Alumina Phase Research Articles

C-S-H seeding activation of Portland and Belite cements: An enlightening in situ synchrotron powder diffraction study

C-S-H seeding in Portland cements is well known from basic scientific works and field applications. Moreover, this activation approach could be beneficial for low-CO2 cements under development where a general drawback is poor mechanical strengths during the first week of hydration. However, a mechanistic understanding of the different processes taking place when seeding is still not developed. Here, we contribute to this knowledge gap by studying one commercial Portland cement and two industrial-trial belite cements. Three different admixtures are employed, viz. two types of commercial C-S-H seeding and triisopropanolamine as a typical alkanolamine. A multitechnique approach is employed including calorimetry, ultrasonic pulse velocity, thermal analysis and Rietveld analysis of laboratory X-ray powder diffraction data. Chiefly, an in situ X-ray synchrotron diffraction study has allowed mapping out the evolution of every crystalline phase. Furthermore, the use of an internal standard permitted to measure the changes in the overall amorphous content. In a nutshell, alite and belite (phases) hydrations are not significantly accelerated by C-S-H seeding for the three studied cements. Conversely, sulphate and aluminate phase dissolutions are enhanced. Faster ettringite crystallisation contributes to the observed improved mechanical properties at early ages. Moreover, a synergistic effect between C-S-H seeding and alkanolamine addition is proved. The importance of these findings for the possible acceleration of low-CO2 cement hydration is discussed.

The influence of sulfate addition on hydration kinetics and C-S-H morphology of C3S and C3S/C3A systems

A comprehensive understanding of hydration kinetics, sulfate requirement and phase assemblage of Portland cement is crucial for further optimization of early-age performance, especially when part of cement is replaced by mineral additions in blended cements. Gypsum is added to PC to control the reaction of the aluminate phase. The addition of gypsum also enhances the reaction of C3S independently of the interaction with the aluminates. In this study, the influence of gypsum in C3S and C3S/C3A hydration was studied in terms of the morphology and composition evolution of the C-S-H formed. The addition of gypsum modifies morphology of the C-S-H (needle length) precipitated and increase the nucleation density. The extent of the enhancement is dependent on the specific surface area (reaction rate) of C3S. In C3S/C3A systems, no differences in C-S-H morphology were observed before and after the aluminate peak.

Thermoluminescence characteristics of different phase transitions from nanocrystalline alumina

Nanocrystalline boehmite material was synthesized using the hydrothermal method. Different annealing temperatures have been used to transform boehmite into different alumina phases to study the effect of different phase transitions on the thermoluminescence properties of alumina. XRD analysis was carried out to investigate the crystal structure of the different alumina phases. The thermoluminescence glow curves for different alumina phases showed different structures; however, the sensitivity was almost constant for all the phase transitions of alumina over the applied dose ranging from 0.55 to 330 Gy.

Wear behavior and microstructure of alumina-mullite-zirconia composites prepared by a novel method: Coating of zircon powder by aluminum alkoxide

The formation of the mullite phase is the main challenge in the preparation of alumina-mullite-zirconia (AMZ) composites. To overcome this limitation, a novel method based on the coating of zircon powder with aluminum alkoxide was proposed in this study. Reaction sintering of alumina and coated zircon was carried out at 1630 °C for 3 h. The microstructural, physical, mechanical, and tribological properties of samples were compared with the conventional AMZ composites prepared by common mixing of alumina and zircon. The microstructural analysis indicated the higher alumina phase of the sample prepared with the conventional method. On contrary, the samples prepared with the proposed method included higher percentages of mullite phase. In terms of mechanical properties, the conventional AMZ samples performed better. However, due to the beneficial effect of the mullite phase in tribological applications, the samples prepared with this new method show superior wear resistance. Especially, the samples prepared from 30 wt% aluminum alkoxide exhibited the best wear resistance. The delamination and adhesive wear mechanisms govern the wear process.

The mechanism of basic oxygen furnace steel slag retarding early-age hydration of Portland cement and mitigating approach towards higher utilization rate

The incorporation of steel slag usually leads to retardation on the early-age hydration of cement, which limits its use as supplementary cementitious material in cement or concrete. Aiming to a better understanding of the retarding mechanism, the influences of two representative basic oxygen furnace steel slags on setting behaviour , hydration kinetics , aqueous and solid composition during early age were investigated. Different from the existing mechanisms, this study highlights the role of aluminate phase contained in steel slags in the retardation of hydration. The incorporation of steel slag with high aluminate phases content reduces C 3 S/aluminate phase ratio significantly. This reduction in C 3 S/aluminate phase ratio leads to undersulfation in the system, where aluminate phases react prior to C 3 S, resulting in flash setting, reduced C 3 S reaction, and thus the hindered early-age strength gain. This study also proposes an efficient way to alleviate the retarding effect of steel slag on early-age hydration by increasing the sulfate supply, which allows the system to react in a properly sulfated manner.

Photoluminescence properties of Al-doped ZnO synthesized via facile sol-gel route

Pure and Al-doped ZnO nanoparticles are synthesized by hydrothermal technique. The structural and phase analysis of the prepared powder is attained by X-rays diffraction and concluded that Al doping increases the crystallite size and causes compressive stress formation in the material network. Fourier transforms infrared spectroscopy reveals the formation of the aluminum oxide phase but with a tiny size that XRD cannot detect. The impact of different ratios of Al doping influence on powder morphology is carried out using transmission electron microscopy, and the optical bandgap of all synthesized products with the doping ratio is investigated by UV–visible reflectance. The electronic defects localized in the material band-gap are studied via photoluminescence spectroscopy and inferred that Al doping alters significantly the electronic defects nature present in the band-gap. In this study, the intrinsic oxygen vacancies are removed by replacing Zn when Al atoms are introduced into the material network. • Al-doped zinc oxide and Pure ZnO nanopowder has been successfully prepared by using the sol-gel method. • The effect of Al doping ratio on morphological, optical and structural properties has been investigated. • From XRD analysis we concluded that Al doping increases yields to a little increase of a crystallite size and to a compressive stress formation in the material network. • The Al doping yields causes an optical band-gap broadening because of the electronic concentration enhancement. • Fourier transform Infra-red (FTIR) spectroscopy reveals the formation of aluminium oxide phase but with a very small size that cannot be detected by XRD.

Recycling of aluminum dross and silica fume wastes for production of mullite-containing ceramics: Powder preparation, sinterability and properties

The improper disposal of industrial wastes causes environmental pollution so their recycling for fabrication of new products became an interesting research issue. In this work, sintered mullite-containing ceramics were prepared from aluminum dross and silica fume (up to 40 wt%) waste materials after sintering up to 1500 °C. Before sintering, the starting waste materials were converted into nano powders by mechanical milling alloying method up to 15 h. The obtained waste nano powders were investigated using different techniques as X-ray diffraction (XRD), transmission electron microscope (TEM) and scanning electron microscope (SEM). On the other hand, phase identification by XRD, physical properties determination (bulk density and apparent porosity), microstructure by SEM, mechanical and electrical properties of sintered bodies were investigated. The results revealed that mullite phase was formed in higher amounts with increasing both sintering temperature (1500 °C) and silica fume content. At 1300 °C, amorphous mullite was formed in addition to the alumina phase. It is also noted that the apparent porosity and bulk density were reduced with increasing silica content. However, they exhibited opposite trend when the temperature increased from 1300 into 1500 °C. Moreover, with increasing the mullite content, the microhardness, compressive strength, Younges modulus and electrical conductivity were decreased and reached 10.2 GPa, 216.9 MPa, 119.7 GPa and 4.9 × 10 −12 S/m, respectively, for the sample that contained higher amount of mullite, while the fracture toughness was improved and reached to 3.44 MPa m0.5.

Tailoring Lithium Aluminate Phases Based on Thermodynamics for an Increased Recycling Efficiency of Li-Ion Batteries

The increased usage of Li-ion batteries (LIBs) requires innovative recycling processes, allowing recovering Li with a high recycling efficiency. Different research groups have developed several recycling technologies in the past. One promising technology is the pyrometallurgical combined with the hydrometallurgical process route. To increase the recycling efficiency of this technology, it is of high relevance to artificially engineer high lithium-containing phases by tailoring slag solidification with process parameters and composition. The significant challenge thereby is the complex interrelationship between composition, process temperature, and high lithium-containing phases such as LiAlO2. Especially, the influence of Mg impurities in the feed is crucial for the phase formation. Consequently, the question arises to what extend the Mg content should be artificially adjusted in combination with CaO as a slag builder to design LiAlO2. To overcome this challenge, a comprehensive thermodynamic model framework is presented in this work, which allows tailoring the slag composition and temperature such that the LiAlO2 amount can be engineered. The model is validated successfully by experimental data. For the first time, this study presents the role and potential of combined MgO and CaO tailoring for this complex system to gain high amounts of the desired LiAlO2. This lays a valuable base for significantly increasing the recycling efficiency in the pyrometallurgical recycling of LIBs by taking into account unavoidable feed impurities.

Effect of alkali and sulfate on early hydration of Portland cements at high water to cement ratio

In this work, the hydration kinetics and the development of concentrations in the pore solution of cement clinker, neat and composite cements pastes were investigated.Early alite hydration is mainly limited by the nucleation and growth of the hydrates (C-S-H, CH, ettringite). Only after 4 to 10 h, when the main alite reaction starts, the dissolution rate of the alite becomes the limiting factor. The kinetics of the reaction of C3A with anhydrite is initially limited by the slow nucleation and formation of ettringite and only later by the dissolution rate of calcium aluminate phase. The change of the mechanisms corresponds to the initiation of the portlandite precipitation. The slower reaction of clinker sample cannot be explained by the high Al concentration alone but rather by the effect of available sulfate; high sulfate concentrations accelerate the hydration. Higher alkali hydroxide concentration does not show an accelerating effect on PC hydration.

A cost-effective way of sintering Ce3+:YAG based ceramic phosphors for high power/high brightness phosphor-converted solid state light sources

Transparent or translucent Ce3+:YAG single-phase ceramics and Ce3+:YAG/Al2O3 ceramic composite were prepared by simple sintering in flowing oxygen at atmospheric pressure. The sintered samples presented similar optical transmittance spectra as those sintered in vacuum except the oxygen vacancies related absorption band peaked at 320 nm. SEM morphology of the ceramic plates showed dense microstructures when sintered at about 1700 °C, and the mechanism of densification in oxygen atmosphere was analyzed. In addition, the chromaticity and luminescence spectra of the light source prototypes combining blue LD and the as-prepared ceramics were measured, the effect of the optical transmittance, Ce3+ doping concentration and the secondary phase alumina on the color coordinates and luminous efficacy were also discussed. This process obviates the need for expensive sintering equipment and extra heat treatment steps, which may pave the way for obtaining Ce3+:YAG based ceramic phosphors for practical applications through a cost-effective method.

Ammonia decomposition over Ru catalysts supported on alumina with different crystalline phases

The effect of the crystalline phase of alumina on ammonia decomposition over alumina-supported ruthenium (Ru) catalysts was examined. Various Ru catalysts supported on different alumina (α-Al2O3, κ-Al2O3, θ-Al2O3, δ-Al2O3, η-Al2O3, and γ-Al2O3) were prepared using a wet impregnation method. The prepared catalysts were characterized using inductively coupled plasma-optical emission spectroscopy, N2 physisorption, X-ray diffraction, transmission electron microscopy, and CO chemisorption. The Ru dispersion for Ru/Al2O3 was reduced without a calcination step in the following order: Ru/α-Al2O3 < Ru/κ-Al2O3 < Ru/θ-Al2O3 < Ru/δ-Al2O3 < Ru/η-Al2O3 < Ru/γ-Al2O3. Among them, Ru/α-Al2O3 showed the highest catalytic activity for ammonia decomposition. Moreover, the effect of calcination temperature before the reduction step was investigated. Ru particle size and morphology were confirmed to change through the calcination process. Ru/κ-Al2O3 with Ru particle sizes ranging between 7 and 8 nm exhibited the highest ammonia decomposition rate among Ru/Al2O3 catalysts calcined at different temperatures and reduced at 573 K. Therefore, Ru dispersion and its morphology, which can be controlled by the support and calcination temperature, are critical factors for hydrogen production through ammonia decomposition over Ru/Al2O3 catalysts.

Hydration behavior and formation of strätlingite compound (C2ASH8) in a bio-cement based on tri-calcium silicate and mono-calcium aluminate for dental applications: influence of curing medium

BackgroundThe aim of this work was to study the influence of simulated body fluid (SBF) and Pseudomonas aeruginosa (in Mueller Hinton Broth, MHB media) on the hydration reactions, formation of strätlingite (C2ASH8) and morphology of a composite material based on nano-size tri-calcium silicate (C3S) and tri-calcium aluminate (CA) phases. The two experimental phases were prepared in laboratory by firing the required molar ratios of chemically pure reactants by solid-state reactions at elevated temperatures. Investigation of setting time, micro-hardness, pH of immersion solution, X-ray diffraction analysis, infrared spectroscopy, scanning electron microscopy and cytotoxicity were also carried out.ResultsThe results emphasized that the hydrated compounds of C3S phase can partly solve the problem of conversion process through the formation of the more stable and highly mechanical stratlingite hydrate (C2ASH8) plates that results in an increase in hardness values for samples cured in distilled water (DW) at 37 °C. The SBF solution showed more strength loss than those cured in Pseudomonas aeruginosa microbe (MHB) medium bacterial solution. SEM and EDAX analyses emphasized the formation of hydroxyapatite at 37 °C.ConclusionsThe prepared cements are promising for dental application.

Improved additive manufacturing of silicon carbide parts via pressureless electric field‐assisted sintering

High solids loading silicon carbide (SiC)-based aqueous slurries containing only .5 wt. % organic additives were utilized to create specimens of various geometries via an extrusion-based additive manufacturing (AM) technique. Pressureless electric field-assisted sintering was performed to densify each specimen without deformation. The combination of these techniques produced parts with >98% relative density despite containing only 5 wt.% oxide sintering additives. After sintering, specimens contained only the α-SiC and yttrium aluminum perovskite phases. This suggests the evolution of a nonequilibrium yttrium aluminate phase, as well as transformation from β-SiC to α-SiC. The fabrication method presented in this work has advantages over other AM techniques commonly used with SiC, because it does not require significant organic additives nor additional postprocessing steps such as chemical vapor infiltration or polymer impregnation and pyrolysis.

Best references for the QPA of Portland cement

Cement references were reviewed and whole pattern methods were developed for the quantitative phase analysis (QPA) of Type I Portland Cements. A set of control references were established for phase identification and quantitative analysis using laboratory diffractometers. Both RIR and Rietveld whole pattern fitting methods were used in the analyses. A block refined, parameter restricted, Rietveld method produced the best QPA results by comparison with known mixtures. Similar to prior literature findings, care has to be taken because of the severe peak overlap of the major calcium silicate and calcium aluminate phases in Portland cement and the complexity of the chemistry and structures involved. Two of the four major phases identified are doped supercells and the major C3S phase is also disordered.

Investigation of sulfate attack on aluminum phases in cement-metakaolin paste

Al-containing phases such as stratlingite, C-A-S-H, and TAH play an essential role in studying the sulfate attack of MK blended cement paste. This paper investigates the sulfate attack of various aluminate hydrate products commonly formed in metakaolin (MK) blended cement paste and the evolution of aluminate hydrate products of the cement-MK paste under sulfate attack. The results show that both the synthesized stratlingite and the stratlingite produced by the cement-MK paste have excellent sulfate attack resistance. Additions of 30% and 50% MK consume all portlandite produced by the cement hydration and result in the formations of C-A-S-H, stratlingite, TAH, and C4AH13. For the 30% MK blended cement paste under sulfate attack, C4AH13 reacts with SO42− to form ettringite at the beginning because of its poor sulfate resistance. Once C4AH13 is consumed, SO42− directly targets the stratlingite, C-A-S-H, and TAH due to no portlandite in the system; C-A-S-H and TAH are the two primary contributors of Ca2+ ions and Al3+ ions to the ettringite formation. The ettringite formation is suppressed due to the absence of CH, and the existence of stratlingite, C-A-S-H, and TAH reduces the content of reactive Al that can react with sulfate.

Chloride-binding capacity of cement-GGBFS-nanosilica composites under seawater chloride-rich environment

The effects of granulated blast furnace slag (GGBFS) and nano-silica (NS) on the chloride-binding capacity of cement paste after 6-month exposure to seawater chloride-rich solutions were investigated in this paper. The pH, chloride-binding ratio (CBR), leaching behavior, and phase transformation were investigated by various experimental and analysis methods. Thermodynamic modeling was also used to study the phase assemblages for the Portland cement-GGBFS-NS composites exposed to the NaCl and MgCl2 solutions. It was found that for all cementitious composites, more chlorides were bounded in samples exposed to the salt solutions with sodium ions than that with magnesium ions. Proper additions of GGBFS and NS can enhance the chloride-binding capacity of cementitious composites. The results confirm that the addition of GGBFS can improve the chemical chloride-binding capacity because of the increased amount of chloroaluminate. The increased amount of hydrated gels in the cementitious composites with GGBFS also improved the physical chloride-binding capacity. The addition of NS increased the physical chloride-binding capacity due to the more formation of C-S-H/C-A-S-H gels, while the excessive addition of NS left less aluminum phase available for the formation of chloroaluminate, thus further decreased the chemical chloride-binding capacity. Magnesium ions in solutions increased the amount of chloride in the diffuse layer of C-S-H gels and hydrotalcite. The related results provide novel insight into the influences of GGBFS and NS on the chloride-binding capacity of cementitious composites under chloride-rich environments.

Recycling bagasse and rice hulls ash as a pore‐forming agent in the fabrication of cordierite–spinel porous ceramics

Bagasse and rice hulls ash are both waste materials. In recent years, in order to meet environmental protection, these materials have been recycled in the production of porous ceramics. A solid-state reaction mechanism of calcined alumina and talc was used to prepare cordierite–spinel porous ceramics. Talc was added from 30 to 60 wt.% at the expense of alumina and sintered at 1400°C for 2 h. The effect of bagasse and rice hulls ash (as a pore forming agent) on the densification parameters, cold crushing strength (CCS), and pore size distribution was also studied. The phase composition (X-ray diffraction) and microstructure (scanning electron microscopy) of sintered samples were investigated. The results showed that the main phases present in the samples are cordierite, corundum, spinel, and sapphirine. In the sample with a higher amount of talc additions (60 wt.%), only the formation of the cordierite and spinel phases was observed. The bulk density of the samples and the apparent porosity ranged from 1.77 to 2.26 g/cm3 and from 28.6% to 48.21%, respectively. The CCS of the samples ranges from 13.9 to 36.3 MPa. The microstructures of the sintered samples were observed for the formation of cordierite phase, alumina phase, and spinel phase in an excellent crystallization and phase arrangement.

Orthogonal analysis and mechanism of compressive strength and microstructure of the metakaolin-fly ash geopolymer

It is essential to give easier preparation and maintenance methods for geopolymers to facilitate practical application of metakaolin-fly ash based geopolymers (MK-FA geopolymers) in the engineering field. In this article, 16 sets of samples were prepared by orthogonal experiments design (OED) to quantify the order of contribution of three hypothetical influence factors in the growth of compressive strength. Test results revealed that the promotion of the calcium phase was significant. The calcium phase and fly ash acted synergistically during the early stage of strength development, with the former accelerating the hydration process of the precursor materials and the latter providing additional silicon and aluminum phases, which facilitated the formation of the polymeric skeleton and the densification of the matrix. In the later stages of the experiment, the promotion of gel particle formation by calcium effectively mitigated the disadvantage of reduced average reactivity due to the incomplete hydration of fly ash. Moreover, the highest compressive strength was obtained for the MK-FA geopolymer prepared according to the theoretical optimum ratio derived from the orthogonal analysis, which demonstrated the correctness and feasibility of the OED.

Enhanced removal of fluoride from water through precise regulation of active aluminum phase using CaCO3.

Coagulation with aluminum salts is an important method for fluoride removal from groundwater. However, the hydration of aluminum salts generating a large number of H+ usually leads to limited defluorination performance due to the optimum pH of active aluminum phase for fluoride removal around 5.5-6.5. In this work, enhanced fluoride removal from groundwater through precise regulation of active aluminum phase by CaCO3 was investigated. Precipitation products were characterized by XPS, FTIR, XRD, and SEM, respectively, and the mechanism of the high fluoride removal efficiency was discussed and compared with the traditional coagulation of Al2(SO4)3. In the Al2(SO4)3 + CaCO3 (ASCC) system, CaCO3 can stably regulate the pH at the optimum range for active aluminum phase existence and has the best fluoride removal effect. CaCO3 accurately regulated the activity of the aluminum phase by slowly releasing OH- and fine tuning pH, thereby achieving effective fluoride removal. Undissolved CaCO3 particles exist as the carrier of defluorination flocs to accelerate precipitation and improve stability. The work here provides a new method for fluoride removal and may shed light on the application of CaCO3 coagulants for other pollutants.

Preparation of α-alumina/γ-alumina/γ-alumina-titania ceramic composite membrane for chloride ion removal

It is a universally acknowledged truth that ceramic membranes with high heat and chemical resistance play a crucial role in the nanofiltration (NF) process under harsh conditions. Although the ceramic membranes are challenging to use for salt filtration due to pore size and surface charge constraints. In this study, sol-gel and dip-coating techniques are used to make alumina–titania composite ceramic membranes with an γ-alumina interlayer for salt rejection. Then, particle size distribution was determined using dynamic light scattering (DLS), and membrane surface quality and morphology were examined using scanning electron microscopy (SEM). The microstructural properties of the membrane have been investigated using X-Ray Diffraction (XRD). A dead-end module is used to measure water permeability and salt rejection. After calcination at 450 °C, SEM shows the crack-free membrane surfaces. The interlayer and top layer membrane thicknesses were estimated to be 1.5 and 0.5 μm, respectively. In addition, XRD measurements show an alumina crystal structure for the interlayer and anatase, rutile, and alumina phases for the composite membranes. Finally, it was shown that the amount of chlorine rejection was pH-dependent, with the rejection rate increasing from 12.7% to 67% as the pH climbed from 4 to 12. Overall, this research provides a composite ceramic nanofiltration for high-pressure desalination, and it is shown that the membrane's surface does not alter after five cycles, which allows for a long-term chemical process and it could be a promising candidate in the wastewater treatment field.