Alkalis from aggregate

The decomposition of dolomite aggregate in the reaction with alkalis from cement, is also ranked among the cause of concrete destruction. On the other side, the... 

The soluble alkali content in cement is of great importance for the aggregate expansion reaction. Sodium and potassium occur in cement in the form of sulphates and form the solid solutions in CjA and C2S (see Sect. 2.5.5.3). Sodium and potassium sulphates are very quickly dissolved. Alkalis from C3A solid solution are easily soluble too, while those from CjS are practically inactive, because of the slow CjS hydration. The release of alkalis is also affected by distribution of particular phases in the polymineral cement grain one can imagine C3A encapsulated in brownmil-lerite coating its reaction with water will be retarded. 

There are generally more alkalis in fly ash than in Portland cement. Most of these alkalis occur in the vitreous phase and hence only a part is water soluble, usually about 0.1% Na20g [60]. On the other side one should remember that the glass is the most reactive component of fly ash in cement paste. It is not clear what part participate in the pozzolanic reaction and what is released to the pore solution. However, there is a dominant opinion that the release of alkalis to the solution from fly ash occurs slowly and hence they cannot participate in the reaction with aggregates [60]. Therefore Hobbs [110] proposes to take only 0.2% Na20 as an income of alkalis from fly ash, when the total alkalis content in concrete is calculated. However, better effect are showing fly ash with low alkalis and CaO content [60, 121]. 

At the end of forties several cases of hmestone aggregate concretes destmction have been documented in North America. Initially this was attributed to the poor aggregate resistance to fieezing and thawing. However, already in 1957 Swenson [140] proved that the limestone aggregate swelling was the result of reaction with alkalis from cement. 

There are various hypotheses explaining this aggregate expansion mechanism. The most wide spread refer to the swelling of clay minerals and of osmotic pressure formation [141,142]. All these hypotheses agree that this phenomenon relates to the reaction of soluble alkalis from cement with the aggregate leading to the decomposition of dolomite ... 

Fly ash addition is diminishing also the risk of reaction of alkalis from cement with aggregate, which was discussed in Sect. 6.4. The fly ash addition, which allows the reduction of OH ions concentration in the liquid phase of the paste to 0.3 mol/1 is protecting concrete against destraction [140]. This corresponds to about 40% of fly ash addition to cement with average alkalis content (Na2O =0.92%) or about 30% in the case of cement with lower alkalis content (Na2O =0.68%). 

The experiments on alkali iodides, PEOx-Nal or PEOx-Lil [316-318] were performed on PEO chains of 23 or 182 (-CH2-CH2-O-) monomers and Orion ratios between 15 and 50. The incoherent scattering from protonated polymers was measured using INI 1C, which yields the intermediate scattering function of the self-correlation. The experiments were performed in the homogeneous liquid phase where the added salt is completely dissolved and no crystalline aggregates coexist with the solution, i.e. at temperatures around 70 °C. 

The different types of admixtures, known to reduce alkali-aggregate reactions, can be divided into two groups those that are effective in reducing the expansion due to the alkali-silica reaction, and those that lower expansions resulting from the alkali-carbonate reaction. For the alkali-silica reaction, reductions in the expansion of mortar specimens have been obtained with soluble salts of lithium, barium and sodium, proteinaceous air-entraining agents, aluminum powder, CUSO4, sodium silicofluoride, alkyl alkoxy silane,... 

Information from previous work suggests that air-entrainment offers a measure of protection against alkali-aggregate expansion. An air-entrainment of 3.6% can cause a 60% reduction in expansion [10]. In Fig. 6.2 the... 

Non-alkaline liquid accelerators are fairly new to the international market, therefore the bibliography on their use is still scarce. They were conceived to solve some classical problems stemming from the use of alkaline accelerators, such as caustic alkalis, hazardous conditions in underground work, risk of alkali-aggregate reaction, risk of handling conventional accelerators with extremely high pH level, and reduction of latter-age strength. 

In the absence of a suitable solid phase for deposition and in supersaturated solutions of pH values from 7 to 10, monosilicic acid polymerizes to form discrete particles. Electrostatic repulsion of the particles prevents aggregation if the concentration of electrolyte is below ca 0.2 N. The particle size that can be attained is dependent on the temperature. Particle size increases significantly with increasing temperature. For example, particles of 4—8 nm in diameter are obtained at 50—100°C, whereas particles of up to 150 nm in diameter are formed at 350°C in an autoclave. However, the size of the particles obtained in an autoclave is limited by the conversion of amorphous silica to quartz at high temperatures. Particle size influences the stability of the sol because particles <7 nm in diameter tend to grow spontaneously in storage, which may affect the sol properties. However, sols can be stabilized by the addition of sufficient alkali (1,33). 

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