Alkali silicate gels

In pozzolanic reaction, the alkali silicate gel is formed in an environment rich in Ca and, except in a narrow zone close to the reacting surface, is quickly converted into C-S-H. In ASR, it is formed in an environment poor in Ca ", and massive outflows of gel may result. The cement paste cannot supply Ca " fast enough to prevent much of this gel from persisting for long... 

Siliceous gel zeolite A synthetic, inorganic exchanger produced by the aqueous reaction of alkali with aluminum salts. 

Role of alkali and NH cations in the crystallization of ZSM-5 Introduced in an aqueous (alumino) silicate gel (sol), the bare alkali cations will behave in various ways firstly, they will interact with water dipoles and increase the (super) saturation of the sol. Secondly, once hydrated, they will interact with the aluminosilicate anions with, as a result, the precipitation of the so formed gel (salting-out effect). Thirdly, if sufficiently small, they also can order the structural subunits precursors to nucleation species of various zeolites (template function-fulfilled by hydrated Na+ in the case of ZSM-5 (11,48)). ... 

Solutions of monosilicic acid may also be obtained by careful hydrolysis of tetrahalo-, tetraalkoxy-, or tetraacyloxysilanes by electrolysis or acidification of alkali silicate solutions or by ion exchange (qv). By operating under carefully controlled conditions at low temperature and pH, solutions may be obtained that remain supersaturated with respect to amorphous silica for hours at temperatures near 0°C. Eventually, however, polymerization reactions involving the formation of siloxane linkages occur, leading ultimately to the formation of colloidal particles and further aggregation or gel... 

Titanium-containing materials have been synthesized from strongly alkaline suspensions obtained by mixing TiCU with alkali silicate and sodium aluminate solutions. The gel obtained was homogenized and mixed in an autoclave at... 

In the first place, we learned more about the formation of nickel hydrosilicates under certain circumstances from an investigation of Van Eijk van Voorthuysen and Franzen (2). These investigators made a number of preparations by combining boiling dilute solutions of nickel nitrate and alkali silicate in various proportions. In order to find out to what extent co-precipitation is required for the formation of hydrosilicate structures, acid was added to a nickel hydroxide suspension in a silicate solution by which silica is precipitated, or conversely, alkali was added to a suspension of silica gel in a nickel nitrate solution. Some of the preparations were subjected to a hydrothermal treatment at 250° C. for 50 hrs. with a sufficient quantity of water for developing the best possible structure. 

Silica gel based on alkali silicates 100-800 micro, meso medium low... 

It has been shown that a plot of Si chemical shifts (Figure 12.03 (a), (b) and (c)) in a variety of alkali-silicate glasses, crystalline silicates and gels... 

Figure 12.03. chemical shifts versus the parameter P for (a) alkali silicate glasses, (b) other complex silicate glasses and (c) combined glasses, gels and crystalline silicates. Best fits are indicated by dashed lines (After Prabakar et al., 1991)... 

Alkali aluminosilica gels from which zeolites usually are crystallized represent heterogenous colloidal systems consisting of liquid and solid phases which strongly differ in their chemical compositions. In those cases when aluminosilica gels are obtained under equal conditions and from the same initial materials—for example, from solutions of the silicates and aluminates—the results of their crystallization reproduce well,... 

Two reactions can convert alkali silicate glass into a solid mass with which contaminated sludges can be bound (1) by adding acid to form silica gel, whereby the evaporation of water does not occur (alternative to vaporization ) and (2) reaction with multi-valent metal ions (e.g., calcium chloride) while forming aqueous metal silica gel, where heavy metals are precipitated and are mechanically bonded into the gel structure. The CHEM-FIX-Process is primarily used in the United States for inorganic contaminants the Belgian SOLIROC-Process contains additives to make it usable for the solidification of organic wastes. 

Alkali-silica reaction is an expansive reaction in concrete that can occur when a solution of sodium or potassium hydroxide reacts with a siliceous aggregate to form a gel of hydrated alkali silicate. 

The alkali limiting criterion, for example 0.6% Na O in cement, is linked with the aim to reduce the concentration of sodium and potassium ions in the concrete pore solutiou Simultaneously, these ions cause a substantial decrease of Ca(OH)2 concentration in the capillary solution. Some doubts arouse as the calculation of alkali content in the form of sodium equivalent is concerned hence it is known that the potassimn silicate gel reveals lower expansion that the sodium silicate. 

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