Mechanism of retardation

Jennings et al. (J33) found that with C3S, the retarding effect of sucrose was greatest if it was dissolved in the mixing water. This contrasts with the situation for cement. They concluded that the retarder was incorporated into the initial product and impeded its transformation into a second product. This explanation is compatible with that discussed above if incorporation is taken to mean adsorption on nuclei or growing crystals of the second product. Although there may well be a phase transformation, this evidence does not seem to demand one. Even without one, the retarder would be more effective the sooner it was added, because a smaller area would need to be poisoned. The situation with cement differs due to the competing effect of the aluminate phase. 

The broad conclusion indicated by the evidence considered in this section is that organic retarders are substances that are readily adsorbed on to the surfaces of growing particles of hydration products, especially C S H, and which do so in preference to complexing Ca ions in aqueous solution. The monodentate mode of attachment proposed by Thomas and Birchall (T49) explains the differences in behaviour among sugars, but its applicability in this respect to a wider range of substances has yet to be examined. It does not appear to account for the observation that the effective retarders are compounds in which two oxygen atoms are located on adjacent carbon atoms, or are otherwise able to approach each other closely. 

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The kinetics and mechanism of retardation and inhibition has been reviewed by Bamford,173 Tiidos and Foldes-Bere/snieh,174 Eastmond,1 5 Goldfinger el /. 6 and Bovey and Kolthoff.1 n... 

The retarder may be a free radical which is too unreactive to initiate a Polymer chain (e.g. triphenyl methyl or diphenyl-picrylhydrazyl). The mechanism of retardation is simply the combination or disproportionation of radicals. 

Role of Ascorbates At low levels 100 ppm) ascorbic acid has been shown to catalyze development of WOF as shown by increased TBA values 0> 3). At higher levels (>1,000 ppm), however, ascorbic acid retards oxidation (3). Although the mechanism of retardation is not known, it Was been suggested that high levels of ascorbates may upset the balance between... 

The mechanism of retardation of olefin polymerization is discussed in detail in... 

In the long distance (time) limit, the D plot makes another plateau whose value decreases on addition of HA. The decrease is described by Ogston s law (Eq. (33.20)) and this behavior is explained with a mechanism of retardation by the mesh which act as a continuous medium with friction. Therefore, the distance or time dependence is negligible to form a plateau. 

The object of this research work is the investigation of the mechanism of retardation of gypsum plaster by the addition of citric acid and its influence on the properties of the fresh and hardened mortar. The significance of this work lies in the explanation of the retardation mechanism and its close connection with the microstructure and hence with the mortar properties. 

The mechanism of retarded autoxidation in the presence of stabilizers of both preventive and chain-breaking type thus includes the possible participation of the antioxidant in all stages of the process (I) ... 

The mechanism of retardation by soluble phosphates is explained by the precipitation of insoluble tricalcium phosphate lay er on the surface of cement grains, impermeable for water [96]. For example the calcium dihydrophosphate Ca(H2P04)2 11,0 reacts with Ca(OH)2 in the strongly supersaturated layer surrounding the alite grains and the Ca3(P04)2 is produced. 

The mechanism of retarding effect has not been explained. There are many hypotheses which highlight this phenomenon only partially. The problem is more complicated and particular retarders differ as the delaying mechanism is concerned. Some of them affect the hydration in many ways. For example in the presence of triethanoloamine the reaction of gypsum with CjA is accelerated but the l dration of CjS and calcium hydroxide formation is hampered [94]. 

As additives in the polymer industry, borates are valued primarily as fire retardants. The relevant chemical compounds are zinc borates, which have the important property that they tend to retain their waters of hydration at relatively high temperatures. This fact likely plays a role in the mechanism of retarding fires. Fires involving plastics containing zinc borates spread more slowly and they also produce less smoke. 

With these figures we calculate that the surfactant bank should have arrived at the effluent end of the core at 0.87 pore volumes of fluids injected provided that adsorption of surfactant on the rock was the only mechanism of retardation. The surfactant bank actually arrived, as measured by C/C = 0.5, at 0.98 pore volumes of chemical slug injected. To attribute that additional 0.11 pore volumes of surfactant retardation to increased adsorption requires a mechanism by which the presence of oil in the rock, at waterflood residual saturation and less, more than doubles the amount of surfactant absorbed by the rock. Furthermore, since the concentration of surfactant in the effluent liquids rises above the concentration of surfactant in the chemical slug, any mechanism based on adsorption would have to include a condition under which previously adsorbed surfactant could be produced in the presence of full-strength chemical slug. We believe, rather, that the greater retention of surfactant in the presence of oil is due to the formation inside the core of microemulsion phases which are richer in surfactant and often are more viscous than the chemical slug. 

S. P. Radko and A. Chrambach. Mechanisms of retardation of rigid spherical particles with 3 to 1,085 nm radius in capillary electrophoresis, using buffered polyacrylamide (molecular weight 5 x 10 ) solutions. Electrophoresis, 17 (1996), 1094-1102. 

The kinetics and mechanism of retardation and inhibition has been reviewed by Bamford, Tiidos and Foldes-Berezsnich, Eastmond, Goldfinger et and Bovey and Kolthoff Gommon inhibitors include stable radicals, oxygen, certain monomers, phenols, qirinones, phenothiazine, nitro and nitroso compoimds, and certain hansition metal salts. Some inhibition constants (kjkp) are provided in Table 6. Absolute rate constants (i ) for the reactions of these species with simple carbon-centered radicals are sirmmarized in Table 7. 

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