Igor Medveď

Bio: Igor Medveď is an academic researcher from Czech Technical University in Prague. The author has contributed to research in topics: Cement & Diffusion (business). The author has an hindex of 7, co-authored 26 publications receiving 275 citations. Previous affiliations of Igor Medveď include University of Constantine the Philosopher.

Simultaneous DSC and TG analysis of high-performance concrete containing natural zeolite as a supplementary cementitious material

Abstract: Natural zeolite is a pozzolan active material used as a supplementary cementitious material to improve the final properties of concrete. In this paper, the thermal properties of hardened high-performance concrete containing natural zeolite in the amount varying from 0 to 60 mass % of the cement binder are studied. Using the differential scanning calorimetry and thermogravimetry, the hydration and pozzolanic reaction in the concrete are investigated in dependence on the amount of the added natural zeolite. The investigation is performed in the temperature range from 25 to 1000 °C with a rate 5 °C min−1 in an argon atmosphere. We found out that the temperature and enthalpy of liberation of physically bound water, C–S–H gels, and ettringite decomposition (all occurring from 50 to 300 °C) almost do not change with an amount of the natural zeolite in the studied samples. On the other hand, for portlandite (420–510 °C) and calcite decomposition (580–800 °C), they decrease with an amount of the natural zeolite. Finally, the last modification at temperature about 857 °C was attributed to the crystallization of wollastonite.

Kinetics of thermal expansion of illite-based ceramics in the dehydroxylation region during heating

Abstract: The kinetics of the illite dehydroxylation is investigated using data from non-isothermal thermodilatometric experiments. An illitic clay with more than 70 % of illite was thermally treated under several heating rates from 1 to 15 °C min−1 in the dehydroxylation region. Model-free and nonlinear model-fitting methods were used to analyze the data, and their results are discussed. The dehydroxylation of the trans-vacant and cis-vacant illite layers can be described by the Avrami–Erofeev model with the exponent values 1.010 and 2.1, respectively, and the apparent activation energies of 119 and 184 kJ mol−1, respectively. The obtained kinetic parameters were used for the calculation of an optimal heating regime in order to achieve a constant thermal expansion rate in the dehydroxylation region for an illite-based ceramic body.

Determination of a partial phase composition in calcined gypsum by calorimetric analysis of hydration kinetics

Abstract: A calorimetric method for the determination of a partial phase composition in calcined gypsum is presented The method is based on the separation of the reaction heat of the transformation of β-anhydrite III to β-hemihydrate from the main hydration reaction transforming β-anhydrite III to dihydrate Using the known hydration heats of β-anhydrite III to dihydrate and β-hemihydrate to dihydrate, the content of β-calcium sulfate anhydrite III and β-calcium sulfate hemihydrate as well as the amount of non-hydrating parts in gypsum can be determined Practical application of the method is illustrated on three industrial products

Influence of calcite in a ceramic body on its thermophysical properties

Abstract: We study thermal expansion, mass changes, heat capacity, and thermal diffusivity and conductivity for a ceramic body with (10 and 20 mass%) and without waste calcite content, using the TDA, TG, DTA, DSC, and flash method. The measurements were performed (a) for green samples either isothermally or by a linear heating up to a temperature 600, 1,050, or 1,100 °C, depending on the measurement method; (b) at the room temperature for samples preheated at 100, 200,…, 1,100 °C. In case (a) we show that a high calcite content may double the energy consumption during the anorthite creation at 950 °C. On the other hand, calcite has a slight positive effect on the final contraction and quite substantial effect on the thermal conductivity in the range 150–550 °C, decreasing it even by 50 %. In case (b) a positive impact of calcite on the final contraction is about 10 times higher than in case (a). A clear effect of calcite on the thermal diffusivity occurs in case (b) only above 600 °C, resulting in a rather different behavior for the 10 and 20 mass% calcite content.

Mathematically modeling fixed-bed adsorption in aqueous systems

Abstract: Adsorption is one of the widely used processes in the chemical industry environmental application. As compared to mathematical models proposed to describe batch adsorption in terms of isotherm and kinetic behavior, insufficient models are available to describe and predict fixed-bed or column adsorption, though the latter one is the main option in practical application. The present review first provides a brief summary on basic concepts and mathematic models to describe the mass transfer and isotherm behavior of batch adsorption, which dominate the column adsorption behavior in nature. Afterwards, the widely used models developed to predict the breakthrough curve, i.e., the general rate models, linear driving force (LDF) model, wave propagation theory model, constant pattern model, Clark model, Thomas model, Bohart-Adams model, Yoon-Nelson model, Wang model, Wolborska model, and modified dose-response model, are briefly introduced from the mechanism and mathematical viewpoint. Their basic characteristics, including the advantages and inherit shortcomings, are also discussed. This review could help those interested in column adsorption to reasonably choose or develop an accurate and convenient model for their study and practical application.

Model for Surface Diffusion of Adsorbed Gas in Nanopores of Shale Gas Reservoirs

Abstract: Surface diffusion plays a key role in gas mass transfer due to the majority of adsorbed gas within abundant nanopores of organic matter in shale gas reservoirs Surface diffusion simulation is very complex as a result of high reservoir pressure, surface heterogeneity, and nonisothermal desorption in shale gas reservoirs In this paper, a new model of surface diffusion for adsorbed gas in shale gas reservoirs is established, which is based on a Hwang model derived under a low pressure condition and considers the effect of adsorbed gas coverage under high pressure Additionally, this new model considers the effects of surface heterogeneity, isosteric sorption heat, and nonisothermal gas desorption Results show that (1) the surface diffusion coefficient increases with pressure and temperature, while it decreases with activation energy and gas molecular weight; (2) contributions of viscous flow, Knudsen diffusion, and surface diffusion to the total gas mass transfer are varying during the development of shal

Implications of Apparent Pseudo-Second-Order Adsorption Kinetics onto Cellulosic Materials: A Review

Abstract: The pseudo-second-order (PSO) kinetic model has become among the most popular ways to fit rate data for adsorption of metal ions, dyes, and other compounds from aqueous solution onto cellulose-based materials. This review first considers published evidence regarding the validity of the mechanistic assumptions underlying application of the PSO model to adsorption kinetics. A literal interpretation of the model requires an assumption that different adsorption sites on a solid substrate randomly collide with each other during a rate-limiting mechanistic step. Because of problems revealed by the literature regarding the usual assumptions associated with the PSO model, this review also considers how else to account for good fits of adsorption data to the PSO model. Studies have shown that adsorption behavior that fits the PSO model well often can be explained by diffusion-based mechanisms. Hypothetical data generated using the assumption of pseudo-first-order rate behavior has been shown to fit the PSO model very well. In light of published evidence, adsorption kinetics of cellulosic materials is expected to mainly depend on diffusion-limited processes, as affected by heterogeneous distributions of pore sizes and continual partitioning of solute species between a dissolved state and a fixed state of adsorption.