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Thermal study of unaltered and altered dolomitic rock samples from ancient monuments

The case of Villarcayo de Merindad de Castilla la Vieja (Burgos, Spain)

Journal of Thermal Analysis and Calorimetry
Authors: J. L. Perez-Rodriguez, A. Duran, and L. A. Perez-Maqueda

Abstract

In this study, the decomposition behaviour of unaltered and altered dolomitic rock samples used in Cultural Heritage buildings was studied by simultaneous TG–DTA experiments at different atmospheres, X-ray diffraction in a high-temperature chamber, and evolved gas analysis. The components of dolomite rock samples and hydrated calcium oxalate formed during the alteration processes of the rocks were characterized, and the decomposition mechanisms of these components were determined. The TG–DTA experiments carried out at CO2 atmosphere were used to determine the carbonate compounds in the rock samples. The TG–DTA study characterized the presence of organic compounds formed during the biological degradation of the rock samples, possibly responsible of the hydrated calcium oxalate formation.

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Abstract  

The cyclic and Controlled Rate Thermal Analysis method (CRTA) has been used. The two rates automatically selected in the cyclic curve are small enough to allow the two states of the sample to be compared have nearly the same reacted fraction. Thus, the activation energy can be calculated without previous knowledge of the actual reaction mechanism. Provided that the activation energy,E, is known, a procedure has been developed for determining the kinetic law obeyed by the reaction by means of master curves that represent the values of the reacted fraction, α, as a function of−E/R(1/T-1/T 0.5),T 0.5 being the temperature at which α=0.5. This procedure has been tested by studying the thermal decomposition reaction of BaCO3.

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Abstract  

It has been demonstrated that a single plot of the values of Δlnα1/2/Δln(1-α) (taken from a single α−T curve obtained under a controlled linear increase of the reaction rate) as a function of the corresponding values of Δ(1/T)/Δln(1−α) permits the simultaneous determination of both the activation energy and the kinetic model in accordance with a solid state reaction.

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Summary The SCTA methods for the kinetic analysis of solid-state reactions have been reviewed. It has been shown that these methods present two important advantages with regards to the more conventional rising temperature experiments. Firstly, they have a higher resolution power for discriminating among the reaction kinetic models and, secondly, SCTA is a powerful tool for minimizing the influence of the experimental conditions on the forward reaction.

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Abstract  

Roman ancient mortars have been widely studied, in connection with both diagnosis and application required for restoring. Thermoanalytical experiments performed on mortars from Pompeii and Herculaneum provided a very good understanding of the technology employed. The mortars from Pompeii were obtained by the proper mixing of lime and marble grains while mortars of Herculaneum by lime and silicates compounds. The position of the endothermic peak of calcite decomposition showed important variations in the different samples studied, which was assigned to the different crystallinity and particle sizes. Experiments under CO2 flow confirmed the presence of magnesium calcium carbonates.

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Journal of Thermal Analysis and Calorimetry
Authors: L. A. Perez-Maqueda, C. Maqueda, J. L. Perez-Rodriguez, J. Subrt, Z. Cerny, and V. Balek

Abstract

Acid leaching of vermiculite is an interesting procedure to prepare high surface area porous silica. Thermal behaviour of unground and ground vermiculite leached with HCl solutions has been studied by TG, DTA, ETA and high temperature XRD. Important differences have been observed in the thermal behaviour of unground and ground vermiculite after the acid treatments. Thus, for the acid-treated unground vermiculite, dehydrated vermiculite, enstatite and cristobalite were formed during the heating, while for the acid-treated ground vermiculite only iron oxides and cristobalite phases were observed. Structural modifications due to acid treatment were responsible for changes in the transport properties determined by ETA for the vermiculite samples.

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Abstract  

The results obtained in this work point out that both SIA and CRTA traces, simulated assuming the same values of the reaction rate, are identical provided that either 'n order’ or diffusion controlled kinetics are concerned. The α-T CRTA plots of solid state reactions fitting an Avrami-Erofeev kinetic model show that the temperature decreases with increasing α until reaching a minimum at a value of the reacted fraction α = αmin characteristic of the Avrami-Erofeev exponent. The shapes of the corresponding SIA diagrams are quite different and a very long isothermal step is obtained. Moreover, the reaction rate is not maintained constant during the isothermal period but it shows a maximum at a value of α depending on the Avrami-Erofeev coefficient. It is important to point out that this αmax value agrees with the corresponding αmin calculated from a CRTA curve by assuming the same Avrami-Erofeev exponent.

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Summary  

The differences on the thermal behaviour (DTG-DTA) of antigorite sample measured before and after sonication have been studied. Sonication treatment produces negligible changes in the structure of the material but substantial textural modifications. These modifications produce changes in the thermal behaviour of antigorite sample. Thus, it has been observed a decrease in the dehydroxylation temperature as measured by DTG and DTA effects. For sonication treatments longer than 20 h, two new effects of dehydroxylation are observed, the intensity of these two new effects increases with the sonication time showing a modification in the release of structural OH. It has been also observed that the formation of forsterite takes place simultaneously with the dehydroxylation of the antigorite. The high temperature exothermic effect is due to the recrystallization of forsterite and not to the formation of forsterite as traditionally assumed. Modifications in the thermal dehydroxylation of antigorite observed in this study are related to the pronounced decrease in particle size obtained by sonication.

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Dependence of the preexponential factor on temperature

Errors in the activation energies calculated by assuming that Ais constant

Journal of Thermal Analysis and Calorimetry
Authors: J. M. Criado, L. A. Pérez-Maqueda, and P. E. Sánchez-Jiménez

Summary  

The dependence of the preexponential factor on the temperature has been examined and the errors involved in the activation energy calculated from isothermal and non-isothermal methods without considering such dependence have been estimated. It has been shown that the error in the determination of the activation energy calculated ignoring the dependence of Aon Tcan be rather large and it is dependent on x=E/RT, but independent of the experimental method used. It has been also shown that the error introduced by omitting the dependence of the preexponential factor on the temperature is considerably larger than the error due to the Arrhenius integral approach used for carrying out the kinetic analysis of TG data.

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