Alumina Phase Research Articles

Electrochemical responses of carbon steel in ordinary Portland cement blended with calcium sulfoaluminate cement

This work investigated the passivation and chloride-induced corrosion of carbon steel exposed to an extract solution (PCSA) of ordinary Portland cement (OPC) blended with calcium sulfoaluminate (CSA) cement. As evaluated by EIS and XPS measurements, carbon steel gained inferior passivity by forming a thinner and less protective passive film in PCSA solution compared with that in the pure OPC solution. However, after chloride attack, comparable corrosion resistance was confirmed for steel exposed to both solutions. Furthermore, enhanced pitting corrosion resistance was identified for steel after 30 d of immersion in PCSA solution, which was mainly attributed to the formation of an Al-rich layer on the steel surface as well as the reduced chloride concentration in solution by the unreacted aluminum phase.

Catalytic carbon-bed sintering of CNTs/calcium aluminate cement and its effects on thermal mechanical properties of refractory castables

To improve the dispersion of carbon nanotubes in refractory castables and exert the strengthening effect of carbon nanotubes on castables, the core-shell CNTs/CAC (carbon nanotubes/calcium aluminate cement) was synthesized by the catalytic carbon-bed sintering method. The effects of catalyst content on the microstructure of CNTs/CAC powders were evaluated by Raman spectroscopy, SEM, and HRTEM. The results suggested that the optimal addition amount of catalyst was 0.75 wt%. The diameter of the bamboo-like carbon nanotubes was ∼50 nm and the length of the carbon nanotubes was ∼120 μm. Carbon nanotubes and small calcium aluminate particles were interspersed with each other and wrapped in a compact calcium aluminate shell with a thickness of 3∼5 μm. Moreover, the DFT calculation was employed to investigate the growth mechanism of carbon nanotubes. Electrons were transferred from Ni to H, O, and C atoms, increasing bond length and weakening the strength of C–H and CO bonds which was conducive to the growth of carbon nanotubes. The formation of the core-shell structure was in direct connection with the liquid-phase eutectic phase. Calcium carbonate and alumina reacted preferentially to form C12A7, and then C12A7 continued to react with alumina to form CA and CA2. With the increase of sintering temperature, a large amount of liquid phase was formed. Carbon nanotubes were wrapped by the liquid phase of calcium aluminate to form a core-shell structure. The CNTs/CAC cement bonded Al2O3–SiC castables had great mechanical properties and thermal shock resistance. Compared with Al2O3–SiC castables bonded with Secar71, the HMOR and thermal shock residual strength ratio of Al2O3–SiC castables bonded with CNTs/CAC cement were increased by 24.7% and 22.1%, respectively.

Aluminium Matrix Composite Materials Reinforced by 3D-Printed Ceramic Preforms.

This article employed the fused deposition modelling (FDM) method and gas-pressure infiltration to manufacture alumina/AlSi12 composites. Porous ceramic skeletons were prepared by FDM 3D printing of two different alumina powder-filed filaments. The organic component was removed using a combination of solvent and heat debinding, and the materials were then sintered at 1500 °C to complete the process. Thermogravimetric tests and DTA analysis were performed to develop an appropriate degradation and sintering program. Manufactured skeletons were subjected to microstructure analysis, porosity analysis, and bending test. The sintering process produced porous alumina ceramic samples with no residual carbon content. Open porosity could occur due to the binder's degradation. Liquid metal was infiltrated into the ceramic, efficiently filling any open pores and forming a three-dimensional network of the aluminium phase. The microstructure and characteristics of the fabricated materials were investigated using high-resolution scanning electron microscopy, computer tomography, hardness testing, and bending strength testing. The developed composite materials are characterized by the required structure-low porosity and homogenous distribution of the reinforcing phase, better mechanical properties than their matrix and more than twice as high hardness. Hence, the developed innovative technology of their manufacturing can be used in practice.

Understanding the adsorption of calcium sulfate on tetracalcium aluminoferrite and tricalcium aluminate surfaces

AbstractCalcium sulfate (CaSO4), an essential retarder in cement, retards the hydration of tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF) phases. However, its retarding mechanism remains unclear. This paper focused on the adsorption of CaSO4f on C4AF and C3A surfaces based on isothermal calorimetry, the measurement of the ionic concentrations in a diluted system, and density functional theory to enhance the understanding of the retardation mechanism. The results showed that the retarding effect of CaSO4 on C4AF was stronger than that on C3A due to the slower CaSO4 consumption rate, lower driving force for CaSO4 adsorption, and surface coverage of Fe(OH)3 gel. The adsorption of CaSO4 hindered Ca dissolution more markedly on C4AF than C3A, which was pronounced on Fe‐free C4AF surfaces. The adsorption of CaSO4 weakened the affinity of water on C4AF and C3A surfaces, lowering the driving force for H2O adsorption. The adsorption of H2O and CaSO4 promoted the dissolution of Al on the [AlO6] octahedral surface of C4AF, which may be responsible for the maintenance of a higher Al concentration in the solution. Based on the above results, the adsorption of CaSO4 on initial C4AF and C3A hydration was explained.

Partial proton ordering during phase transition in Friedel's salt

Friedel's salt, also known as Cl-Hydrocalumite, (Ca2[Al(OH)6]· Cl·2H2O), is one of the calcium aluminates phases (AFm phases). It can accommodate radioactive chlorine isotopes, serving as a potential material in radioactive waste applications. Earlier X-Ray diffraction and nuclear magnetic resonance (NMR) experiments demonstrated the existence of the first-order phase transition between 303 K and 313 K. The phase transition involves an orientational ordering of water molecules while lowering the temperature. However, such behavior was not properly reproduced in computer simulations using classical molecular dynamics. Here we apply density functional theory and ab initio molecular dynamics to study structural properties and rotational dynamics of water molecules in the interlayer space of Cl-AFm. The transition is characterized by the partial proton ordering in the low-temperature phase, which was not previously observed in experiments and computer simulations.

The Effect of Zirconia Content on Physical and Mechanical Properties of Al<sub>2</sub>O<sub>3</sub>/ZrO<sub>2</sub> Composites

The structural, physical and mechanical properties of alumina composites reinforced with various zirconia contents were studied. Zirconia with specific stress-induced toughness mechanisms (from tetragonal to monoclinic) can improve its mechanical properties. The raw materials were commercial products of alumina (Al2O3) and zirconia (ZrO2) with gamma alumina (γ-Al2O3) and monoclinic zirconia (m-ZrO2) phases, respectively. In this study, alumina and zirconia powders containing 0, 10, 20, 30, and 40 wt% were mechanically activated and sintered at 1400°C for 3 h. Fourier transform infrared spectroscopy (FTIR) characterization was used to confirm the functional groups in the sample. Phase analysis of the sintered samples was carried out via X-ray diffraction (XRD). Composite characterization includes diameter shrinkage, density, and Vickers hardness. Corundum (α-Al2O3), monoclinic zirconia (m-ZrO2) and tetragonal zirconia (t-ZrO2) phases were the observed phases in the sintered sample. The Al2O3/ZrO2 60:40 sample had the largest shrinkage in pellet diameter, apparent density, and Vickers hardness, at 8%, 4.35 g/cm3, and 1.33 HVN, respectively.

Carbon nanofibers and polyvinyl-alcohol fiber hybrid-reinforced high-performance concrete: Mechanical property, chloride penetration resistance, and material characterization

The hybridization of nanomaterials and macroscopic fibers is currently a popular topic. The aim of this study was to explore the application potential of incorporating carbon nanofibers (CNFs) into concrete and to analyze the effect of CNFs dispersant on concrete slump and early strength. Polyvinyl alcohol (PVA) fibers and CNFs were used to prepare composite fiber-reinforced (CNFs-PVA) concrete to improve the continuity of concrete properties at the micron/nanometer scale. The engineering properties such as compressive strength, flexural strength and resistance to chloride ion erosion performance of the concrete were investigated, and the microstructure of concrete was characterized using X-ray diffraction (XRD), scanning-electron microscopy (SEM), and X-ray computed tomography (X-CT). The results showed that CNFs could improve the early strength. The combined use of CNFs and PVA increased the 28-day compressive and flexural strength by 23.38% and 28.73%, respectively, compared to the control group. Correspondingly, the electric flux and Migration rate of chloride ions (DRCM) were minimized. From the SEM and XRD test results, it is clear that CNFs and PVA fibers can: (i) bridge low-density C-S-H to form higher-density C-S-H gels, (ii) form longer chain-like fibers with enhanced load transfer and chloride binding capacity, (iii) act as internal curing agents to enhance hydration at the interface, (iv) promote the formation of the aluminum phase hydration products and calcium carbonate phase, and (v) CNFs and PVA refine the pore size by increasing the gel pores and reducing the percentage of microcracks. The combined results indicate that the improved performance of CNFs-PVA is due to the combined effect of bridging and nucleation of hydration products by CNFs, adsorption of hydration products by CNFs-PVA, and promotion of pore filling by the generated hydrides such as calcium carbonate and ettringite. These findings provide new insights for the design of high-performance concrete materials.

Alkali fusion of bauxite refining residue (red mud-RM) to produce low carbon cements

This paper creates hydraulic binders using waste and a low energy input. Cements are produced with a bauxite refining residue (red mud-RM), blended with limestone and lime, and fused at temperatures from 600 to 1200 °C. The Saudi RM investigated has significant Al and Si but low Ca. Therefore, lime (CaO) and limestone (CaCO3) are used, as a source of calcium, to harvest cementing hydrates.When calcining RM alone, reactive aluminium phases begin to form at c.300 °C. However, at c.900 °C, they turn into crystalline corundum (Al2O3), a more stable and less reactive phase. It is hoped that the Ca provided by the lime/limestone will react with the Al in the RM during fusion, to form reactive silicates and aluminates rather than inert corundum. Both types of fusion produced calcium silicates and aluminates with cementing properties. However, lime fusion required higher temperature. Limestone fusion produces cementing phases at lower temperature than lime fusion, due to the lower decomposition temperature of CaCO3 when compared to CaO. High temperature is required to break down CaO (melting point = 2572 °C), whereas CaCO3 decomposes at 600 °C and disappears at 850 °C. Despite the top alkali fusion temperature being much lower than the CaO melting point, the results demonstrate that calcium was released from the lime and entered reactions forming calcium silicates and aluminates. This is probably due to the high alkali content of the RM acting as a flux and lowering the decomposition temperature of the CaO.

Silver Catalysts Supported on High Surface Area α‐Alumina: Effect of Carbohydrate Template Size and Heat Treatment on Phase Purity

Abstractα‐Alumina is a non‐porous metal oxide with applications in ceramics and catalysis. Introducing pores into this material to create catalytically relevant surface area is challenging due to phase transitions over a wide temperature range. Current synthesis strategies involve hard templates such as synthetic polymers, e. g. polymethylmethacrylate (PMMA). Here, we compare cellulose and carbonized glucose as low‐cost and natural alternative templates for high surface area α‐alumina with a two‐step heating method. Quantitative XRD was used to methodically investigate alumina phase purity. Increasing the template size in the range of 220–1000 nm improved α‐alumina purity from 75 to 98 %, while maintaining high surface areas (21–29 m2 g−1). Phase purity increases substantially by prolonging the calcination time. The synthesized high surface area α‐alumina was studied as support for silver catalysts in the epoxidation of ethylene and allowed high silver loadings. Ethylene oxide selectivity increased with enhanced α‐alumina phase purity. Our 30 wt % silver catalyst on pure high surface area α‐alumina did not show loss in selectivity compared to a 15 wt % silver catalyst on commercial non‐porous α‐alumina. This shows the potential of carbohydrate templates, the importance of templating parameters and the benefits of pure high surface area α‐alumina as support for silver catalysts.

CHARACTERIZATION OF ALUMINA MICROSPHERES PRODUCED BY COMBUSTION TORCH REMELTING

The present work investigates the thermal treatment of alumina powder with a purpose built oxygen-Liquefied Petroleum Gas torch, as a of low-cost alumina microsphere production process. In this dry process, irregularly shaped alumina particles are re-melted, acquiring a spherical shape due to surface tension. Microspheres of the stable alpha alumina phase were produced starting from two different alumina powders, both having a granule size of approximately 80 µm. In one case the initial granules are formed of micrometric crystalline particles while in the other the crystalline particles are nanometric. In both cases the resulting microspheres present similar surface texture and the predominant crystalline phase is ?-alumina. Microspheres smaller than 42 µm show a dendritic morphology on the surface with minor presence of ?-alumina. In contrast, microspheres larger than 103 µm present a smoother surface with rhombohedral grains and presence of ?-alumina.

Investigation of the Microstructure and Mechanical Properties in Friction Stir Welded Dissimilar Aluminium Alloy Joints via Sampling Direction

This research study investigates the influence of sampling direction on the microstructure and mechanical properties of dissimilar joints formed by friction stir welding (FSW). The specimens were cut in two directions: perpendicular (transverse) and parallel (longitudinal) to the FSW joint. The tests conducted included X-ray diffraction (XRD), macrostructure, microstructure, tensile, microhardness, and fractography analysis. Different phases were noted in the XRD patterns and explained, with the aluminum phase being the dominating one. The results further showed that the transverse dissimilar joint exhibited higher microhardness compared to the longitudinal dissimilar joint, which is consistent with the respective grain sizes. Moreover, the ultimate tensile strength of the longitudinal joint exceeded that of the transverse joints, showing a substantial 47% increase. Similarly, the elongation of the joints followed a similar trend, with the longitudinal joint displaying a significant 41% increase in elongation compared to the transverse joint. Fractographic analysis revealed ductile fracture behaviour in all joints.

Insight into the role of β-cyclodextrin on the hydration of white Portland cement

Impacts of β-cyclodextrin (β-CD) on hydration of white-Portland-cement (WPC) are studied by calorimetry, ICP-OES, TOC, TG, XRD and time domain NMR. Strong retardation effect of β-CD has been confirmed in both WPC and pure C3S pastes. Much stronger retardation in C3S pastes suggests the aluminate phases (C3A and ettringite) in WPC pastes mitigates the retardation effect of β-CD on C3S hydration, probably by adsorbing β-CD molecules and thus reducing β-CD concentration in the aqueous solution. Together with precipitation experiments of pure CH and C-S-H, it is concluded that β-CD remaining in the aqueous solution of the WPC pastes blocks CH and C-S-H formation and the induction period is remarkably extended as additional time is needed to clean up the aqueous solution by slowly formed hydrates e.g., AFt. NMR results suggest high dosage of β-CD promotes C-S-H precipitation directly in aqueous solution in acceleration period, besides the surface of cement grains.

Irradiation effects on microstructure, mechanical properties, and lead-bismuth eutectic corrosion resistance of alumina coating

A comprehensive evaluation including microstructure, mechanical properties, and lead-bismuth eutectic corrosion resistance of amorphous alumina coatings after different doses of Au2+ irradiation was done using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nanoindentation, SRIM software, and eRDF Analyzer. The results showed that irradiation decreased the surface roughness and induced crystallization in the middle of the coating and at the interface. The middle crystallization might be dominated by electron energy loss, while the interface had to consider the influence of defects and doping. The regions that remain amorphous after irradiation also showed differences in topological bonding and nanovoid. The changes in topological bonding hardened the coating, while the nanovoids generated by atomic rearrangement weakened the mechanical properties. The irradiation deteriorated the lead-bismuth corrosion resistance of the coating due to the void formation and alumina phase transformation. In addition, enhanced temperature sensitivity at the interface was observed after irradiation.

Performance enhancement and characterization of limestone calcined clay cement (LC3) produced with low-reactivity kaolinitic clay

This study investigated the possibility of using a low-reactivity kaolinitic clay (LRKC), calcined to 800 °C, in the formulation of LC3. White ordinary Portland cement (WOPC), LRKC, and limestone powder (LP) of two fineness levels were used to prepare LC3 mixes with and without hemihydrate. The normalized heat evolution was found to increase in the LC3 mixes because of the filler effects provided by LRKC and LP. Without hemihydrate, an intense aluminate peak associated with the formation of AFt, AFm, and C-(A-)S-H occurred early and overlapped the C3S peak. Hemihydrate at 3 wt% delayed the aluminate peak, and more ettringite was formed, yet carboaluminates were reduced due to preferential AFt formation under limited aluminate phases from WOPC and LRKC. The best-performing LC3 mix exhibited a 90-day strength of 12.3% lower than the reference mix, and the strength of all the LC3 mixes converged regardless of the limestone content and fineness.

Properties of various CaO-Al2O3-TiO2 refractories and their reaction behaviours in contact with Ti6Al4V melts

Vacuum induction melting technology is an important method for producing high-quality titanium alloy. The key challenge lies in the development of refractories with excellent chemical stability for titanium alloy corrosion. In this work, CaO-Al2O3-TiO2 refractories are proposed as the potential materials for Ti6Al4V alloy smelting. Four CaO-Al2O3-TiO2 refractories with different TiO2 contents were prepared and their reaction behaviours in contact with titanium aluminum melts were investigated in detail. When the refractory has a suitable content of CaTiO3 phase, it promotes the diffusion and mass transfer rate of ions and improves the sintering reaction activity, leading to good high temperature mechanical properties and corrosion resistance to titanium aluminum alloy. However, when the TiO2 component is excessive, CA6 has been completely wrapped by CaTiO3, and some CA6 grows abnormally, resulting in a decrease in high temperature mechanical properties of the refractory. Moreover, during the smelting of Ti6Al4V alloy, Al component reacts with CaTiO3 to produce metallic Ti and calcium aluminate phase with low melting point, resulting in degradation of refractory.

Silica and kaolin reinforced aluminum matrix composite for heat storage

Abstract This study used aluminum scraps to produce a secondary aluminum metal matrix for heat storage analyses. Silica and kaolin reinforced aluminum metal matrix composites were successfully produced via stir casting. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed for phase and microstructure characterization. XRD revealed alumina (corundum), aluminum and kyanite phases while SEM indicated pores in the composites. Density, average specific heat (from 30 to 200°C), thermal conductivity, and hardness tests were carried out. Total heat energy stored per kg, from 30 to 200°C, was obtained. The inclusion of 7.5 and 15 wt% kaolin increased the specific heat of the matrix from 474.3 to 564.57 J·kg−1·°C−1 and 474.3 to 679.03 J·kg−1·°C−1, respectively. Likewise, adding 7.5 and 15 wt% silica sand increased the thermal conductivity of the matrix from 154.99 to 175.62 W·m−1·°C−1 and 154.99 W·m−1·°C−1 to 181.38 W·m−1·°C−1, respectively. The addition of 7.5 wt% silica sand and 7.5 and 15 wt% kaolin increased the hardness value of the matrix from 72.11 to 73.11 HB, 72.11 to 81.38 HB, and 72.11 to 82 HB, respectively. Hardness of the composites reinforced with kaolin is higher than that of the composites reinforced with silica sand. This is attributed to the higher molecular weight of kaolin. Significant increase in specific heat and thermal conductivity was achieved.

Radiation Shielding Properties of Synthetic Ca-Al2O3 Polymer Based Composites

The optical and radiation shielding capabilities of Ca-Al2O3polymer-based composites were explored by combining synthesized aluminum oxide (Al2O3) with calcium (Ca) obtained from different shells. The obtained powders were dispersed into polymeric materials in a ratio of 2:1 via the melt-mixing process and then cast in a 10cm x10cm mold with a thickness of 10 mm and allowed to set at ambient temperature. The X-ray transmission through the composite samples was investigated using a well-collimated point source, and a linear attenuation coefficient was estimated. X-ray fluorescence results revealed calcium contents of 32 %, 37 %, and 34 % for snail shells, seashells, and periwinkle shells, respectively. The X-ray diffraction result confirmed the Al2O3polymorphs at approximately 32oand 57oand the aluminum phase at 46o. The X-ray photon transmittance of the composites is quite low at 40 keV to 60 keV and comparatively higher at 100 keV to 200 keV. For Ca-Al2O3polymer-based composites with calcium contents sourced from seashells, periwinkle shells, and snail shells, the maximum linear attenuation coefficients were 1.0 cm-1,0.79 cm-1, and 0.65 cm-1, respectively. The half-value layer (HVL) and relaxation length of all the samples under investigation decreased abruptly at 40 keV and 60 keV and grew linearly as the energy increased from 100 keV to 200 keV. A Ca-Al2O3polymer-based composite with calcium contents sourced from seashells was found to have higher x-ray attenuating characteristics than other composites.

Process for recovery and value addition of silica mineral values of distinctly siliceous mining rejects

The mining and metal industries generate a variety of solid wastes from mining activities to finished products. Mining rejects of non-ferrous industries such as bauxite, manganese, gold, zinc, lead and copper mines are underutilized and low in demand as potential raw materials for metallurgical use. Largely these materials are in bulk use for landfilling and building materials without recovering relevant mineral values. Major reject materials such as jarosite, kaolinite, khondalite, alumogoethite and saprolite are rich sources of alumina and silica-bearing mineral phases with varying compositions of iron oxide as major constituents. In the present study, the chemical and mineralogy of some of these materials were assessed. The technical scope for recovering one of the low-cost mineral phases in the rejects such as silica was ascertained for the potential value addition in view of the large scope of utilization in glass, glass products, molten sand, cement, fibreglass, ceramic enamel, sandblasting, refractory and light concrete.The recovered silica was used as a low-cost precursor for making value-added nano-silicate suitable for various industrial applications. The paper discussed the significance of the chemical composition and mineralogy of the raw materials. The process steps involved in the selective beneficiation of silicate mineral phases are used for the stepwise removal of iron oxide and alumina followed by the preparation of sodium silicate. Chemical precipitation and separation of nano-silica followed by characterization of silica nano-particles by scanning electron microscopy (SEM) and inductively coupled plasma (ICP)-MS analysis was also elaborated. These SEM studies inferred that the silica particles size ranges from nanometer to micrometre depending on various factors such as acid concentration, ageing time and the nature of the precursor used. ICP-MS analysis revealed the purity of nano-silicate prepared from the reject material. The leached solution consisting of aluminium has been utilized for making alpha alumina based on the IPR-protected in-house process route to follow the concept of producing zero waste.

Influence of NH4BF4 on the alumina phase transition and morphology

The spherical alumina powders including regular morphology, uniform particle size, smooth surface, and reduced particle wear generally give the finished products with the excellent performances. This research presents a novel method to produce the spherical alumina by employing gibbsite precipitated by the Bayer process as raw material mixed with NH4BF4 at 1000 °C, nano α-Al2O3 was characterized by the well-crystallized and monodispersed spherical alumina single crystal particles. The results provide a novel economical method to prepare the spherical alumina from gibbsite.

Enhancement in compressive strength of carbide slag activated ground granulated blast furnace slag by introducing CaCl2 and NaCl

Alkali-activated materials was the green and low carbon cementitious materials. In this study, the calcium chloride (CC) and sodium chloride (SC) were used as accelerator to promote the compressive strength of carbide slag (CS) activated ground granulated blast-furnace slag (GGBFS). The compressive strength of samples was investigated and the related mechanism was comprehensively characterized with hydration heat, XRD, TG-DTG, SEM, 29Si NMR and MIP in details. These results showed that 5% SC and CC significantly improved early strength with an increase of 58.8% and 127.1% at 3 days, respectively. Interestingly, the presence of CC also increased the 28-day and 90-day strength from 16.6 MPa and 30.4 MPa to 50.2 MPa and 70.5 MPa, respectively. These results citified that the addition of CC and SC accelerated the consumption of CS at early age. At late age, the enhancing effect of SC was lower than CC, reflecting the Ca2+ made a greater contribution to strength development than Na+. The addition of CC still accelerated the rapidly precipitation of aluminium phase in the form of hydrocalumite, which could improve further dissolution and hydration of GGBFS at late age. Therefore, the addition of CC produced more hydration products that would refine microstructure and enhance strength at late age. These findings suggested a novel way to improve the mechanical properties of alkali-activated GGBFS.