Aggregation reactions

Figure 3 Effect of seeding and inhibitors on aggregation reaction. The lag phase (curve c) is characteristic of reactions in which formation of nuclei for polymerization is an unfavorable process. Addition of preformed nuclei or seeds" (curve a) abolishes the lag phase. Inhibitors may affect the formation of nuclei and influence eitherthe lag phase, the extension of the nuclei changing the growth phase, or both (curve d). The inhibitor example (curve d) acts more strongly at nuclei formation than on the slope or plateau level of the growth phase.

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,... 

Fig. 6.2 Influence of air entrainment on expansion due to alkali-aggregate reaction (Ramachandran). 

The ions should not take part in the alkali-aggregate reaction to form other expansive materials. 

The aluminate-based admixtures undoubtedly provide the best wet-mix shotcrete, making it possible to build thick linings even in overhead work. The high alkali content and the consequent health hazard are the main constraints to their more widespread use. Furthermore, there is concern that the admixture may promote the alkali-aggregate reaction in concretes containing reactive aggregates, as well as sulfate attack [117]. 

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. 

Nomachi, H., Takada, M. and Nishibiyashi, S. (1989). 8th International Conference on Alkali-Aggregate Reaction, Kyoto, Japan, 211-15. 

Ohama, Y., Demura, K. and Wada, I. (1992). 9th International Conference on Alkali-Aggregate Reaction in Concrete, London, 420-31. 

Oberholster, R.E. and Westra, W.B. (1981). Proceedings 5th International Conference on Alkali-Aggregate Reaction in Concrete, Capetown, S. Africa, 10. 

Farbiarz, J., Schuman, D.C., Carrasquillo, R.E. and Snow, P.G. (1989). Proceedings 8th International Conference on Alkali-Aggregate Reactions in Concrete, Olso, Norway. 241-6. 

When the irradiation of metal ion solutions is performed in the presence of the ligands CO or PPhs, metal reduction, ligandation, and aggregation reactions compete, leading to reduced metal complexes and then to stable molecular clusters, such as Chini clusters Pt3(CO)6] with m = 3-10 (i.e., 9-30 Pt atoms) [97], or other metal clusters [98]. The synthesis is selective and m is controlled by adjusting the dose m decreases at high doses). The mechanism of the reduction has been determined recently by pulse radiolysis [99]. Molecular clusters [Pt3(CO)6]5 have been observed by STM [100]. 

The above analysis actually refers to the simplest case of aggregation of non-ionic surfactants. For monovalent, ionic surfactants the aggregation reaction becomes ... 

Kronman, M. J., Andreotti, R. E. and Vitals, R. 1964. Inter-and intramolecular interactions of a-lactalbumin. II. Aggregation reactions at acid pH. Biochemistry 3, 1152— 1160. 

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