Ústav technologie stavebních hmot a dílců

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Mechanical Behavior of Oxide Dispersion Strengthened Steel Directly Consolidated by Rotary Swaging
(MDPI, 2024-10-23) Kocich, Radim; Kunčická, Lenka; Dvořák, Karel; Král, Petr
Among the main benefits of powder-based materials is the possibility of combining different constituents to achieve enhanced properties of the fabricated bulk material. The presented study characterizes the micro- and sub-structures and related mechanical properties of ferritic steel strengthened with a fine dispersion of nano-sized Y2O3 oxide particles. Unlike the typical method of preparation via rolling, the material presented herein was fabricated by direct consolidation from a mixture of powders using the versatile method of hot rotary swaging. The mechanical properties were evaluated at room temperature and also at 1300 degrees C to document the suitability of the prepared steel for high-temperature applications. The results showed that the imposed shear strain, i.e., swaging ratio, is a crucial parameter influencing the microstructure and, thus, material behavior. The workpiece subjected to the swaging ratio of 1.4 already exhibited a sufficiently consolidated structure with ultra-fine grains and featured high room-temperature microhardness values (up to 690 HV0.5), as well as a relatively high maximum flow stress (similar to 88 MPa) when deformed at the temperature of 1300 degrees C with the strain rate of 0.5 s(-1). However, the dispersion of oxides within this sample exhibited local inhomogeneities. Increasing the swaging ratio to 2.5 substantially contributed to the homogenization of the distribution of the Y2O3 oxide particles, which resulted in increased homogeneity of mechanical properties (lower deviations from the average values), but their lower absolute values due to the occurrence of nucleating nano-sized recrystallized grains.
Testing of Action of Direct Flame on Concrete
(Hindawi, 2015-01-22) Bodnárová, Lenka; Válek, Jaroslav; Novosad, Petr
The paper states results of experimental exposition of concrete test specimens to direct flame. Concrete test specimens made from various mixtures differing in the type of aggregate, binder, dispersed reinforcement, and technological procedure were subjected to thermal load. Physicomechanical and other properties of all test specimens were tested before exposition to open flame: density, compressive strength, flexural strength, moisture content, and surface appearance. The specimens were visually observed during exposition to open flame and changes were recorded. Exposed surface was photographically documented before thermal load and at 10-minute intervals. Development of temperature of the specimens was documented with a thermocamera. After exposition to thermal load and cooling down, concrete specimens were visually observed, network of cracks was photographically documented, and maximal depth of spalled area was measured.
Stabilization of the ammonia in SNCR fly ash - the influence of tannins presence on the preparation of an autoclaved aerated concrete
(IOP Publishing, 2019-08-14) Dlabajová, Lucie; Lédl, Matěj; Beranová, Denisa; Opravil, Tomáš
Thanks to the legislative regulations on NOx emissions, a selective non-catalytic reduction (SNCR) technology had been introduced to a coal combustion process in power plants. The valuable by-product, fly ash, contains ammonia in the form of soluble salts, e.g. NH4HSO4 and (NH4)2SO4. After mixing SNCR fly ash with cement, thanks to the rise of pH, the toxic ammonia releases and contaminates the working area, so the presence of these salts is undesirable in an autoclaved aerated concrete (AAC) manufacturing process. A possible solution is the conversion of soluble ammonium salts to an insoluble form. Tannins are a class of polyphenolic biomolecules, which react with the ammonium ions to form insoluble compounds. The AAC samples were mixed using SNCR fly ash and two different ammonium binding additives - pure tannic acid and the cost-effective animal food supplement containing chestnut tannins. The influence of additives on the hydration process of the starting mixture was studied by isoperibolic calorimetry. The results suggest that the cost-effective source of tannins retards the hydration. The presence of insoluble compounds was studied by the infrared spectroscopy.
The Physical Properties Of Cementitious Composites For Applications In A High-Temperature Energy Storage Device
(Czech Technical University in Prague, 2019-05-12) Šťastník, Stanislav; Bodnárová, Lenka
The article was focused on experimental verification of the identification of thermal characteristics of high-performance cementitious composites at elevated temperatures. The purpose of the research was to verify the properties, and consequently the suitability of these special cement composites for the production of a heat storage device for the long-term accumulation of thermal energy. The theoretical prerequisites for the identification of thermal characteristics and the practical procedure of experimental verification of the characteristics were presented. A modified procedure for the determination of temperature conductivity by the hot wire method was suggested. Subsequently, the experimental verification and the results of the measured thermal characteristics were listed – and the temperature conductivity , the thermal conductivity [w/m.K], the volumetric heat capacity and the specific heat capacity c [J/kg.K] by the modified method were determined.
Effect of Inorganic SiO2 Nanofibers in High Strength Cementitious Composites
(EDP Series, 2019-04-08) Bodnárová, Lenka; Hela, Rudolf; Sedláček, Daniel
The paper deals with the verification of the effect of the addition of inorganic SiO2 nanofibers to cement composites. In the first stage, a stable suspension of SiO2 nanofibers was prepared in an aqueous medium. It is important to distribute nanofibers so that the nanofibers do not appear in the form of clumps and at the same time do not get damaged during the dispersion process. The ultrasonification process was used for dispersion. The dispersed suspension of SiO2 nanofibers and water was dosed together with the superplasticizing admixtures into the dry components of the cement composite and the components were homogenized. The properties of the cement composite with SiO2 nanofibers have been tested – compressive strength, flexural strength, density. Composites with the addition of SiO2 nanofibers at a dose of 0.008 % by weight of cement exhibited an increased compressive strength of up to 33 % and a 19 % greater flexural strength at doses of 0.016 and 0.032 % of cement weight than the reference sample without nanofibers. The presence of SiO2 nanofibers in the composite was monitored by scanning electron microscopy (SEM).

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