Study on the Production Factors in the Process of Production and Properties of Fly Ash-Based Coarse Aggregates

An optimization study was carried out for the sustainable production of coarse aggregates from fly ash and alkaline solution, considering the combined effect of alkaline solution and production process. The trial mixes during the process of producing the artificial aggregates were designed through Taguchi’s experimental design method. The combined effect of alkaline solution (geopolymerisation) and production process (pelletization factors) along with engineering properties of the produced coarse aggregates was evaluated using response indices at different curing ages. Furthermore, the influence of each individual factor of geopolymerisation and pelletization on the engineering properties was determined through grey relational analysis to identify the most influencing factors in the production of coarse aggregates. The results obtained from grey relational analysis indicate that the properties of produced aggregates are governed mostly by geopolymerisation. It is also observed that water content of 20% by mass of fly ash is found to be essential for the suitable production of coarse aggregates and factors such as Na2O content and curing regime improved the engineering properties.

Perlite deposits of the Central Slovakia Volcanic Field (Western Carpathians): Geology and properties

Perlites in the Central Slovakia Volcanic Field are associated with with rhyolite dykes, cryptodomes, extrusive domes, coulées and volcanoclastic rocks of the Jastrabá Fm. (12.3–11.4 Ma). From numerous occurrences only the Lehôtka pod Brehmi (LPB) and Jastrabá (JST) represent deposits of economic interest. The LPB deposit exploits a pile of extruded hyaloclastite breccia composed of grey porous and dark dense fragments. The JST deposit exploits glassy rhyolite breccia composed of grey porous fragments associated with an extrusive dome/coulée. The perlites at both deposits are peraluminous, calc-alkaline of the high-K type, poor in phenocrysts (around 5 %) of plagioclase, biotite and minor amphibole (LPB) or sanidine/anorthoclase (JST). Glass at both deposits is silica rich (75.4–79.5 wt. % dry) with Al2O3, K2O and Na2O as other major constituents. It is inhomogeneous showing domains enriched in Na2O or K2O. Glass water content (3.0–6.0 wt. %) shows a weak positive correlation with its silica content and a negative correlation with its Na2O content. Perlites show porosities of 5–16 % (dark dense), 16–30 % (grey porous) and 30–44 % (pale grey ­pumiceous). Narrow stretched pores represent remnants after outgassing of ascending magma while open undeformed pores grew at a low pressure before quenching. The transformation of volcanic glass into perlite took place owing to the hydration by heated fluids of meteoric origin. The hydration was supported by a significant porosity with inter­connected pores and by sustained elevated temperature. Perlites at both deposits show a low content of tightly-bound water and a low Na/K ratio. These properties are responsible for their relatively low degree of expansion. On the other hand, due to the same reason, the perlites have a good mechanical stability.

Experimental and Theoretical Study of Radiation Shielding Features of CaO-K2O-Na2O-P2O5 Glass Systems

The gamma radiation shielding ability for CaO-K2O-Na2O-P2O5 glasses were experimentally determined between 0.0595 and 1.41 MeV. The experimental MAC results were compared with theoretical results obtained from the XCOM software to test the accuracy of the experimental values. Additionally, the effect of increasing the P2O5 in the glass composition, or reducing the Na2O content, was evaluated at varying energies. For the fabricated glasses, the experimental data strongly agreed with the XCOM results. The effective atomic number (Zeff) of the fabricated glasses was also determined. The Zeff values start out at their maximum (12.41–12.55) at the lowest tested energy, 0.0595 MeV, and decrease to 10.69–10.80 at 0.245 MeV. As energy further increases, the Zeff values remain almost constant between 0.344 and 1.41 MeV. The mean free path (MFP) of the fabricated glasses is investigated and we found that the lowest MFP value occurs at the lowest tested energy, 0.0595 MeV, and lies within the range of 1.382–1.486 cm, while the greatest MFP can be found at the highest tested energy, 1.41 MeV, within the range of 8.121–8.656 cm. At all energies, the KCNP40 sample has the lowest MFP, while the KCNP60 sample has the greatest. The half value layer (HVL) for the KCNP-X glasses is determined. For all the selected energies, the HVL values follow the order of KCNP40 < KCNP45 < KCNP50 < KCNP55 < KCNP60. The HVL of the KCNP50 sample increased from 0.996 to 2.663, 3.392, 4.351, and 5.169 cm for energies of 0.0595, 0.245, 0.444, 0.779, and 1.11 MeV, respectively. The radiation protection efficiency (RPE) results reveal that decreasing the P2O5 content in the glasses improves the radiation shielding ability of the samples. Thus, the KCNP40 sample has the best potential for photon attenuation applications.

Influence of Glass Composition on the Luminescence Mechanisms of CdSe Quantum Dot-Doped Glasses

<div> <div> <div> <p>In this work, we characterized the electronic structure of CdSe quantum dots embedded in a series of x Na2O, (1–x) SiO2 glass matrices (x = 0, 0.25, 0.33 and 0.5). We analyzed the impact of the glass matrix composition on both the atomic structure of the quantum dot (QD) and the QD/glass interface, as well as the luminescence mechanisms, using density functional theory (DFT) calculations. The increase of Na2O content in the glass matrices was found to promote the formation of Cd–O and Se–Na interfacial bonds, and disrupting the Cd–Se bonds network. In particular, we show that the glass composition directly affects the nature of the highest occupied molecular orbitals (HOMO). According to the atomic structure, the band gap distribution and the density of states calculation, we find that there is significant reconstruction of the QD, and that the picture sometimes proposed of a “pristine QD” surrounded by glass is not realistic. The introduction of CdSe QD significantly decreased the bandgap of the glass compared to pristine glasses, and the interfacial bonds greatly contributed to the frontier orbitals without forming midgap states. We propose a new energy diagram, quite different from the traditional model, to explain the luminescence of CdSe quantum dot- doped glasses, originating from the intrinsic emission of this hybrid system {QD + glass}. These results improve our understanding of the luminescence of CdSe quantum dot-doped glasses, explaining the reason for the poor quantum efficiency and broad emission linewidth compared with their colloidal counterparts. </p> </div> </div> </div>

Influence of Glass Composition on the Luminescence Mechanisms of CdSe Quantum Dot-Doped Glasses

<div> <div> <div> <p>In this work, we characterized the electronic structure of CdSe quantum dots embedded in a series of x Na2O, (1–x) SiO2 glass matrices (x = 0, 0.25, 0.33 and 0.5). We analyzed the impact of the glass matrix composition on both the atomic structure of the quantum dot (QD) and the QD/glass interface, as well as the luminescence mechanisms, using density functional theory (DFT) calculations. The increase of Na2O content in the glass matrices was found to promote the formation of Cd–O and Se–Na interfacial bonds, and disrupting the Cd–Se bonds network. In particular, we show that the glass composition directly affects the nature of the highest occupied molecular orbitals (HOMO). According to the atomic structure, the band gap distribution and the density of states calculation, we find that there is significant reconstruction of the QD, and that the picture sometimes proposed of a “pristine QD” surrounded by glass is not realistic. The introduction of CdSe QD significantly decreased the bandgap of the glass compared to pristine glasses, and the interfacial bonds greatly contributed to the frontier orbitals without forming midgap states. We propose a new energy diagram, quite different from the traditional model, to explain the luminescence of CdSe quantum dot- doped glasses, originating from the intrinsic emission of this hybrid system {QD + glass}. These results improve our understanding of the luminescence of CdSe quantum dot-doped glasses, explaining the reason for the poor quantum efficiency and broad emission linewidth compared with their colloidal counterparts. </p> </div> </div> </div>

Protective and decorative properties of titanium glass enamels

It is known that enamel coatings with enhanced protective and decorative properties can be fabricated on the basis of boron-silicate glass frits with an increased content of TiO2 in their composition. Opacity and white color of enamel coatings are due to their crystallization at firing temperatures. Titanium glass enamels are not pure white visually, and show yellow and blue shades depending on their chemical composition. The purpose of this work was to establish the influence of basic components on the water resistance and color characteristics of titanium enamel coatings. Our experimental study was focused on the glass frits with the chemical composition described by the generalized formula (76–n–m)SiO2nB2O3mNa2O24MexOy, where MexOy is the total content of TiO2, ZrO2, Al2O3, MgO, СaO, P2O5, and K2O. It was found that the glass frits with the following content of basic components (mol.%): Na2O 12–13, K2O 3, B2O3 10–11, SiO2 49–51 demonstrated the highest water-resistant properties. The color characteristics of titanium enamel coatings prepared on the basis of the mentioned glass frits differ significantly from the reference white light A; the titanium enamel coatings under consideration have a yellow-green tint. The highest deviations of the enamel coating color purity and color tone from a standard of white color were observed when increasing the Na2O content in the glass frits composition at the expense of SiO2. The color of titanium coatings with the highest water resistance has a deviation from the standard of white color within 4–6%.

Mitigating the Drying Shrinkage and Autogenous Shrinkage of Alkali-Activated Slag by NaAlO2

The shrinkage of alkali-activated slag (AAS) is obviously higher than ordinary Portland cement, which limited its application in engineering. In this study, the effects of NaAlO2 in mitigating drying shrinkage and autogenous shrinkage of AAS were studied. To further understand the shrinkage mechanism, the hydration products and microstructures were studied by X-ray diffraction, scanning electron microscopy and nitrogen adsorption approaches. As the partial substitution rate of NaAlO2 for Na2SiO3 increased, the drying shrinkage and autogenous shrinkage reduced significantly. The addition of NaAlO2 could slow down the rate of hydration reaction and reduce the porosity, change the pore diameter and the composition of generated paste and cause more hydrotalcite and tetranatrolite generated—which contributed to reduced shrinkage. Additionally, raising the Na2O content rate caused obvious differences in drying shrinkage and autogenous shrinkage. As the Na2O content elevated, the drying shrinkage decreased and autogenous shrinkage increased. A high Na2O content would cause complete hydration reactions and provoke high autogenous shrinkage. However, incomplete hydration reactions left more water in the paste, and the evaporated water dramatically influenced drying shrinkage. The results indicate that addition of NaAlO2 could greatly mitigate the drying shrinkage and autogenous shrinkage of AAS.

Setting and Hardening Behaviour of Alkali-Activated Landfilled Fly Ash–Slag Binder at Room Temperature

A geopolymer is normally considered an environmentally friendly binder due to the utilisation of industrial wastes. This study focusses on the potential of geopolymer preparation at room temperature from landfilled fly ash (LFA) which has been discharged to the land for more than three years. To accelerate the reaction process, 20–30 wt.% LFA was replaced by ground-granulated blast-furnace slag (GGBS). The effect of water glass modulus, Na2O content, water-to-solid ratio, and GGBS content on the setting time and strength development of the binder was discussed. Results showed that to activate LFA, the optimal value of the sodium silicate modulus for alkaline solution was 1.4–1.6 with a Na2O content of 10%, and the water-to-solid ratio was 0.4. In addition, the setting time of the binder reduced with increasing content of GGBS replacement, and the compressive strength increased due to the coexistence of C–(A)–S–H and zeolite-like phases. The maximum compressive strength of the binder was 29.2 MPa after 56 days of curing. The relatively low strength was likely due to the absence of the Q4 unit with a three-dimensional structure.