Structural investigation relating to the cementitious activity of bauxite residue — Red mud

The cementitious behavior of red mud derived from Bauxite-Calcination method was investigated in this research. Red mud were calcined in the interval 400–900 °C to enhance their pozzolanic activity and then characterized in depth through XRD, FTIR and ²⁹Si MAS-NMR techniques with the aim to correlate phase transitions and structural features with the cementitious activity. The cementitious activity of calcined red mud was evaluated through testing the compressive strength of blended cement mortars. The results indicate that red mud calcined at 600 °C has good cementitious activity due to the formation of poorly-crystallized Ca₂SiO₄. The poorly-crystallized Ca₂SiO₄ is a metastable phase which will transform into highly-crystallized Ca₂SiO₄ with the increase of calcination temperature from 700 °C moving to 900 °C. It is the metastable phase that mainly contributes to the good cementitious activity of red mud. This paper points out another promising direction for the proper utilization of red mud.     

Reticulated Porous Foam Ceramics with Different Surface Chemistries

The aim of this study was to discuss the influence of different filter surface chemistries on the properties of foam filters. For reliable results, it is essential to ensure comparable structural properties (cell size and strut thickness) for all different surface chemistries (Al₂O₃, MgAl₂O₄, 3Al₂O₃·2SiO₂, SiO₂, and TiO₂) possess the same structural properties (cell size and strut thickness). Filters made of 100% of the investigated materials and alumina skeletons coated with the investigated materials were prepared. The coated alumina samples were sintered in one and two steps. The processing route with two sintering steps resulted in improved mechanical properties and comparable shrinkage and strut thickness. The 100% bulk foams possessed different pore sizes due to the differences of the material shrinkage. In this study, a comparison of the experimental investigated properties of the ceramic foam filters and the theoretically calculated values for foam materials derived from the bulk material properties is established.     

Investigation of photocatalytic properties of TiO2 nanoparticle coating on fly ash and red mud based porous ceramic substrate

Titanium dioxide is one of the best semiconducting photocatalysts available for photocatalytic cleaning applications. Especially nano-sized TiO₂ particles deposited on porous substrates can be utilized as a filter for solid and liquid media. On the other hand, red mud and thermal plant fly ash are hazardous wastes that are produced in large quantities. Recycling/reuse of these waste material in a porous ceramic production would be beneficial both for environmental and economical issues. In the present study, a porous substrate was produced from red mud and fly ash with varying ratios and additives of H₃BO₃, CaCO₃, and MgCO₃ for lowering the melting temperature and porosity formation. Sintered ceramics were then coated with nano-sized TiO₂ particles by the sol-gel method. Ultrasonic dispersion of nano-sized TiO₂ nanoparticles was also utilized as an alternative method for impregnation of nanoparticles into the porous structure of the ceramic substrate. Finally, photocatalytic activities and degradation of methylene blue (MB) under UV radiation of substrates were investigated. According to the SEM investigations, the sol-gel method was observed to be a better way of nanoparticle deposition because deposited particles are homogenous throughout the ceramic body. Also, this method provides lower particle sizes than the ones that were deposited by the ultrasonic dispersion method. This results in higher surface area and better photocatalytic activities.     

Polymer-derived ceramic tapes with small and negative thermal expansion coefficients

A polysiloxane filled with ß-eucryptite and/or SiC was used for the processing of polymer derived ceramic tapes. The combination of both fillers in varying proportions allowed to tailor the overall bulk thermal expansion and the flexural strength of the resulting composite materials simultaneously. Incorporation of SiC increased noticeably the flexural strength of the samples and influenced the phase changes resulting from the interactions between the β-eucryptite filler and the polymer derived ceramic matrix. Changes in the phase composition and changes of the unit cell parameters of β-eucryptite because of the formation of solid-solutions with silica originating from the SiO₂ constituent of the polymer derived ceramic matrix were observed by Rietveld refinement. Tapes resulting from this process possess a sufficient mechanical stability and their coefficient of thermal expansion can be adjusted from slightly positive to moderate negative values.     

Investigation of a high stable β-cristobalite ceramic powder from CaO–Al2O3–SiO2 system

The room temperature stabilized β-cristobalite ceramic powder has great potential for use in production of engineering ceramic materials due to its high resistance to thermal shock, low expansion coefficient, high chemical resistance and low density. However, the use of this material is not common in ceramic industries. The problem is shown to be the instability of β-phase during milling. The applied external force leads to phase transformation to α-cristobalite and thus the material shows poor thermal stability and so on. In this study, a reliable β-cristobalite ceramic powder from CaO–Al₂O₃–SiO₂ ternary system was investigated at different compositions and under various sintering temperatures and sintering times. The phase stability of the powder sample was investigated by milling for 50 h using a planetary mill. The crystalline phases were examined by X-ray and FTIR analysis and the results were discussed with respect to the phase homogeneity through the particle mass.     

Novel glass ceramic foams materials based on red mud

Glass ceramic foams were prepared using red mud and fly ash with added CaCO₃ as foaming agents. The aim of the present work was to investigate the possibility of adding red mud, an alkaline leaching waste, in the raw material for the preparation of glass ceramic foams. The results of mineralogical analyses as well as the microscopic examination showed that the use of the red mud affect the mineralogical characteristics and structures of the as produced foams. The influence of amount of red mud on the bulk density and compressive strength of samples was further evaluated. The experimental results showed that relatively low bulk density foams (0.33–0.41 g/cm³) could be obtained by using low sintering temperature (760–840 °C) when the red mud/fly ash ratio does not exceed 40:60. The reduction of sintering temperature or, above all, the reduction of the holding time, was found to limit the coalescence and significantly improve the compressive strength of the foams (0.33–2.74 MPa).     

Assessment of mechanical and microstructural properties of geopolymers produced from metakaolin,silica fume,and red mud

Geopolymers have been studied as viable alternative to traditional Portland cement-based products, given the use of industrial by-products as raw materials. This work evaluated the mechanical and microstructural properties of geopolymeric mortars produced with sodium hydroxide solution, metakaolin, silica fume, and red mud. The mixtures were produced by means of dosages with different molar ratios and curing conditions. The raw materials were characterized by granulometry, chemical, mineralogical, and thermal analysis. The characterization of mortars was performed by scanning electron microscopy (SEM) and axial compressive strength tests. The precalcination at 850°C of the red mud was sufficient to make it more reactive and suitable for use in geopolymers. Noteworthy, the best mechanical strengths of metakaolin mortars for curing at 50°C, and with the lowest SiO₂/Al₂O₃ ratios. In the mortars with incorporated red mud, there was a decrease of strength at thermal curing conditions and with the increase of residue content, whose microstructure indicates the formation of more pores in the geopolymer matrix. The thermal curing promoted the formation of sodalite crystals, and the significant presence of Na particles on the surface suggests that part of the added NaOH did not react with the precursors.     

Low hydrothermal temperature synthesis of porous calcium silicate hydrate with enhanced reactivity SiO2

This study presents a novel approach for the synthesis of porous calcium silicate hydrate (CSH) at a low hydrothermal temperature of 110 °C based on enhanced reactivity SiO₂ (i.e. silica/polyethylene glycol (PEG₂₀₀₀) composites) as the source silica material. The as-prepared CSH materials exhibited a porous microstructure with a large number of small mesopores. The porosity formation mechanism of CSH was apparent that cavitation, resulting from sonication, enabled PEG₂₀₀₀ (via intercalation on silica) to break apart Si–O–Si structural units, thereby enhancing SiO₂ reactivity at a low hydrothermal temperature. In addition, the presence of PEG₂₀₀₀ effectively prevented the aggregation of particles during the formation process of the porous CSH solid. The low temperature synthesis proposed herein represents a viable and effective method for the further development of porous CSH as a functional ceramic material.     

Polymer-derived Co2Si@SiC/C/SiOC/SiO2/Co3O4 nanoparticles: Microstructural evolution and enhanced EM absorbing properties

In this work, porous core-shell structured Co₂Si@SiC/C/SiOC/SiO₂/Co₃O₄ nanoparticles were fabricated by a polymer-derived ceramic approach. The in situ formation of mesopores on the shell, microstructural, and phase evolution of resulting nanoparticles were investigated in detail. The obtained nanoparticles-paraffin composites possess a very low minimum reflection coefficient (RC_min) −60.9 dB, broad effective absorption bandwidth 3.50 GHz in the X-band and 15.5 GHz in the whole frequency range (from 2.5 to 18 GHz). The results indicate outstanding electromagnetic wave (EMW) absorbing performance among all the reported cobalt-based nanomaterials, due to the reasons as follows: (a) The unique core-shell structure as well as complex phase composition of SiC/C/SiOC/SiO₂/Co₃O₄ in the shell, result in a large number of heterogeneous interfaces in the nanoparticles; (b) Nanoparticles have both dielectric and magnetic loss; (c) Mesopores in the shell prolong the propagation path of EMW, thereby increasing the absorption/reflection ratio of EMWs. Thanks to the material structure design, the resulting core-shell structured cobalt-containing ceramic nanoparticles have great potential for thin and high-performance EMW absorbing materials applied in harsh environment.     

Co–SiO2 aerogel-coated catalytic walls for the generation of hydrogen

Cobalt–silica (Co–SiO₂) aerogel coatings were successfully grown on the walls of ceramic monolith channels, thus resulting in structured catalytic wall materials useful for gas–solid reactions. The preparation involved the synthesis in situ of SiO₂ gels by the sol–gel hydrolysis and condensation of tetraethylorthosilicate (TEOS), quenching at the gelation point, incorporation of Co by impregnation, and extraction of the solvent by supercritical drying. Characterization of the aerogel-coated monoliths revealed an excellent dispersion and homogeneity of the aerogel and good adherence properties. The catalytic performance of these materials in the ethanol steam reforming reaction addressed to obtain hydrogen was studied at atmospheric pressure by carrying out consecutive cycles at 473–773–473 K with a C₂H₅OH:H₂O  1:3 (molar) mixture and stability tests, and the results were compared with those obtained over monoliths prepared by conventional washcoating methods from Co–SiO₂ xerogel. Co–SiO₂ aerogel catalytic walls were about four times more active for hydrogen generation at 623 K than conventional monoliths. An unusual rapid activation of aerogel-coated monoliths was attained at 580–590 K, even after several cycles and oxidation treatments at 563–613 K, which was attributed to highly dispersed cobalt particles and higher effective diffusivity of reaction species due to high porosity and larger average pore size. The reproducible low-temperature activation of Co–SiO₂ aerogels supported on ceramic monoliths may be useful for the practical application of fuel reformers to the on-site generation of hydrogen from ethanol.     

Embedded SiO2 interface in SiCnws/Ba0.75Sr0.25Al2Si2O8 ceramic with enhanced electromagnetic absorption at elevated temperature

For high-temperature electromagnetic absorption materials, higher polarization loss is needed to balance the impedance mismatch due to greater conduction loss at elevated temperatures. Here, a SiO₂ interface was introduced into a SiCnws/BSAS ceramic based on wide bandgap and low dielectric constant characteristics of SiO₂. The interface structure was tailored by changing the SiO₂ content. When the SiO₂ content reached 15 vol%, three-phase interlaced interfaces were formed, which produced many nano-heterointerfaces that increased the polarization loss by 77.5 %. The optimized SiCnws/SiO₂-BSAS ceramic achieved enhanced electromagnetic absorption from 298 K to 873 K, and its effective absorption bandwidth reached 4.1 GHz at 873 K. The electromagnetic absorption mechanism was analyzed from the perspectives of electron transport and space charges. This heterointerface design strategy provides a new method for the development of high-temperature electromagnetic absorption materials.     

Oxidation behavior of medium-entropy (Y1/3Yb1/3Lu1/3)2O3 modified SiC ceramic at 1700 °C: Experimental and theoretical study

The medium-entropy oxide (Y_1/3Yb_1/3Lu_1/3)₂O₃ with a body-centered cubic structure was successfully synthesized by solid-state reaction process, and then it was introduced into SiC ceramic to study its effect on the oxidation behavior of SiC ceramic at 1700 °C. The (Y_1/3Yb_1/3Lu_1/3)₂O₃-modified SiC ceramic exhibited better oxidation resistance than its individual oxides (Y₂O₃, Yb₂O₃, and Lu₂O₃) modified SiC ceramic. The experimental and calculated results all indicate that the rare-earth atoms had the tendency to diffuse into the SiO₂ structure and occupy the interstitial positions within SiO₂ structure. The introduction of medium-entropy oxide (Y_1/3Yb_1/3Lu_1/3)₂O₃ reduced the initial oxidation rate of the ceramic samples (1?3 h), and enhanced the stability of SiO₂ structure, thus resulting in a better oxidation resistance at 1700 °C.     

Waste-bearing foamed ceramic from granite scrap and red mud

Using granite scrap and red mud as raw materials, SiC as foaming agent, powder sintering method was used to prepare closed-pore foamed ceramic. The effects of the ratio of red mud and granite scrap, foaming agent content, sintering temperature and holding time on the crystalline phase, pore structure, and performance of foamed ceramic were systematically studied. The results showed that, when the content of red mud was 10 wt%, together with 1.0 wt% SiC addition, the heating rate was 5°C/min, the foamed ceramic sintered at 1130°C for 30 min exhibited optimal properties, including bulk density of 483.11 kg/m³, porosity of 77.27%, compressive strength of 1.62 MPa and water absorption of .49%. Based on these properties, it possessed broad potential application prospects in the fields of sound and thermal insulation, lightweight construction materials. In this study, the utilization ratio of industrial solid waste was 100%, realized the comprehensive utilization of granite scrap and red mud, and provided a new idea to realize their low-cost utilization by preparing foamed ceramic with associated economic and environmental benefits.     

Stability of (Cr)CaO.SnO2.SiO2 pink pigment in ceramic frits

Abstract

Incorporation of the (Cr) CaO.SnO₂.SiO₂ pigment in ceramic glazes yields colours with a red component the hue of which depends on the nature of the frits used to produce the glazes. Several samples were prepared by adding this pigment to different frits commonly used in the manufacture of glazed ceramic tiles, each sample subsequently being fired at the appropriate maturing temperature. A study of the resulting glazes shows that variations in colour arise as a result of the three possible phenomena which may develop in firing depending on the frit composition used. These three phenomena are pigment dissolution in the glassy phase, devitrification of crystalline phases whose nature differs from that of the pigment, and immiscible glassy phase separation.     

Bauxite ‘red mud’ in the ceramic industry. Part 2: production of clay-based ceramics

Some potential uses of red mud as a raw component in clay mixtures for ceramic bodies production are presented. The influence of increasing amounts of red mud on the forming procedure, sintering and final properties was analyzed. Samples were produced by uniaxial pressing and slip casting. Two different clays are used as basic materials, the former being currently employed for the production of bricks by extrusion, the second — almost pure Kaolin — for high quality ceramic manufacturing. In both cases the addition of red mud led to more deflocculated solid–water systems and an increase of the critical moisture content. Mixtures prepared with the first clay and red mud loads up to 50% were fired at 850°C. The red mud content did not influence the sample porosity while determining a strength decrease attributed to the inertness of red mud at the working temperature. Samples produced using the second clay and red mud (0 – 20%) were fired at 950 and 1050°C. The addition of red mud determined increases of density and flexural strength which can be accounted for by the formation of a larger amount of glassy phase at higher red mud contents. The results of this work indicate excellent perspectives for using ‘red mud’ as raw material in mixtures with clay for the production of ceramic bodies.     

Wettability and infiltration of Si melt on SiO2-Si3N4 composite ceramic

Non-wettable material with Si melt is preferred to manufacture crucible in order to avoid adhesion between Si and traditional fused silica crucible. In this work, wetting and infiltration behaviors of Si drop on porous/dense SiO₂-Si₃N₄ ceramic and SiO₂ ceramic were systematically studied via the sessile drop technology and microstructural analysis method. The porous SiO₂-Si₃N₄ ceramic exhibited non-wetting behavior with stable contact angle of about 130 °. Nevertheless, dense SiO₂-Si₃N₄ ceramic and SiO₂ ceramic displayed wetting features with Si drop. For both SiO₂-Si₃N₄ ceramics, three kinds of infiltrations were observed, including infiltration under Si drop, infiltration under substrate surface (beyond drop) and infiltration on substrate surface. Notably, the infiltration under Si drop had the slowest speed with tiny infiltration depth. The above non-wetting behavior and tiny infiltration under drop of porous SiO₂-Si₃N₄ ceramic were closely related to material pore characteristics and Si/substrate interfacial reaction.     

Synthesis and characterization of novel saponified guar‐graft‐poly(acrylonitrile)/silica nanocomposite materials

The combination of carbohydrates with silicon‐based ceramic materials offers attractive means of production for high performance materials. Present article describes the synthesis of novel nanocomposites out of SiO₂ and saponified guar‐graft‐poly(acrylonitrile) (SG). Tetraethoxysilane was used as the precursor for silica and growth of SiO₂ phase was allowed concurrently in the presence of SG. The material so obtained was thermally treated at 80°C, 160°C, 500°C, and 900°C to study the effect of thermal curing on its properties. During the curing process, silanol surface groups of silica globules reacted to create the reinforced SiO₂‐SG substance. It was observed that at 900°C, the SiO₂ phase crystallized out in tetragonal shape (similar to Cristobalite form of silica) in presence of SG. The chemical, structural and textural characteristics of the composites were determined by FTIR, XRD, TGA‐DTA, SEM and BET studies. The materials were also evaluated as efficient Zn²⁺ metal binder. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 536–544, 2007     

EAFD-loaded vitreous and glass–ceramic materials

SiO₂, Na₂O and CaO were mixed and co-melted with electric arc furnace dust waste. The resulting vitreous materials, produced by quenching at ambient atmosphere, were transformed into glass–ceramics by two-stage heat treatment, under thermal conditions that were determined by differential thermal analysis. X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry and transmission electron microscopy were employed to investigate the physical properties of all products. It was found that whilst wollastonite (CaSiO₃) separates from the parent matrix as the dominant crystalline phase in all glass–ceramic products, the crystallization mode depends on the batch composition. Leaching tests evidenced that vitreous products were chemically durable. Devitrification did not significantly affect leach resistance so glass–ceramic materials retain the leach resistance that was achieved by vitrification.     

Resin-silica composite nanoparticle grafted polyethylene membranes for lithium ion batteries

To avoid the peeling-off of ceramic nanoparticles (NPs) from polyolefin membranes usually occurred in commercially available ceramic NPs coated polyolefin separators for lithium batteries, we propose a simple one-pot in-situ reaction method to modify commercial polyethylene (PE) separators by surface grafting 3-Aminophenol/formaldehyde (AF)/silica (SiO₂) composite NPs. The AF/SiO₂ composite NPs form self-supporting connected pores on the modified layer of the separator surface, which ensures the transportation of Li⁺. Moreover, the PE@AF/SiO₂ separators has higher electrolyte wettability and compatibility than neat PE separators attributed to the plentiful polar functional groups in the AF/SiO₂ layer and AF/SiO₂ composite NPs, resulting in higher lithium ion transference number (= 0.62) and ionic conductivity (σ = 0.722 mS cm⁻¹). More importantly, the discharge capacity, capacity retention rate and coulombic efficiency (136.2 mA h g⁻¹, 87.9% and 99%, respectively) after 200 cycles of Li|NMC half batteries with PE@AF/SiO₂ separators, are all more excellent than that with the pure PE separator (125 mA h g⁻¹, 83.1% and 85%, respectively). Our results show that the PE@AF/SiO₂ separators obtained by this modification method have higher electrochemical stability in the lithium battery system.     

Effect of La2O3 on the oxidation resistance of SiC ceramic at 1973 K: Experimental and theoretical study

The effect of La₂O₃ on the oxidation resistance of SiC ceramic at 1973 K was investigated using isothermal oxidation test and first-principles calculations. The SiC ceramic with La₂O₃ shows a better oxidation resistance compared with that without La₂O₃ due to the in situ formed La₂Si₂O₇ in SiO₂ glass layer after oxidation at 1973 K. First-principles calculations based on density functional theory were applied to analyze the solution behaviors of La atom in the surface of SiO₂ and La₂Si₂O₇. The solution energy of La atom in SiO₂ (0 1 1) is −19.05 eV, which is far less than −4.19 eV in La₂Si₂O₇ (2 0 1) with a La vacancy, thus resulting in that La atom in La₂Si₂O₇ (2 0 1) diffuses into SiO₂ (0 1 1). The SiO₂ lattice with an interstitial La atom is more stable than that with a substitutional La atom and the interstitial La breaks the nearest Si–O bond to form La–O and La–Si bonds, which is beneficial to improving the high-temperature stability of SiO₂. Experimental and theoretical results indicate that the formation of refractory La₂Si₂O₇ phase enhances the stability of SiO₂ glass layer, so as to protect SiC ceramic from further oxidation at 1973 K.