František Šoukal

Blended cements consisting of Portland cement–slag–silica fume–metakaolin system

AbstractIn the present study, hydration of four-compound systems consisting of Portland cement and three supplementary cementitious materialsn: silica fume, blast-furnace slag, and metakaolin, has been investigated by means of isothermal calorimetry and thermal analysis. Substitution of Portland cement was achieved at 25, 30, and 35xa0mass% by cementitious supplementary materials. Owing to the high specific surface, the silica fume eventuated in more distinctive demonstration of pozzolanic reactions. It decreased C/S ratio and led to the formation of hydration phases with C/Sxa0≅xa01, which is demonstrated by wollastonite crystallization at higher temperatures. Enhanced formation of gehlenite hydrate was proved by DTG when blended cements included higher content of metakaolin and blast-furnace slag. Pozzolanic materials increased the compressive strength of particular blended samples and thus overcame the dilution effect especially at long term of curing. Presented quaternary blended samples in comparison with ordinary Portland cement thus allowed the formation of more thermal stable hydration products and can be considered as promising materials for the development of special concrete also for hydrothermal applications.

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.

Performance of G-Oil Well cement exposed to elevated hydrothermal curing conditions

G-Oil Well cement was modified by blending it with blast furnace slag and silica fume at various ratios. The hydration was carried out under the hydrothermal conditions (200xa0°C and 1.2xa0MPa) up to 7xa0days. TG and DTG were performed on cured pastes to identify the hydrated products, their quantity and their stability under given hydrothermal curing conditions. The microstructure of samples was observed by a scanning electron microscope. The mechanical compressive strength was determined and the pore structure was analyzed using mercury intrusion porosimeter. It was found out that the compressive strength values of blend G-Oil Well cements markedly increased with increasing blast furnace/silica ratio. The pore structure was consolidated, as demonstrated by the displacement of pore size distribution to the region of micro and nano pores.

Influence of temperature on early hydration of Portland cement–metakaolin–slag system

The hydration of ternary blended cements comprising Portland cement, metakaolin and blast-furnace slag was studied at different temperatures of 30, 40, 50 and 60xa0°C by isothermal calorimetry. Phase composition of hydration products was determined by thermal analysis and X-ray diffraction for sample hydrated at 60xa0°C. Hydration involves different sequential and successive reactions in which mechanism, kinetics and products are strongly influenced by temperature. Activation energy for each exothermic reaction was determined. The linear trend of heat flow maxima for 2nd, 3rd and 4th exotherm versus temperature was revealed. The first peak is related to formation of mostly amorphous aluminate hydrates with no evidence of crystalline ettringite. Formation of first crystalline ettringite detected by X-ray diffraction was correlated with the distinct 3rd peak that occurs after 2nd “silicate” peak that corresponds to formation of calcium silicate hydrates and calcium hydroxide. The 4th peak recognizable at temperatures above 40xa0°C is related to the conversion of ettringite to hemicarbonate.

Effect of Nanoparticle Modification on Static and Dynamic Behaviour of Foam Based Blast Energy Absorbers

The effect of multi-wall carbon nanotubes and nanosilica (nano SiO2) content on physico-mechanical properties of glass microspheres-epoxy resin composite, designed for blast energy absorbing applications, was evaluated experimentally. Specific porous lightweight foam with high volume fraction of microspheres (70 vol.%) was prepared and modified by 1 to 5 vol.% of multi-wall carbon nanotubes and nanosilica. Compressive and flexural strength tests were conducted at quasi-static load. Split Hopkinson Pressure Bar apparatus was used to obtain high velocity characteristics of the materials. The relative absorbed energy was calculated to assess the relation between the composition of the material and its shock wave attenuation capacity. The mixtures containing nanosilica exhibited an increasing trend in the energy absorption capacity with increasing nanoparticle content up to 4 vol.%. The addition of carbon nanotubes also increased absorbed energy (again up to 4 vol.%, a significant drop was observed at higher concentrations). Comparing the values of the relative absorbed energy, the carbon nanotubes composites prevail over the nanosilica modified ones.

High temperature properties of MDF composite based on calcium aluminate cement and polyvinyl alcohol

The macro-defect-free composites belong to the well-known group of promising materials consisting of inorganic binder and organic polymer. MDF composites exhibit unusual mechanical properties, especially the flexural strength that can reach over 200xa0MPa. Moreover, the MDF composites based on calcium aluminate cement have a good temperature resistance because of the Al2O3 content in the cement. This paper deals with the preparation and high temperature characterization of MDF composite based on calcium aluminate cement combined with polyvinyl alcohol that could enable its utilization as a refractory material in industrial kilns. The composition of the MDF mixture has been optimized for an easy high-shear processing and flexural strength of the resulting 7-days cured material has been studied under laboratory condition and after heating at 240, 300, 600, 1,000, and 1,500xa0°C. The structure changes during the heating have been observed by SEM and the course of processes during the heating has been investigated by TG–DTA–EGA, TMA and heating microscopy.

The Characterization of Fixation of Ba, Pb, and Cu in Alkali-Activated Fly Ash/Blast Furnace Slag Matrix

The fixation of heavy metals (Ba, Cu, Pb) in an alkali-activated matrix was investigated. The matrix consisted of fly ash and blast furnace slag (BFS). The mixture of NaOH and Na-silicate was used as alkaline activator. Three analytical techniques were used to describe the fixation of heavy metals—X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), and X-ray powder diffraction (XRD). All heavy metals formed insoluble salts after alkaline activation. Ba was fixed as BaSO4, and only this product was crystalline. EDS mapping showed that Ba was cumulated in some regions and formed clusters. Pb was present in the form of Pb(OH)2 and was dispersed throughout the matrix on the edges of BFS grains. Cu was fixed as Cu(OH)2 and also was cumulated in some regions and formed clusters. Cu was present in two different chemical states; apart from Cu(OH)2, a Cu–O bond was also identified.

Solid-state synthesis of SrY2O4 and SrSm2O4

The course of solid-state synthesis of SrY2O4 and SrSm2O4 is described in this work. The processes during the thermal treatment, the mechanism and kinetics, the reactivity with water and the thermal stability of products were investigated by thermal analysis, isothermal calorimetry, infrared spectroscopy and scanning electron microscopy. Since solid-state synthesis does not show any significant effect on DTA, the course of synthesis was observed by the expansion of specimen. The kinetics of formation of SrY2O4 and SrSm2O4 is driven by the rate of reaction of 2nd order (g(α)xa0=xa0(1xa0−xa0α)−1xa0−xa01) and requires the Ea of 450 and 918xa0kJxa0mol−1, respectively. The heat released by the reaction of SrSm2O4 with water is significantly higher than that of SrY2O4. The introduction of Al2O3 to SrY2O4 led to the formation of monoclinic phase of Sr6Y2Al4O15. The hydraulic activity of these phases can be applied for the preparation of special inorganic cements.

Investigation of Subterranean Termites Nest Material Composition, Structure and Properties

Termites (Isoptera1) are colourless or white insect sometimes called “white ants2” but they are intimate relatives of cockroaches. The organization of termite community definitely belongs to the most complicated in the nature hence termites are described as the eusocial3 insect. Termites are distributed all over the world from the 47° Northern latitude to the 47° Southern latitude, but they are extremely abundant in the tropical rainforest4. Until today there are recognized about 3 000 termite species in the 9 families but two of them became extinct (Fig. 1). Only species of the genera Archotermopsis, Hodotermopsis, Zootermopsis and Reticulitermes can be found also in the temperature regions. Only two species5 are native of the temperature region of Europe. Besides the known kinds of termites an existence of other more than 1000 species is supposed [1-6]. The foundations of thermite taxonomy were established by Holmgren [7-9]. The following studies provided by Snyder [10], Grasse [11] and Emerson [12] define seven families of termites which are today generally accepted. Many of the conventional phylogenetic reconstructions

The influence of pH buffers on hydration of hydraulic phases in system CaO–Al2O3

The present paper deals with the study of pH buffer influence (pHxa0+xa0ions) on hydration course of tricalcium aluminate (C3A), monocalcium aluminate (CA), monocalcium di-aluminate (CA2) and dodecacalcium hepta-aluminate (C12A7). These calcium aluminate phases were synthesized at 1450xa0°C (CA and C3A), 1600xa0°C (CA2) and 1360xa0°C (C12A7), respectively. The purity of synthesized products was controlled by X-ray diffraction (XRD). Then, different calcium aluminate phases were hydrated at 25xa0°C in excess of water at pH 6 and with buffers to keep the pH values at 9, 11 and 13. The evolution of hydration heat was monitored by isothermal calorimetry for 48xa0h. After this period, hydration was stopped and the products were cross-characterized with different techniques: XRD, high-temperature X-ray diffraction, combined differential thermal analysis–thermogravimetry–effluent gas analysis. It was found out that the hydration course and the nature of hydration products of different calcium aluminate phases are strongly dependent on the hydration environment (pHxa0+xa0ions).