Alkali silicate reaction

Gifford, P.M., and Gillot, J. (1996) Alkali-silicate reaction (ASR) and alkali-carbonate reaction (ACR) in activated blast fiunace cement (ABFSC). Cement and Concrete Research 26,21-26. 

Hobbs, D. W. (1988) Alkali-Silicate Reaction in Concrete, Thomas Telford Ltd, London, UK. 

Alkali-silicate reaction. The alkali-silicate reaction has not received general recognition as a separate entity. The alkali-silicate reaction was proposed by Gillott.E l The rocks that produced this reaction were grey wackes, argillites, and phyllites containing vermiculites. 

Although it has heavy weight and it needs complicated connection system to beams or floors, which sometimes brings difficulties to fix it. Besides it, recently cracks on concrete surface, due to alkali silicate reaction phenomena, became closed up, and it has been a major problem that the durability of concrete should be taken more careful attention and be revalued. 

The mechanism of the inhibitive action of LiOH proposed by Stark et al. [7] is attributed to the formation of lithium silicate that dissolves at the surface of the aggregate without causing swelling [7], In the presence of KOH and NaOH the gel product incorporates Li ions and the amount of Li in this gel increases with its concentration. The threshold level of Na Li is 1 0.67 to 1 1 molar ratio at which expansion due to alkali-silica reaction is reduced to safe levels. Some workers [22] have found that when LiOH is added to mortar much more lithium is taken up by the cement hydration products than Na or K. This would indicate that small amounts of lithium are not very effective. It can therefore be concluded that a critical amount of lithium is needed to overcome the combined concentrations of KOH and NaOH to eliminate the expansive effect and that the product formed with Li is non-expansive. 

Pederson, L. R., McGrail, B. P., McVay, G. L., Petersen-Villalobos, D. A. Settles, N. S. 1993. Kinetics of alkali silicate and aluminosilicate glass reactions in alkali chloride solutions Influence of surface charge. Physics and Chemistry of Glasses, 34, 140-148. 

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... 

Several studies on the quaternary systems of CaO-SiOj-HiO with NajO or K,0 have been reported (K19,S52,M52). Alkali greatly lowers the concentrations of CaO in the solution and raises those of Si02. The solid phase compositions are difficult to study. Determinations based on changes in concentration on adding CH to alkali silicate solutions are subject to considerable experimental errors, while direct analyses of the solid are difficult to interpret because the alkali cations are easily removed by washing. Suzuki ei al. (S52) considered that they were adsorbed. Macpheeef /. (M52) reported TEM analyses of the C-S-H in washed preparations obtained by reaction ofCjS (lOg) in water or NaOH solutions (250 ml). The C-S-H obtained with water had a mean Ca/Si ratio of 1.77 that obtained with 0.8 M NaOH had a mean Ca/Si ratio of 1.5 and a mean NujO/SiOj ratio of 0.5. These results do not appear to be directly relevant to cement pastes. The pore solutions of the latter may be 0.8 M or even higher in alkali... 

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... 

Aqueous solutions of alkali silicates can also be produced in an energetically favorable reaction by reacting sand with sodium hydroxide e.g. in autoclaves with stirrers under pressure and high temperatures. 

Of the alkali silicates with Na20 nSi02. only sodium metasilicate, Na2Si03, with n = 1, has industrial and commercial importance. It is either produced by melting sand with sodium carbonate in a 1 1 molar ratio or by reaction of sand with solid sodium carbonate in rotary or drum furnaces. 

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. 

This describes the specific curing behavior of fly ash, cement dusts, and certain steel works byproducts, is based on the reaction of silicate and aluminous materials with quick lime. Here too, as with the above-mentioned additives, a higher pH causes the precipitation of metal hydroxides and carbonates. The British SEALOSAFE-Process uses fly-ash plus Portland cement, or alkali silicate glass and Fe/Al hydroxides to solidify a broad spectrum of wastes. In the POZ-O-TEC-Process, the wastes from flue gas scrubbers are solidified together with grate ash and fly-ash. The pozzolanic processes have the advantage of excellent longterm stability however, the products solidify rather slowly and are susceptible to acids. 

The alkali silicate glasses are easily corroded by aqueous acids and the main reaction is an exchange between H3O+ of the solution with the alkali M+ of the glass the corrosion rate increaseswith the ionic radii ofM+, that is, K > Na" " > Li" ". The chemical durability is greatly increased by addition of divalent CaO or trivalent AI2O3 oxides, leading to commercial soda-lime-silica compositions. 

Limestones generally do not contain sufficient reactive silica or silicates to cause expansion, and damaging alkali-carbonate reaction has rarely been reported. The reactions involving carbonate rocks can be either expansive or non-expansive and are more likely to occur when the limestone contains appreciable quantities of dolomite and clay minerals [8.1]. ASTM C586 [8.8] gives a test method for determining the potential alkali reactivity of carbonate rock aggregates. 

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. 

According to the hypothesis of Kawamura et al. [132] the expansion of concrete is limited when Ca(OH)2 in the paste is available . These auAors found that the sodium or potassium silicate formed as a result of alkali-sihca reaction, in the presence of fly ash contains much more lime. This complies with much earlier view presented by Veibeck and Gramlich [133] that the availabihty of Ca + ions on the... 

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