Search Results (33)

This study explores the synergistic potential of ladle slag (LS) and sunflower shell fly ash (SSFA) in alkali-activated binder systems, focusing on their chemical and mineralogical characteristics and the influence of SSFA addition on the mechanical performance of LS-based pastes. X-ray fluorescence and XRD analysis revealed that LS is rich in CaO and latent hydraulic phases such as γ-belite and mayenite, while SSFA is dominated by K₂O, SO₃, and KCl/K₂SO₄ phases, reflecting its biomass origin. Infrared spectroscopy and thermal analysis confirmed the presence of carbonate, hydroxide, and hydrate phases, with SSFA exhibiting more complex thermal behavior due to volatile-rich composition. When used alone, LS produced weak binders; however, a 10 wt% SSFA addition tripled compressive strength to nearly 30 MPa, indicating a significant activation effect. Further increases in SSFA content led to strength reduction, likely due to increased porosity and excess salts. Microstructural analysis showed that SSFA promotes the formation of AFm phases such as Friedel’s salt and hydrocalumite, altering hydration pathways and enhancing early strength through chemical activation and carbonation processes. The findings highlight the potential of combining LS and SSFA as a sustainable binder system, offering a waste-derived alternative for low-carbon construction materials. Full article

Treating bauxite residue as an alternative source of metals for iron and aluminum industry is an approach to promote circular economy in metal industries. Reduction of metal oxides with a H₂-based process is an important step for the decarbonization of metal industry. In this study, bauxite residue (BR) pellets were prepared and were reduced with different H₂-H₂O gas compositions at different temperatures, which yielded with various degrees of reduction. The bauxite residue pellets were made from a mixture of bauxite residue and Ca(OH)₂ powders and sintered at 1150 °C. Hydrogen reduction was carried out on the oxide pellets using a resistance furnace at elevated temperatures in controlled reduction atmosphere of H₂-H₂O gas mixtures, which resulted in the reduction of iron oxides in the pellets. Unreduced and reduced pellets were subsequently heated to 1400 °C to study their sintering behavior during H₂ reduction using differential thermal analysis (DTA) and thermogravimetric analysis (TGA) techniques to investigate the evolution of phases related to slag formation. Equilibrium module of Factsage™ was utilized to analyze results of thermal analysis. Both chemical and physical changes that occurred during the sintering process of the H₂-reduced BR pellets were successfully detected by TG–DTA analysis, and the initial slag- and gas-phase formation were detected to occur from 900 °C and 1180 °C, respectively. One of the most notable chemical reactions to occur during the analysis was formation of mayenite at 810 °C, which is easily leachable, providing potential for recovery of alumina. Full article

This study achieved the preparation of budget-friendly stratified Ca-Al adsorbents using a simplified precipitation synthesis route with subsequent pyroprocessing, showing superior defluoridation capabilities in aqueous environments. The structural properties and defluoridation performance of the adsorbents were systematically investigated by optimizing critical synthesis parameters, including calcium-to-aluminum molar ratios, the solution pH during co-precipitation, and calcination temperature. Characterization results revealed that the optimal sample (prepared at a Ca/Al ratio of 2:3, initial pH of 10, and calcination temperature of 600 °C) exhibited a high specific surface area, ordered mesoporous structure, and abundant surface hydroxyl groups, facilitating efficient fluoride adsorption. Batch adsorption experiments demonstrated significant effects of adsorbent mass, solution pH, and initial fluoride concentration on removal efficiency. The isothermal adsorption characteristics conformed to the Langmuir model, complemented by pseudo-second-order kinetic compliance, which jointly confirmed chemisorption-dominated monolayer coverage. Notably, the maximum adsorption capacity reached 263.33 mg g⁻¹, surpassing most comparable adsorbents reported in the literature. The material maintained a superior fluoride removal performance across a wide pH range (4~12) and exhibited superior recyclability. Rapid adsorption kinetics were observed, with equilibrium achieved within 60 min. The material showed a good removal effect in actual fluoride-containing smelting wastewater, which further proved its application potential. In addition, the analysis of the adsorption mechanism showed that the removal of fluoride was mainly achieved through the coordination between fluoride and metal ions and the ion-exchange reaction with surface hydroxyl groups. These findings suggest that the adsorbent has significant prospects for practical water quality fluoride removal applications. Full article

Iron and alumina can be separated from bauxite residue and calcite self-hardened reduced pellets through simultaneous magnetic separation and alkali leaching. Bauxite residue and calcite self-hardened pellets were reduced non-isothermally by hydrogen gas to obtain metallic iron. Thereafter, the fine grounded reduced pellet powder was leached with a simultaneous magnetic stirrer, while two different leaching processes were applied. In a magnetic stirring alkali-leaching process, the simultaneous leaching and magnetic separation by Na₂CO₃ solution was carried out. In another process, the reduced pellets were leached into water with gradual magnetic separation followed by the addition of Na₂CO₃ solution. X-ray diffraction, scanning electron microscopy, X-ray fluorescence, and inductively coupled plasma mass spectroscopy were used to conduct phase analysis, microstructural analysis, compositional analysis, and elemental analysis of the leaching solutions, respectively. It was found that, there was an increase in iron in the magnetic fraction as compared to a nonmagnetic fraction in both the leaching processes; however, the iron recovery is more noticeable in the magnetic alkali leaching process. The recovery of alumina depends upon the amount of mayenite formation during reduction. The greater the amount of mayenite and the lower the amount of gehlenite, the greater the alumina recovery will be. The simultaneous alkali leaching and magnetic separation lead to greater unlocking of the reduced matrix and to greater iron and alumina recovery compared to magnetic water leaching followed by alkaline leaching. Full article

This paper describes the impact of hydrothermal conditions on the strength properties and hydration processes of belite cement mortar samples. The belite-rich binder was synthesized by sintering the initial mixture of raw materials (granite cutting waste, the silica-gel waste from AlF₃ production, and natural materials) in a high-temperature furnace at a temperature of 1150 °C for 2 h. The prepared clinker consists of larnite, mayenite, srebrodolskite, ye’elimite, and gehlenite. To control hydration kinetics and optimize the hardening of belite cement mortar, the produced clinker was blended with 7.5% of gypsum. The mechanical properties were assessed by curing the standard prisms (following the EN 196-1 standard, cement/sand = 1:3, W/C= 0.67) under water-saturated conditions in a stainless steel autoclave. The curing process was performed in a temperature range of 90 °C to 200 °C at various hydrothermal curing durations (6–48 h). The results indicated that the curing conditions highly influence the compressive strength evolution of belite cement mortar and the formed mineralogy of hydrates. The highest compressive strength value (exceeded 20 MPa) was obtained at 200 °C, i.e., when the main belite cement mineral was entirely hydrated and recrystallized into 1.13 nm tobermorite. The microstructural evolution and the phase assemblage during the hydrothermal curing were determined by X-ray diffraction analysis and differential scanning calorimetry. Full article

In this study, the isothermal reduction of bauxite residue-calcite sintered pellets by hydrogen at elevated temperatures and different gas flow rates was investigated. A thermogravimetric technique was applied to study the kinetics of the direct reduction by H₂ at 500–1000 °C. It was observed that iron in sintered oxide pellets mainly exists in the form of brownmillerite, srebrodolskite and fayalite. The reduction of brownmillerite, the dominant Fe-containing phase, with hydrogen produces mayenite, calcite and metallic iron. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray fluorescence (XRF), BET surface area, pycnometer and mercury intrusion porosimeter analyses were adopted on reduced pellets to interpret the experimental results. The order of the reduction process changes from first-order reaction kinetics to second-order with an increasing reduction temperature. The change in reaction order may be due to sintering at higher reduction temperatures and corresponding physical and microstructural changes in pellets. The activation energy of reduction was calculated as 55.1–96.6 kJ/mol based on the experimental conditions and using different kinetic model equations. From the experimental observations, it was found that 1000 °C with 60 min is the most suitable condition for bauxite residue-CaO sintered pellets’ reduction with hydrogen. Full article

Calcium aluminate of a mayenite structure, 12CaO∙7Al₂O₃ (C12A7), is widely applicable in many fields of modern science and technology. Therefore, its behavior under various experimental conditions is of special interest. The present research aimed to estimate the possible impact of the carbon shell in core-shell materials of C12A7@C type on the proceeding of solid-state reactions of mayenite with graphite and magnesium oxide under High Pressure, High Temperature (HPHT) conditions. The phase composition of the solid-state products formed at a pressure of 4 GPa and temperature of 1450 °C was studied. As is found, the interaction of mayenite with graphite under such conditions is accompanied by the formation of an aluminum-rich phase of the CaO∙6Al₂O₃ composition, while in the case of core-shell structure (C12A7@C), the same interaction does not lead to the formation of such a single phase. For this system, a number of hardly identified calcium aluminate phases along with the carbide-like phrases have appeared. The main product of the interaction of mayenite and C12A7@C with MgO under HPHT conditions is the spinel phase Al₂MgO₄. This indicates that, in the case of the C12A7@C structure, the carbon shell is not able to prevent the interaction of the oxide mayenite core with magnesium oxide located outside the carbon shell. Nevertheless, the other solid-state products accompanying the spinel formation are significantly different for the cases of pure C12A7 and C12A7@C core-shell structure. The obtained results clearly illustrate that the HPHT conditions used in these experiments lead to the complete destruction of the mayenite structure and the formation of new phases, which compositions differ noticeably depending on the precursor used—pure mayenite or C12A7@C core-shell structure. Full article

Experimental research was carried out to produce pellets from bauxite residue for the further extraction of iron and alumina. Bauxite residue and limestone with three different mixture compositions were pelletized experimentally via agglomeration followed by drying and sintering at elevated temperatures. X-Ray diffraction (XRD) and scanning electron microscopy (SEM) were used for the phase and microstructural analysis, respectively. Tumble, abrasion, and breaking load tests were applied to determine the strength of the pellets. For measurement of porosity and surface area, mercury porosimetry and BET surface area methods were applied. It was found that at 1100 °C sintering temperature, all the three sintered pellet compositions have a moderate porosity and low strength, but the reverse result was found when 1200 °C sintering temperature was applied. Moreover, for the pellets sintered at 1150 °C high strength and proper porosities were obtained. In the sintered pellets, iron present in form of brownmillerite (Ca₂Fe_1.63Al_0.36O₅), srebrodolskite (Ca₂Fe₂O₅), and fayalite (Fe₂SiO₄), while alumina present mostly in gehlenite (Ca₂Al₂SiO₇) and little fraction in mayenite (Ca₁₂Al₁₄O₃₃) and brownmillerite phases. The identified phases are the same for of the three pellets, however, with variations in their quantities. Porosity and mechanical properties of pellets are inversely related with both varying sintering temperature and composition. It was found that with more CaCO₃ use in pelletizing, higher porosity is obtained. However, with increasing sintering temperature the strength of the pellets increases due to clustering of particles, while porosity decreases. Full article

The evolution of the structure and the phase composition of a dispersed mayenite at its interaction with metallic aluminum was studied in a temperature range from 900 to 1400 °C in both argon and air atmospheres. The aluminum loading was varied from 0 to 50 wt%. It was found that the addition of aluminum significantly affects the stability of the mayenite and other calcium aluminate phases within the studied temperature range. The formation of the electride state registered by the appearance of a characteristic electron paramagnetic resonance (EPR) signal from F⁺-like centers (g~1.994) in an argon atmosphere was shown to take place already at 1150 °C due to an aluminothermic reduction of this material. The super-narrow (H_p-p < 0.5 G) EPR spectra from F⁺-like centers, which were recently observed for the core–shell structures of the C12A7@C type only, were registered for mayenite for the first time. The results obtained in the present study testify firstly towards the possibility of significantly diminishing the temperatures required for the formation of the electride state in such systems and secondly towards the ability to stabilize the size of small electride nanoparticles within the synthesized calcium aluminate matrix. Full article

Pd-containing catalysts based on highly dispersed aerogel-derived mayenite were prepared via two approaches. The Pd@C12A7 sample was obtained through the addition of Pd nitrate solution to a fresh Ca(OH)₂-Al(OH)₃ gel. Pd/C12A7 was synthesized through conventional wet impregnation of the aerogel-derived mayenite. The evolution of the textural characteristics of the support (C12A7) depending on the calcination temperature was investigated. Pd-containing samples were explored using transmission electron microscopy and spin probe EPR spectroscopy. Using the latter method, the presence of active oxygen species capable of producing nitroxyl radicals from diphenylamine was observed. The activity of these species and the reproducibility of their redox behavior were studied in three cycles of temperature-programmed reduction in both hydrogen and CO atmospheres. A prompt thermal aging technique was used to access and compare the activity of the samples towards CO oxidation. The state of Pd species before and after the aging procedure was studied via UV–Vis spectroscopy. It was found that the dispersion of PdO was higher in the case of the Pd/C12A7 catalysts compared to the Pd@C12A7 sample. This is why the Pd/C12A7 catalyst demonstrated higher activity in CO oxidation and better reducibility in TPR cycles. Full article

The memory (memristive) properties of an electride material based on polycrystalline mayenite (C12A7:e⁻) were studied. The phase composition of the material has been confirmed by such methods as XRD, TEM, Raman, and infrared spectroscopy. The electride state was confirmed by conductivity measurements and EPR using a characteristic signal from F⁺—like centers, but the peak at 186 cm⁻¹, corresponding to an electride with free electrons, was not observed explicitly in the Raman spectra. The temperature dependence of current–voltage characteristics in states with low and high resistance (LRS and HRS) has been studied. In the LRS state, the temperature dependence of the current has a non-Arrhenius character and is described by the Hurd quantum tunnelling model with a Berthelot temperature of 262 K, while in the HRS state, it can be described in terms of the Arrhenius model. In the latter case, the existence of two conduction regions, “impurity” and “intrinsic”, with corresponding activation energies of 25.5 and 40.6 meV, was assumed. The difference in conduction mechanisms is most likely associated with a change in the concentration of free electrons. Full article

The present paper continues the exploration of the physicochemical and catalytic properties of vanadia-mayenite composites. The samples were prepared by an impregnation of calcium aluminate Ca₁₂Al₁₄O₃₃ (mayenite, C12A7) with a solution of vanadium precursor. Pure mayenite and V/C12A7 nanocomposites were characterized by Raman and diffuse reflectance UV–Vis spectroscopies. The reducibility of the samples was examined in temperature-programmed reduction experiments performed in a hydrogen atmosphere. The catalytic performance of vanadium-containing systems was studied in the non-oxidative dehydrogenation of ethane. As found, the low-loaded sample (5%V/C12A7 sample) contains vanadium predominantly in the form of Ca₃(VO₄)₂, while for the 10%V/C12A7 sample, two types of calcium vanadates (Ca₂V₂O₇ and Ca₃(VO₄)₂) are registered. The presence of these phases defines the spectroscopic characteristics and the redox properties of nanocomposites. Both the samples, 5%V/C12A7 and 10%V/C12A7, exhibit comparable catalytic activity, which is mainly connected with the amount of the Ca₃(VO₄)₂ phase. The uniqueness of the studied catalysts is their excellent tolerance toward coke formation under the reaction conditions. Full article

Calcium aluminates (CA) with a mayenite structure have attracted a growing interest during the last decades. The present paper reports the preparation of vanadia-mayenite composites performed via an impregnation of pure CA with ammonium vanadate solution. The properties of the prepared materials were explored by a low-temperature nitrogen adsorption/desorption technique, X-ray diffraction analysis, transmission electron microscopy, and spin probe method. As revealed, the addition of vanadium significantly affects the textural properties and the porous structure of mayenite. The blockage of micropores by vanadium species is supposed. The spin probe electron paramagnetic resonance technique based on the adsorption of 1,3,5-trinitrobenzene, phenothiazine, and diphenylamine has been applied to study the active sites on the surface of the composite samples. The results demonstrated an increase in the concentration of weak electron-acceptor sites when the vanadium loading was 10 wt%. X-ray diffraction analysis and transmission electron microscopy studies showed that the composites consist of few phases including mayenite, CaO, and calcium vanadates. Full article

The hydrogen reduction of bauxite residue lime pellets at elevated temperatures was carried out to recover iron and alumina from the bauxite residue in a new process route. Prior to the H₂ reduction, oxide pellets were initially prepared via the mixing of an industrial bauxite residue with fine calcite powder followed by calcination and high-temperature sintering. The chemical, compositional, and microstructural properties of both oxide and reduced pellets were studied by advanced characterization techniques. It was found that iron in the oxide pellets is mainly in the form of brownmillerite, and calcium–iron–titanate phases, while upon reduction they are converted to wüstite and shulamitite intermediate phases and further to metallic iron. Moreover, it was found that the reduction at lower temperature of 1000 °C is faster than that at higher temperatures of 1100 °C and 1200 °C. The slower rate and extent of reduction at the higher temperatures is attributed to the porosity loss and reduction mechanism change to a diffusion-controlled process step. In addition, it was found that Al-containing phases in the raw materials are converted mainly to gehlenite in sintered pellets and further to the leachable mayenite phase. The alkaline leaching of selected reduced pellets by a sodium carbonate solution yielded up to 87% Al recovery into the solution, while the metallic iron was not affected. Full article

This article demonstrates the possibility of creating memory devices based on polycrystalline mayenite. In the course of the study, structural characterization (XRD, TEM) of ceramic samples of mayenite was carried out, as well as a study of the spectral (THz range) and electrophysical characteristics. Materials obtained by calcination at high (1360–1450 °C) temperatures in an inert argon atmosphere differ in the degree of substitution of oxygen anions O²⁻ for electrons, as indicated by the data on the unit cell parameters and dielectric constant coefficients in the range of 0.2–1.3 THz, as well as differences in the conducting properties of the samples under study by more than five orders of magnitude, from the state of the dielectric for C12A7:O²⁻ to the conducting (metal-like) material in the state of the C12A7:e⁻ electride. Measurements of the current–voltage characteristics of ceramic C12A7:e⁻ showed the presence of memristive states previously detected by other authors only in the case of single crystals. The study of the stability of switching between states in terms of resistance showed that the values of currents for states with high and low resistance remain constant up to 180 switching cycles, which is two times higher than the known literature data on the stability of similar prototypes of devices. It is shown that such samples can operate in a switch mode with nonlinear resistance in the range of applied voltages from –1.3 to +1.3 V. Full article