Prevention alkali silica reaction

Linking of the preventing action of mineral additions, with the sodium and potassium concentration lowering in the liquid phase, is obvious. The progress of alkali silica reaction is strongly dependent on this concentration, as well as on the related pH of this solution. Low alkali content is a warrant of the lack of concrete expansion with reactive aggregate. 

One approach to preventing expansion due to ASR consists in lowering the alkali content in the concrete mix to sufficiently low concentrations. It is generally accepted that an alkali-silica/silicate reaction in concrete made with Portland cement will not occur if the content of equivalent Na20 (defined as Na2O =Na2O+0.66K2O) in the mix does not exceed 4 or even 3 kg/m. Such low alkali concentrations are usually not achievable with ordinary Portland cement, but may be achieved if a low-alkali Portland cement (see section 2.10) is used instead. 

Finally, our observations regarding the longterm impact of alkali ion exchange on glass dissolution now provide a mechanistic basis for the empirical residual rate of reaction appended to the TST rate law articulated by Grambow (1985). The residual rate was appended to prevent calculated glass dissolution rates from dropping to zero under silica-saturated conditions, which is not in accord with experimental observations. 

A limitation of the hard template method is that the resulting materials must be stable in HF or NaOH solution and that the precursors must not react with the silica template at a high temperature. For example, in the formation of lithium containing transition metal oxides, it is necessary to first form the transition metal oxide as a mesoporous solid to prevent reaction of the alkali metal Li with the silica template and then to react the mesoporous transition metal solid with a lithium source, such as LiOH. 

Three conditions are essential to ASR development alkalis, reactive silica and sufficient moisture. If one of these conditions is not present, the reaction will not occur. Therefore, it is very important to evaluate the reactivity of these rocks before using them as aggregates, by taking preventive measures such as the use of puzzolanic materials. 

In zinc-rich paints some early patents indicate the importance of a silica component. Colloidal silica was reacted with finely divided zinc to form a colloidal zinc silicate in which the excess zinc metal was suspended (654). A water-insoluble binder for zinc-containing coatings was produced by mixing alkali-stabilized colloidal silica with lithium hydroxide in suitable proportions (655). Evolution of gas from mixtures of colloidal silica with zinc powder is prevented by adding an indigoid compounds (656). Another formulation involves quaternary ammonium polysilicate solution milled with lead oxide as the binder for zinc (657). Some of the problems stem from the impurities in the zinc powder which promote reaction with the medium (658). Adhesion. of paints of this type of steel is improved by adding up to 2% of a styrene-acrylic resin dispersion (659). 

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