Kinetics particle size distribution effect

Modified amino acids such as N-acyl-dehydroalanine polymers and copolymers with N-vinyl-N-methyl acetamide seem to be particularly effective [396]. The crystallization kinetics in the presence of polyvinylpyrrolidone and tyrosine have been tested by time-resolved experiments [981]. An influence is evident on the particle size distribution of the hydrate [1433]. 

By definition, the kinetic curve of a cement is the weighted sum of the curves for its constituent phases as they occur in that cement. The reactivities of individual clinker phases were considered in Section 4.5 and some effects of particle size distribution, which is a particularly important variable, in Section 4.1.4. Although many data relating particle size distribution directly to strength exist, much less is known about its relation to degrees of reaction. Parrott and Killoh (P30) presented data indicating that the rate of hydration, as represented by that of heat evolution, was proportional to the specific surface area during the period of hydration in which the rate was controlled by diffusion. 

The kinetics of cement hydration are dominated by the effects associated with the particle size distribution of the starting material, and attempts to explain them in which this is ignored can lead to very misleading results (T41,B98,B105,K37,J27,K38,K39). Even laboratory-prepared samples with close distributions (e.g. 2-5 pm) (K20) are far from monodisperse from the kinetic standpoint. Two approaches to the resulting problems of interpretation will be considered. 

Effect of Particle Size Distribution on Sintering Kinetics... 

Attempts have been made to allow for the influence of particle-size distributions on kinetic behaviour [76-83], but most usually it is assumed (perhaps implicitly) that all reactant particles behave similarly. Allowances for size effects, which are difficult to quantify, are most often based on numerical integration across an assumed or measured distribution of particle dimensions [82,83]. 

Dirksen JA, Ring TA. Fundamentals of crystallization Kinetic effects on particle size distributions and morphology. Chem Eng Sci 1991 46), 2389-2427. 

An approach which circumvents the problem of the effect of particle-size distribution on decomposition kinetics was adopted by Walker et al [26] and by Fox [10], who worked with individual single crystals or collections of noninteracting particles of uniform size. A typical lead azide preparation of the latter type is shown in Figure 4. Walker et al found that sodium and thallous azides do indeed show induction periods which are dependent upon particle size the... 

A pseudo-bulk system is one in which the compartmentalized nature of the locus of polymerization has no effect on any kinetic property (rate, molar mass or particle size distributions). A system in which n is appreciably greater than 0.5 will always be pseudo-bulk there are so many radicals in a particle that the polymerization will be indistinguishable from the equivalent bulk one. However, a system with a low value of n can also be pseudo-bulk, if (for example) radical desorption results in the desorbed radical suffering no other fate except to re-enter another particle [1,3]. It is then apparent that the polymerization process will not see the walls between particles. Because pseudo-bulk kinetics can occur in systems where n 0.5, a pseudo-bulk system is different from the Smith-Ewart Case 3. 

The kinetic hydration of cement is widely studied in the literature and some of these papers reported the chemical, physical and mechanical behaviour [2-5], Simplified models were used by Knudsen[6], Basma et al [7], Schindler and Folliard [8], Bentz [9]. The majority of these models are empirical, based on experimental observations of macroscopic phenomena, and they take into account the effects of curing temperature, water-cement ratio, fineness, particle size distribution and chemical composition of cement [5]. 

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