A. Bezjak

On the determination of rat constants for hydration processes in cement pastes

A procedure has been developed to evaluate the actual rate constants for the hydration of individual phases in polysize cementitious systems. The evaluation is based on the comparison between observed and calculated degrees of hydration, the latter being obtained by taking into account the particle size distribution of the original sample and the effects of possible overlapping of different rate-determining processes. The determination of rate constants for C3S-hydration is described in order to illustrate the proposed procedure.

Hydration of B2O3-stabilized α′-and β-modifications of dicalcium silicate

Abstract Paste hydration of B 2 O 3 -stabilized α′- and β-C 2 S was studied at reaction times between 5 hours and 28 days. Data on the degree of hydration determined by X-ray diffraction at different ages were used to evaluate hydration rate constants. The higher rate of α′-C 2 S hydration resulted in the higher compressive strengths of the α′-C 2 S-alite paste as compared with the β-C 2 S-alite paste. Differences in crystal imperfections of the two C 2 S-polymorphs were determined using electron microscopy and electron diffraction.

Crystallization kinetics of mullite from single-phase gel determined by isothermal differential scanning calorimetry

Abstract Transformation kinetics of single-phase gel with mullite composition was studied by isothermal differential calorimetry (DSC) in temperature range from 937 to 959°C. Single exotherm was observed for annealing temperatures below 947°C, and two overlapped exothermic peaks were seen above this temperature. According to XRD analysis, mullite was the only phase crystallized either under non-isothermal or isothermal heat treatment. Johnson-Mehl-Avrami (JMA) equation for nucleation and growth could not describe mullite crystallization adequately, even below 947°C. Using bimodal JMA-type model, that proposes mullite crystallization in two steps, the fitting was remarkably good in the whole temperature range. Obtained kinetics data do not allow one to characterize the gel, either as typical single phase one (nucleation-controlled process with two rate constants and small apparent activation energies), or as hybrid gel (mullite formation via spinel and high apparent activation energies). The rate constants were an order of magnitude smaller than is proposed for single phase gel. The apparent activation energies, however (Ea1 = 1053 ± 51 kJ/mol, and Ea2 = 1028 ± 22 kJ/mmol, were in great discrepancy to those already cited for single phase gels, but they were in very good agreement with data evaluated for diphasic and hybrid gels. Mullite a-axis length and effective fraction of mullite that is formed in the first and second step of the process provided an insight in the mechanism of mullite crystallization. It is assumed that not the nucleation and crystallization limitations, rather the phase separation is the controlling process in mullite formation from single phase gel under applied experimental conditions.

Kinetic analysis of cement hydration including various mechanistic concepts. I. Theoretical development

Abstract Mathematical relations are given for the determination of kinetic parameters of hydration of individual phases in polysize cement specimens. Equations are derived by considering various mechanistic concepts with two examples developed in particular. In both cases it is assumed that interaction and diffusion through hydrate layer contribute together to the total reaction resistance. In one instance the acceleration is covered by Avrami and Erofeev equation, while in the other the acceleration is interpreted as the consequence of the increasing driving force due to a continuous diminution of the concentration of decisive ionic species in the surrounding solution. The models proposed for mathematical analysis of kinetic curves showing the advancement of hydration in polysize specimens have the following basic characteristic in common: particles belonging to different size groups hydrate obeying at the same time different rate lays, the overall degreee of hydration being the result of simultaneous action of different rate-determining processes.

Resolution of overlapping peaks and the determination of kinetic parameters for the crystallization of multicomponent system from DTA or DSC curves: I. Non-isothermal kinetics

Abstract A new computer program has been created to separate overlapping exothermic and endothermic DTA (DSC) effects, obtained under non-isothermal conditions, occurring due to simultaneous nucleation and growth or more growth processes, and also to calculate the kinetic parameters of the processes represented by each peak. A new graphical method for the initialization of the required parameters and the refinement of the initial values by the least squares method is presented. The procedure for the determination of the kinetic parameters from DTA data is demonstrated on a model system and on a kinetic analysis of mullite formation from diphasic gel.

A kinetic study of hydrothermal reaction in ℓ-C2S-quartz system. I. Determination of rate constants for processes with two acceleration periods

Abstract The kinetics of hydrothermal reaction in l-C 2 S-quartz polysize system was studied by applying a procedure that takes into account the effect of simultaneous action of different rate-determining processes. The experimentally obtained α-t interdependences exhibited two acceleration periods and the analysis has been carried out for the first and the second period independently. It has been shown that during the first acceleration period nucleation and growth, phase-boundary interaction and diffusion process control simultaneously the overall advancement of quartz conversion. At the beginning of the second acceleration period fast diffusion coinciding with crystallisation of xonotlite takes place and, consequently, nucleation and growth of the reaction product has been proved as the rate determining step.

A new approach to solid-state reactions kinetics analysis: the application of assisting functions to basic equations for isothermal conditions

Abstract Functions describing simple models for solid-state reaction kinetics under isothermal conditions were transformed in order to obtain new functions with improved properties suitable for kinetic analysis. The transformations were performed trough multiplying well known functions as Johnson–Mehl–Avrami or Jander’s with assisting functions. In such manner, new functions, Φ , were obtained enabling calculation of kinetic parameters in a simple way. The relationships between the values of the extremes of Φ functions and various kinetic parameters have been established. The calculation of kinetic parameters by application of new equations have been demonstrated on six model systems. Very good agreement between preset and calculated parameters for each model system has been observed. Using the same approach, the rate-determining step, i.e. the reaction model, and proper kinetic equation could also be determined.

Determination of kinetic parameters for nucleation and growth from DTA data

Abstract A mathematical model has been derived for non-isothermal kinetic analysis from overlapped exotherm and endotherm DTA effects occurring due to simultaneous nucleation and growth processes. In the course of the mathematical treatment the refinement of the initial values of the kinetic parameters is performed. The procedure for the determination of kinetic parameters from DTA has been demonstrated on a model system and on a kinetic analysis of mullite formation from diphasic gel.

Importance of factors for ruggedness test in phase solubility analysis

Abstract In the preparation of chemical reference substances for pharmaceuticals it is essential to make an accurate assessment of the content of impurities. The phase solubility analysis (PSA), a method based on intrinsic thermodynamic properties of matter, is employed to detect contaminating substances and to estimate their concentration. Factors that influence the performance of the PSA method, particularly the reproducibility, are varied to find the optimal analytical conditions and to design a sufficiently rugged method. The ruggedness test is carried out by deliberately varying factors: weight of sample, solvent composition, bath temperature, agitation time, agitation speed, amount of clear solution and others. Simple factorial experiments can be applied to develop a procedure for routine determinations of the purity of a substance through the application of precise solubility measurements.

Kinetic analysis of extraction of calcium silicates with methanolic solution of salicylic acid. I. Theoretical part

Abstract A procedure has been developed for determining kinetic parameters of extraction as heterogeneous solid-liquid reaction in the case of polysize specimens. The system γ-Ca 2 SiO 4 - methanolic solution of salicylic acid served to illustrate the proposed procedure. The results of the investigation have revealed that surface interaction is rate-determining for the extraction.