Jaromir Havlica

Possibilities of Utilizing Solid Residues from Pressured Fluidized Bed Coal Combustion (PSBC) for the Production of Blended Cements

Abstract Solid residues produced in pressurized fluidized bed combustion differ in their chemical and phase composition from those produced under atmospheric pressure and even more from conventional fly ashes. They usually exhibit distinct hydraulic/pozzolanic properties and thus may be used, in combination with Portland clinker, as constituents of blended cements. Due to their high SO 3 content, the addition of gypsum to such binders has to be adjusted accordingly.

Possibilities of the use of isoperibolic calorimetry for assessing the hydration behavior of cementitious systems

Abstract The possibilities of a new multicell isoperibolic–semiadiabatic calorimeter for the measurement of the hydration heat evolution of binders in pastes, mortars and concretes of different composition during setting and early hardening are presented. Up to 32 samples can be tested simultaneously. The measurements enable to study the influence of the main constituents as well as minor organic and inorganic additives on hydration, to estimate the start of setting and to calculate the partial heat of hydration during a given time of curing. Due to the simple construction, no special service is demanded.

The synthesis and characterization of an expansive admixture for M-type cements I. The influence of free CaO to the formation of ettringite

The hydration process of expansive admixtures for M-type of expansive or shrinkage compensation cements was studied by thermal analysis supplemented by infrared spectroscopy and X-ray diffraction. The main attention was focused on ettringite formation and on the influence of lime content upon the process. The behaviour of expansive admixture during hydration is significantly affected by the content of lime. The increasing content of lime slows down the rate of ettringite formation and supports the formation of monosulphate (AFm phase) that can be transformed afterwards into ettringite during hardening process. This process may produce better expansion effect because of better transfer of expansion stress after the setting of the paste.

Influence of superplasticizers on the course of Portland cement hydration

A multicell isoperibolic — semiadiabatic calorimeter was used for the measurement of temperature and the determination of the hydration heat evolution at earlier period of cement pastes setting and hardening. The measurements were aimed at the determination of the effect of superplasticizers (SPs) on the course of the Portland cement hydration. Commercial polycarboxylate SP was added to the mixtures and the heat effect was measured. With the increasing content of SP, the hydration temperature increased up to a certain value and then decreased. In case of a sufficient amount of water in the mixture to achieve complete hydration of cement, samples with the highest values of the maximum hydration temperature reached the highest values of the released total heat. If there is not a sufficient amount of water to achieve complete hydration, the samples with the highest values of the maximum hydration temperature reach the lowest values of the released total heat.

Calorimetric determination of the effect of additives on cement hydration process

Possibilities of a multicell isoperibolic-semiadiabatic calorimeter application for the measurement of hydration heat and maximum temperature reached in mixtures of various compositions during their setting and early stages of hardening are presented. Measurements were aimed to determine the impact of selected components’ content on the course of ordinary Portland cement (OPC) hydration. The following components were selected for the determination of the hydration behaviour in mixtures: very finely ground granulated blast furnace slag (GBFS), silica fume (microsilica, SF), finely ground quartz sand (FGQ), and calcined bauxite (CB). A commercial polycarboxylate type superplasticizer was also added to the selected mixtures. All maximum temperatures measured for selected mineral components were lower than that reached for cement. The maximum temperature increased with the decreasing amount of components in the mixture for all components except for silica fume. For all components, except for CB, the values of total released heat were higher than those for pure Portland cement samples.

The influence of the partial pressure of oxygen on the phase stability im the system Ca - Cr - O

Abstract DTA and TG measurements in controlled atmosphere were used for evaluation of temperature dependence of standard Gibbs energy of reaction 4 3 CaCrO 4 + 2 3 Cr 2 O 3 — 4 3 CaCr 2 O 4 + O 2

Sonochemical synthesis of Gd3+ doped CoFe2O4 spinel ferrite nanoparticles and its physical properties

In this work, a facile and green method for gadolinium doped cobalt ferrite (CoFe2-xGdxO4; x=0.00, 0.05, 0.10, 0.15, 0.20) nanoparticles by using ultrasonic irradiation was reported. The impact of Gd3+ substitution on the structural, magnetic, dielectric and electrical properties of cobalt ferrite nanoparticles was evaluated. The sonochemically synthesized spinel ferrite nanoparticles were characterized by X-ray Diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometer (VSM). X-ray diffraction (XRD) study confirmed the formation of single phase spinel ferrite of CoFe2-xGdxO4 nanoparticles. XRD results also revealed that ultrasonic irradiation seems to be favourable to achieve highly crystalline single crystal phase gadolinium doped cobalt ferrite nanoparticles without any post annealing process. Fourier Transform Infrared and Raman Spectra confirmed the formation of spinel ferrite crystal structure. X-ray photoelectron spectroscopy revealed the impact of Gd3+ substitution in CoFe2O4 nanoparticles on cation distribution at the tetrahedral and octahedral site in spinel ferrite crystal system. The electrical properties showed that the Gd3+ doped cobalt ferrite (CoFe2-xGdxO4; x=0.20) exhibit enhanced dielectric constant (277 at 100Hz) and ac conductivity (20.2×10-9S/cm at 100Hz). The modulus spectroscopy demonstrated the impact of Gd3+ substitution in cobalt ferrite nanoparticles on grain boundary relaxation time, capacitance and resistance. Magnetic property measurement revealed that the coercivity decreases with Gd3+ substitution from 234.32Oe (x=0.00) to 12.60Oe (x=0.05) and further increases from 12.60Oe (x=0.05) to 68.62Oe (x=0.20). Moreover, saturation magnetization decreases with Gd3+ substitution from 40.19emu/g (x=0.00) to 21.58emu/g (x=0.20). This work demonstrates that the grain size and cation distribution in Gd3+ doped cobalt ferrite nanoparticles synthesized by sonochemical method, is effective in controlling the structural, magnetic, and electrical properties, and can be find very promising applications.

The synthesis and characterisation of an expansive additive for M-type cements

The expansion effect of laboratory-prepared expansive additives for M-type expansive cement was investigated at the early stage of hydration by the multicell isoperibolic calorimeter and volumetric technique based on Archimedes’ principle. The relative volume changes and heat released during hydration are strongly affected by the content of lime in the expansive additive due to the influence of CaO on the kinetics and mechanism of formation of ettringite. The increasing content of lime favours the formation of monosulphate and its later transformation to ettringite generating expansion stress. The effect of expansive additive on the behaviour of mortar samples was measured as linear elongation of test blocks using Graf-Kaufman dilatometer. Lower or higher content of lime in expansion additive slightly decreases the 7th-day compressive and flexural strength of samples while this effect is negligible for expansive additive with nominal composition of ettringite.

Mechanisms of Macro-Pores Origin in the MDF Composites Fabrication

Paper deals with the description of the MDF composite microstructure and the possible emergence of macro-pores. MDF composites show enhanced mechanical properties, especially flexural strength. Where, the flexural strength can be associated with the elimination of critical cracks in the material. One of the steps of production of MDF composites is high-shear mixing, that just helps to achieve defect-free structure. The primary aim of this work is to specifically describe the origin of the macro-pores in the structure of MDF composites. The secondary one is to find appropriate setup for high-shear mixing equipment in order to eliminate main macro-pores (shear speed, distance between the rollers etc.).

The synthesis and characterization of an expansive additive for M-type cements Part III. the influence of gypsum and water-to-cement ratio on the formation of ettringite

The expansive admixtures for M-type expansive cements were prepared by mixing of aluminate cement and gypsum. Hydrated samples were investigated by thermal analysis, infrared spectroscopy, and scanning electron microscopy. The main attention was focused on the formation of ettringite and the influence of water-to-cement and water-to-gypsum ratio on the process. The results of thermal analysis and infrared spectroscopy indicate high water-to-cement ratio and high water-to-gypsum ratio as the most proper condition for the formation of ettringite. It was observed that the value of water-to-gypsum ratio close to 1.76 supported the formation of monosulfate phase in the expansive mixture. The SEM indicates an oriented crystal growth of ettringite crystals that produces the expansive stresses.