliceous structures in biology may vary consid-... 

Other hydrated matrices such as rayon (7) and layer silicates (8). Moreover, collagen shares with otFer polymers the dis-tTnction that water absorbed in them is usually considered to occur in different states, bound and unbound. 

The formation of hydroxyaluminosilicate species at near-neutral pH is unique. No interactions occur between silicic acid and Ca2+ or Mg2+ at less than pH 10, so that the transport and binding of these cations is unhindered. The interactions of Fe3+ with silicic acid at near-neutral pH are very different from those of aluminum. Acidic solutions containing Fe3+ (10-4 M) with a threefold molar excess of silicic acid remain clear on neutralization no visible precipitate forms. Instead, Fe-O polymers are formed as spherical hydrated ferric oxide particles 10-15 nm in diameter,... 

By acidification of the silicate solutions colloid polymer forms, colloidal hydrated Si02, are slowly separated. Absorbed silicate anions give the colloidal particles a negative charge. Silicates can form complexes with Fe(III) and Mn(II) their importance was mentioned in the section dealing with iron. 

Inorganic colorants listed in 21CFR 178.3297 include aluminum, aluminum hydrate, potassium silicate, aluminum silicate, barium sulfate, bentonite, calcium carbonate, calcium silicate, calcium sulfate, carbon black (channel process, prepared by the impingement process from stripped natural gas), chromium oxide green Cr203, cobalt aluminate (with restrictions), diatomaceous earth, iron oxides, kaolin (modified for use in olefin polymers in amounts up to 40%), magnesium oxides, magnesium silicate (talc), sienna, silica, titanium dioxide, titanium dioxide-barium sulfate, ultramarines, zinc carbonate (limited use), zinc chromate (less than 10%), zinc oxide (limited use), and zinc sulfide (less than 10%). 

First Step. When polymer latexes are mixed with fresh cement mortar or concrete, the polymer partides are uniformly dispersed in the cement paste phase. In this polymer-cement paste, flie cement gel is gradually formed by the cement hydration and the water phase is saturated with calcium hydroxide formed during the hydration, whereas the polymer partides dqrosit partially on the surfaces of the cement-gel-unhydrated-cement partide mixtures. It is likely that the calcium hydroxide in the water phase reacts witit a silica surface of the aggre tes to form a calcium silicate layer.I It is confirmed that tire formation of the calcium hydroxide and ettringite in the contad zone between tire cement hydrates and aggregates is attributed to the bond between them.I lPl... 

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