Davor Rušić

The role of silica fume in the kinetics and mechanisms during the early stage of cement hydration

Abstract The role of silica fume in the reaction kinetics and mechanisms of the early stage hydration of Portland-slag cement–silica fume pastes (W/S=0.5 at 20°C) has been studied. Using differential scanning calorimetry (DSC), the rate of calcium hydroxide (CH) formed during hydration has been determined quantitatively and the percentage of reaction of hydration has been calculated. The kinetic analysis was used to obtain parameters, which were employed to find out the influence of the silica fume content on the reaction rate constant value. The reaction stages have been analysed and explained by kinetic terms. The delay in CH formed during hydration between 8 and 10 h from the beginning of hydration has been noticed and a mechanism has also been discussed. The investigations have shown that the addition of silica fume of more than 8 mass% reduces the duration of the phase boundary interaction as a rate-determined process resulting in the fast diffusion rate-determining process. The results of this study have also revealed evidence of the accelerator effect of silica fume during the first 8 h of hydration when it still exists as chemically inert filler. The pozzolanic reaction between silica fume and CH formed during hydration is occurring after three days of hydration.

Modeling batch kinetics of copper ions sorption using synthetic zeolite NaX.

The sorptive removal of copper ions from aqueous solutions using zeolite NaX has been studied by a batch technique. The influences of solute concentration, temperature and particle size on the sorption process were examined. Several kinetic models were used to test the experimental rate data and to examine the controlling mechanism of the sorption process. Lagergren pseudo-first order, the pseudo-second-order (Ho) and Ritchie second-order models were analyzed using nonlinear regression technique while Weber-Morris model was analyzed using linear least squares method. The obtained results indicated that synthetic zeolite NaX could be used as an efficient material for the sorption of copper ions. A kinetic study has shown that the best fit is achieved when the Ritchie model was applied and that sorption did not involve film or intraparticle diffusion, i.e., they were not the rate controlling steps. The activation energy was found to be 12kJ/mol in the present study.

Kinetic analysis of thermal decomposition of Ca(OH)2 formed during hydration of commercial portland cement by DSC

AbstractIn this paper, evaluation of kinetic parameters (the activation energy – E,the pre-exponential factor – A and the reaction order – n) with simultaneous determination of the possible reaction mechanism of thermal decomposition of calcium hydroxide (portlandite), Ca(OH)2 formed during hydration of commercial Portland-slag cement, by means of differential scanning calorimetry (DSC) in non-isothermal conditions with a single heating–rate plot has been studied and discussed. The kinetic parameters and a mechanism function were calculated by fitting the experimental data to the integral, differential and rate equation methods.To determine the most probable mechanism, 30 forms of the solid-state mechanism functions, f(αc) have been tried. Having used the procedure developed and the appropriate program support, it has been established that the non-isothermal thermal decomposition of calcium hydroxide in the acceleratory period (0.004<αc<0.554) can be described by the rate equation: d αc/dT=A/βexp(−E/RT)f(αc), which is based on the concept of the mechanism reaction:f(αc)=2(αc)1/2.The mechanism functions as well as the values of the kinetic parameters are in good agreement with those given in literature.n

A new approach in mathematical modelling of cement hydration development

Abstract This paper describes development of a kinetic cement hydration model, which observes hydration rate relative to time d α /d t = f ( T , t ). The model established makes it possible to observe hydration rate relative to time and not relative to the reaction degree α , d α /d t = f ( T , α ), which offers a new approach to solving the cement hydration kinetics. The model assumes several simultaneous processes sequentially governing individual hydration segments. The model has been tested by means of experimental α – t data obtained from the DSC plots by quantifying the content of Ca(OH) 2 from the cement–water system. The results indicate that the model established is valid and can be applied well.