Alkali-silica/silicate reaction

Three factors are required to cause damage to concrete by an alkali-silica/silicate reaction ... 

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. 

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. 

Alkali reactivity results from sodium and potassium hydroxides present in cement reacting with silica, silicates, or carbonates in aggregates. Under certain conditions, the reaction can result in expansion of the aggregate and cracking of the concrete. This can lead to mis-alignment of components within structures and corrosion of reinforcement. 

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 right choice of supporting material as well as the choice of suitable properties (pore size, specific surface, chemical surface composition) are important factors influencing the immobilization of the metallocene catalyst and the fragmentation of the support during polymerization. Commercially applied porous silica gels are prepared by neutralization of aqueous alkali metal silicate with acid. The pore structure and pore size distribution can be controlled by the type of chemical reaction and experimental conditions. ... 

Fig. 2. Plot of normalized rate vs. the activity of silicic acid for the LAWABP1 (see Table 1) glass composition at two temperatures (26 and 40 °C). Rates are all computed at steady-state conditions. Boron and Na release rates are identical at low silica activities, then decrease, and become constant at or near saturation with respect to amorphous silica (vertical dot-dashed line). Note that the B rate decreases more than the Na rate. This behaviour can be rationalized as competition between two concurrent reactions alkali-hydrogen exchange and matrix dissolution (see text). Error bars represent 2-

The acidic character of silica is shown by its reaction with a large number of basic oxides to form silicates. The phase relations of numerous oxide systems involving silica have been summarized (23). Reactions of silica at elevated temperatures with alkali and alkaline-earth carbonates result in the displacement of the more volatile acid, C02, and the formation of the corresponding silicates. Similar reactions occur with a number of nitrates and sulfates. Silica at high temperature in the presence of sulfides gives thiosilicates or silicon disulfide, SiS2. 

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... 

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