Cements alkali silica reaction

Potential volume change of cement—aggregate combinations Accelerated detection of potentially deleterious expansion of mortar bars due to alkali-silica reaction... 

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. 

Rodrigues, F,A, Monteiro, P.J.M., and Sposito, G., The alkali-silica reaction. The surface charge density of silica and its effect on expansive pressure. Cement Concrete Res., 29, 527, 1999,... 

Fig. 6.32 Relationship between the active alkalis content in cement and cement content in a concrete vs. the alkali-silica reaction susceptibility. (According to 71 )...

Reduced concrete deterioration due to alkali-silica reaction in mixes in which Portland cement has been partially replaced by fly ash has been widely reported (Hobbs, 1986, 1989 Meland, 1986 Shayan et ai, 1996). Fly ash seems to act mainly as an alkali diluter, lowering the amount of available alkalis in the system. The capability to reduce the alkali-aggregate expansion may vary in different ashes, and depends on their own alkali content and fineness. 

Alkali-silica reaction. The second destructive problem is attributed to a reaction of the silica-rich aggregates (e.g., cherts, obsidian, opal, quartzite), and alkalis present in the cement paste. In theory, any aggregate containing silica has the potential to participate in the alkali-silica reaction. 

Cement type, admixtures, water/cement ratio, compaction, alkali-silica reaction High resolution scanning acoustic microscopy (SAM)... 

In this research project, some mixtures were prepared by composite cement was made by replacement of %10 of weight of cement by pozzolans and different volume fracture of PP fiber in the matrix for studying the effects of this materials to control the amount of expansion due to alkali-silica reaction. 

By replacing the cement Portland with groimd glass type II the expansion rate of the specimens decrease aroimd 15%. According to last researches materials with pozzolanic characteristics can suppress the alkali-silica reaction (ASR) and decrease the expansion... 

Side effects. During chloride extraction, hydroxyl ions are formed around the reinforcing steel, locally increasing the pH and sodium and potassium ions are enriched around the steel. These changes might stimulate aUcah-silica reaction (ASR, Section 3.4). In the framework of COST 521, the possibility of ASR was checked as a side-effect of chloride extraction [28,36,80,81]. The aggregates studied were reactive and the alkali content of the cement was just below the critical values. The results obtained with non-carbonated concrete showed that, under the worst conditions, chloride extraction induced concrete expansion, but no cracking was observed. 

Glass is generally considered to be an inert material however, it is not chemically resistant to hydrofluoric acid and alkali. Expansive reactions between amorphous silica (glass) and alkalis (such as sodium and potassium found in high concentrations in high alkali Portland cement) could have deleterious effects if glass is used in Portland cement concrete structures [208,209]. 

In an alkali silica cement, we see a new setting/hard-ening reaction with quartz becoming a factor. 

The physico-mechanical properties of low-alkali Portland cements do not differ significantly from corresponding cements with normal alkali contents. Their marketing and use make sense only in regions with deposits of rocks that may be susceptible to alkali-silica or dedolomitization reactions, if employed as concrete aggregates. 

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. 

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