Ruža Krstulović

Phase transformation of calcium carbonate polymorphs

Abstract The phase composition of mineral aragonite and synthetic vaterite samples was determined qualititavely by using IR-spectrophotometric and X-ray diffraction analyses. The transformation of aragonite, A, and vaterite, V, into the stable modification calcite, C, was followed using differential scanning calorimetry analysis. In order to determine the kinetics and mechanisms of these phase transformations, a number of experimental DSC curves were elaborated mathematically and the stationary point theory was applied. The activation energy, Ea, and the enthalpy, ΔH , were found to be, respectively, 234.5 ± 5.6 kJ mol −1 and 122 Jg −1 for the phase transformation A → C, and 252.8 ± 48.7 kJ mol −1 and −21.2 Jg −1 for the V → C transformation.

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.

A conceptual model of the cement hydration process

Cement hydration development has been analyzed on the basis of measured data obtained by the microcalorimetrical method. Based on the most appropriate model assumed, thermokinetic analysis has been carried out for the polymineral and polysize system tested on industrial cement samples. A computer program has been developed to determine specific kinetic parameters describing individual hydration processes. The program makes it possible to determine the controlling processes during hydration and their share. Certain kinetic parameters make it possible to observe differences in hydration of pure clinker minerals and of Portland cement in a subtler way.

Durability of the hydrated limestone-silica fume Portland cement mortars under sulphate attack

Abstract The effect of silica fume, emerging as a by-product in production of ferrosilicon, on corrosion resistance to sulphate attack of Na 2 SO 4 and MgSO 4 solutions has been studied in Portland cement mortars containing limestone and mortars containing no limestone. Expansion and changes in the elasticity modulus of mortars as a function of silica fume content have been investigated. The phases formed and the microstructural changes in the mortar exposed to sulphate corrosion have been determined by differential thermal analysis (DTA), X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The presence of monocarbonate and the absence of monosulphate were detected in the mortars containing limestone. The addition of silica fume results in less CH formed by the hydration process and consequently less gypsum and ettringite during the sulphate immersion of mortars. Mortars containing more than 5 mass% of SiO 2 , or simultaneously limestone and more than 2 mass% of SiO 2 , are characterised by a good sulphate resistance and show lower expansion than the control, the sulphate-resisting mortars independent of the type of the sulphate solution.

The properties of Portland cement-limestone-silica fume mortars

Abstract This work has studied the influence of the combined action of silica fume and limestone on strength development, porosity, pore structure and morphological features in the system where 15 wt% of cement was substituted by finely ground limestone. Silica fume was added in amounts of 0, 2, 5, 8, 11 and 15 wt% on a cement basis, respectively. It has been established that limestone addition considerably increases the total porosity of mortars. However, if introduced together with silica fume up to 8 wt% of silica, porosity decreases. More than 8 wt% of silica increases the porosity again. The cement mortar containing 8 wt% of silica fume shows the highest compressive strength, the minimum value of the total porosity, and its pore size distribution curve shows a discontinuous pore structure. Limestone is taken up to the system and reacts with aluminate and ferrite phases from cement. Approximately 5 wt% is available for reaction after 120 days hydration of mortars containing no silica fume. The quantity of limestone incorporated is affected by the silica fume content. The replacement of Portland cement by 15 wt% of silica fume causes reduction both in the amount of cement and in the free CH content available for limestone chemical activity, and in this condition limestone acts only as a filler addition.

Precipitation of calcium carbonate in a calcium nitrate and monoethanolamine solution

Abstract High-purity precipitated calcium carbonate has been produced in the laboratory by carbonation of a solution of calcium nitrate and monoethanolamine. The process was traced by measurements of the change in Ca concentration and in electrical conductivity of the solution. The κ−t and cCu−t curves were analyzed to determine the effect of temperature and concentration of carbon(IV) oxide on the process of carbonation. The samples obtained were mainly of aragonite and vaterite crystal modifications with traces of calcite. The relationship between the mineralogical composition and particle size (morphology) and operating conditions was examined as well.

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.

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.

Investigation of dehydroxylation of gibbsite into boehmite by DSC analysis

The dehydroxylation of gibbsite into boehmite was investigated by means of DSC analysis under non-isothermal conditions in the temperature range 453–673 K at heating rates from 2.5 to 20.0 K min−1. Mathematical analysis of the experimental DSC curves revealed the mechanism and kinetics of the gibbsite dehydroxylation process. The kinetic curvesα=f(t) andα=f(T) are sigmoidal in shape; their inflection points and the νm point of the curvesν=f(T) andν=f(T) are interrelated and are defined by the concept of a stationary point. The activation energy for the first stage of gibbsite dehydroxylation in the temperature range 453–673 K is 132.92±8.33–142.26±8.33 kJ mol−1.

Examination of reaction between the NSF superplasticizer and cement

Abstract The mechanism of the reaction taking place between NSF superplasticizer and Portland cement was examined with spectrophotometric and conductometric techniques, and strength measurements carried out in the presence of 0.1% – 2% of the admixture. The results obtained for electrolyte conductivity, κ, mass of admixture adsorbed to cement surface, and increase in strength, provided a basis for conclusions on the phenomena and effects occurring between the additive and the cement in the reaction medium.