Realizable recycling of coal fly ash from solid waste for the fabrication of porous Al2TiO5-Mullite composite ceramic

As the environment deteriorates, recycling of solid waste has become increasingly important. This study aimed to optimize the use of the Fe₂O₃, SiO₂, and CaO components in coal fly ash and to convert coal fly ash into stable porous Al₂TiO₅-mullite (AT–M) composite ceramic by sintering with AlOOH and TiO₂ additives at high temperatures. The phase composition, microstructure, apparent porosity, corrosion resistance, and mechanical properties of porous AT–M composite ceramics were systematically investigated. Results indicated that the sintered samples exhibited pore size distributions within the 0.16-2.9 μm, apparent porosities of approximately 52.8%, and flexural strength of 29.6 MPa. Corrosion resistance data revealed quality losses in the aqueous NaOH and H₂SO₄ solutions for 10 hours at 0.42% and 2.19%, respectively. After corrosion for 8 hours, the average flexural strength of the samples remained at 21.6 ± 0.53 and 20.84 ± 0.6 MPa, respectively. These findings show that these porous AT–M ceramics may provide enhanced corrosion resistance under alkaline conditions. The porous AT–M composite ceramics may fabricate high-performance composite membrane supports for the high temperature flue gas filtration.     

Substitution of quartz and clay with fly ash in the production of architectural ceramics: A mechanistic study

Quartz and clay are substituted gradually by fly ash using a triaxial ceramic formulation under simulated industrial conditions and the effects of fly ash substitution on the macroscopic properties and microstructures of the sintered ceramics are evaluated systematically. With the substitution of 35 wt% (1250 °C), the ceramic sample exhibited optimal properties, including linear shrinkage of 15.61%, bulk density of 2.39 g cm^-3, water absorption of 0.62% and flexural strength of 41.70 MPa, due to the accelerated densification and fly ash-spurred needle-shaped mullite. The microstructure analysis shows that the sintered matrix consists of three types of particles, quartz-, clay- and feldspar-like particles showing sintering behavior with respect to filling the glassy matrix with preserved morphology, precipitating mullite crystals, and fusing with the surrounding glassy matrix, respectively. The strength of the fly ash - containing ceramics is analyzed by the dispersion-strengthening mechanism and porosity and the results indicate that the fly ash particles affect the mechanical strength due to Griffith flaws when the total porosity is less than 25% and pores at higher total porosity. This study provides a viable strategy to recycle industrial fly ash in the production of architectural ceramics.     

Characterization of sintered coal fly ashes

Çan, Çatala?z?, Seyitömer and Af?in-Elbistan thermal power plant fly ashes were used to investigate the sintering behavior of fly ashes. For this purpose, coal fly ash samples were sintered to form ceramic materials without the addition of any inorganic additives or organic binders. In sample preparation, 1.5 g of fly ash was mixed in a mortar with water. Fly ash samples were uniaxially pressed at 40 MPa to achieve a reasonable strength. The powder compacts were sintered in air. X-ray diffraction analysis revealed that quartz (SiO₂), mullite (Al₆Si₂O₁₃), anorthite (CaAl₂Si₂O₈), gehlenite (Ca₂Al₂SiO₇) and wollastonite (CaSiO₃) phases occurred in the sintered samples. Scanning electron microscopy investigations were conducted on the sintered coal fly ash samples to investigate the microstructural evolution of the samples. Different crystalline structures were observed in the sintered samples. The sintered samples were obtained having high density, low water adsorption and porosity values. Higher Al₂O₃ + SiO₂ contents caused to better properties in the sintered materials.     

Recycling of harmful waste lead-zinc mine tailings and fly ash for preparation of inorganic porous ceramics

The porous ceramics were prepared by directly sintering of lead-zinc mine tailings and fly ash as the raw materials without any additional sintering and foaming agent. The effects of fly ash addition on the crystalline phases, pore structure, physical–chemical porosities and mechanical strength were investigated. The results showed that the bulk density decreased firstly and then increased while the porosity and water absorption presented the opposite tendency with the increase of fly ash content. Meanwhile, the chemical stability improved and the flexural strength had the same variation tendency of the bulk density. The phase evolution of sample with 60 wt% fly ash addition indicated that anorthite phase was formed at low temperature (1000 °C). The thermal behavior illustrated that the foaming process was initiated by the reaction of internal constituents in the lead-zinc mine tailings. Different pore structures indicated different foaming mechanisms that probably occurred at different temperatures. The porous ceramics with 60 wt% fly ash addition exhibited excellent properties, including bulk density of 0.93 g/cm³, porosity of 65.6%, and flexural strength of 11.9 MPa.     

Recycling of coal fly ash for fabrication of elongated mullite rod bonded porous SiC ceramic membrane and its application in filtration

Mullite bonded SiC ceramic membranes were synthesized by a facile solid-state reaction process, using SiC, solid waste fly ash as raw materials and MoO₃ as catalyst for growth of mullite at 1000 °C. The effect of MoO₃ catalyst on mullitization reaction and mullite morphology was investigated. Different pore formers were used to enhance the porosity and to observe its effects on the permeability parameters and filtration characteristics. At room temperature Darcian (k₁) and non-Darcian (k₂) in both water and air flow were measured and clean water flux was determined. The porous SiC ceramics with addition of 5 wt.% MoO₃ exhibited a flexural strength of 38.4 MPa at porosity 36.4 vol% and showed 92% oil removal efficiency from oily wastewater. This technique, combining low-cost materials and the co-sintering at low temperature, can serve as a cost-effective method for the production of high-performance porous SiC ceramic membrnaes for filtration application.     

In-situ growth of mullite whiskers and their effect on the microstructure and properties of porous mullite ceramics with an open/closed pore structure

Porous mullite ceramics with an open/closed pore structure were prepared by protein foaming method combined with fly ash hollow spheres. Both the open porosity and total porosity of samples were enhanced by increasing the hollow sphere content. Mullite whiskers with a diameter of 0.2–4 μm were grown in-situ in the porous mullite ceramics with an AlF₃ catalyst, conforming to a vapor-solid growth mechanism. The pore structure of the porous mullite ceramics was significantly affected by the mullite whiskers which increased the open porosity and total porosity. Moreover, the median pore size was reduced from 65.05 μm to 36.92 μm after the introduction of mullite whiskers. The flexural strength and the thermal conductivity of the samples decreased with increasing total porosity. The porosity dependence of the thermal conductivity was well described by the universal model, providing a reference for the prediction of thermal conductivity of porous ceramics with open/closed pores.     

Effect and mechanism of Fe2O3 decomposition in the preparation of foaming ceramics from industrial solid waste

MgO–Al₂O₃–SiO₂ foam ceramics were prepared by direct sintering of asbestos tailings and coal fly ash by spontaneous bubble process. By X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermodynamic analysis, the effect of Fe₂O₃ decomposition on the preparation of foaming ceramics from industrial solid waste and the pore-forming process was studied. The results show that the foaming effect of foaming ceramics with higher raw iron content is better when the roasting temperature is higher. Increasing the content of iron is beneficial to the formation of cordierite, and increasing the roasting temperature is beneficial to the relative content of spinel in foaming ceramics. Combined with the change in the Fe valence state, content in porous ceramic samples, and thermodynamic analysis of the product, it is found that Fe₂O₃ decomposition is the fundamental cause of ceramic foaming at high temperature. Fe²⁺ produced after decomposition replaces Mg²⁺ into spinel and cordierite structures, and the O₂ produced by decomposition cannot be discharged in time to cause ceramic foaming. The research results can provide theoretical basis for the preparation of industrial solid waste foamed ceramics.     

Combustion synthesis of high flexural strength,low linear shrinkage and machinable porous β-Si3N4 ceramics

Porous β-Si₃N₄ ceramics were prepared by combustion synthesis using Si, α-Si₃N₄ and Y₂O₃ powders as raw materials. The effects of α-Si₃N₄ diluent content in pellet on nitriding rate, shrinkage, porosity and flexural strength of porous ceramics were investigated. The results show that porous β-Si₃N₄ ceramic with porosity of 49 % and flexural strength of 151 MPa can be obtained by combustion synthesis when the content of α-Si₃N₄ diluent in raw materials is equal to that of Si, and the linear shrinkage of porous ceramic is only 2.8 %. In addition, the porous ceramic can be drilled and turned by WC drill and turning tool respectively.     

Microstructure,mechanical properties and toughening mechanisms of graphene reinforced Al2O3-WC-TiC composite ceramic tool material

The low fracture toughness of Al₂O₃-based ceramics limited their practical application in cutting tools. In this work, graphene was chosen to reinforce Al₂O₃-WC-TiC composite ceramic tool materials by hot pressing. Microstructure, mechanical properties and toughening mechanisms of the composite ceramic tool materials were investigated. The results indicated that the more refined and denser composite microstructures were obtained with the introduction of graphene. The optimal flexural strength, Vickers hardness, indentation fracture toughness were 646.31?±?20.78?MPa, 24.64?±?0.42?GPa, 9.42?±?0.40?MPa?m^1/2, respectively, at 0.5?vol% of graphene content, which were significantly improved compared to ceramic tool material without graphene. The main toughening mechanisms originated from weak interfaces induced by graphene, and rugged fractured surface, grain refinement, graphene pull-out, crack deflection, crack bridging, micro-crack and surface peeling were responsible for the increase of fracture toughness values.     

Preparation of self-reinforcement of porous mullite ceramics through in situ synthesis of mullite whisker in flyash body

Self-reinforced porous mullite ceramics were fabricated by a starch consolidation method with flyash, different aluminium sources (Al(OH)₃ and Al₂O₃) and the additive AlF₃ as raw materials. The reinforcement mechanism of needle-like mullite whiskers through in situ synthesis in ceramic body was investigated. The bulk density, apparent porosity and bending strength of the samples were tested. Phase compositions and microstructures of the sintered samples were measured by XRD and SEM, respectively. It showed that AlF₃ as additive was helpful to the formation of mullite whiskers at a low temperature. As the aluminium sources, Al(OH)₃ was more suitable for the preparation of mullite whiskers than Al₂O₃. The in situ synthesized mullite whiskers formed an interlocking structure, which enhanced the mechanical strength of the porous mullite ceramics. Porous mullite ceramics with bending strength of about 100 MPa and apparent porosity of about 55% were made at 1550 °C.     

Porous mullite ceramics derived from coal fly ash using a freeze-gel casting/polymer sponge technique

The objective of this study was to prepare highly porous mullite ceramics with relatively large-sized pores and improved compressive strength using a freeze/gel casting route combined with polymer sponge for recycling of coal fly ash into high value-added ceramics. In this work, a tertiary-butyl alcohol /coal fly ash slurry system with an appropriate addition of Al₂O₃ was used. A reticulated structure with large pore size of 220–300 μm, which formed on burnout of polyurethane was obtained; then, the skeletons consisted mainly of more dense crystalline phases together with a few fine pores (<3 μm). The rod-shaped mullite crystals with an aspect ratio of >3.7 (~4 μm in diameter) seen to have grown within the silicate melts existed. The compressive strength of the sintered porous materials increased in the reverse order of the degree of porosity, i.e. low porosity gave a high compressive strength. The porous materials with an average porosity of 61.6 %, sintered at 1600 °C with 70 wt.% solid loading showed the maximum average compressive strength (~45 MPa).     

Preparation and characterization of porous anorthite ceramics from red mud and fly ash

The porous anorthite ceramics with high porosity, good mechanical strength and low heat conductivity were prepared using red mud and fly ash as raw materials via the pore forming method. The effects of sintering temperature and fly ash on phase evolution, densification, compressive strength, thermal conductivity and microstructure of the ceramic materials were investigated. The results showed that the compressive strength of the porous ceramics had an obvious improvement with the increase in fly ash, and the densification and heat conductivity decreased firstly and then increased. In particular, specimen S2 containing 30 wt% red mud and 40 wt% fly ash sintered at 1150°C had the better performances. It had the water absorption of 18.18%, open porosity of 38.52%, bulk density of 1.29 g/cm³, compressive strength of 42.46 MPa, and heat conductivity of 1.24 W/m·K. X-ray diffraction analysis indicated that mullite, anorthite, α-quartz, and diopside ferrian were the dominant phases in the specimens. Scanning electron microscopy micrographs illustrated that plenty of open pores with strip shape and closed pores with axiolitic shape existed in the specimens. Furthermore, the existence of mullite could prevent crack propagation to enhance the energy of inter-granular fracture. It endowed the porous anorthite ceramics with high porosity, good compressive strength, and low heat conductivity.     

Integrated utilization of high alumina fly ash for synthesis of foam glass ceramic

Due to the numerous increase of the building energy consumption and huge volume of industrial wastes produced in China, the development of thermal insulation materials is quite needed. Herein, foam glass ceramic, a kind of thermal insulation materials, was fabricated by using solid wastes high alumina fly ash and waste glass as the main raw materials. First, in this study the proportion scheme of this research was designed by using Factsage 7.1 and the foaming agent was CaSO₄. Secondly, the decomposition of calcium sulfate and the influence of process parameters, namely the sintering temperature and the foaming agent additive amount, on the microstructure and mechanical properties of foam glass ceramic were investigated. The experimental results showed that when the proposed foam glass ceramic was sintered at between 1180 and 1220?°C, it exerted excellent macro and micro properties. The optimum parameters were 2% CaSO₄ addition and sintering temperature of 1200?°C, and the corresponding bulk density and compress strength values were 0.98?g/cm³ and 9.84?MPa, respectively. Overall these results indicated that the preparation of foam glass ceramic made up a promising strategy for recycling industrial waste into new kind of building insulation materials.     

Mechanical and thermal properties of silicon carbide ceramics with yttria–scandia–magnesia

Two different SiC ceramics with a new additive composition (1.87 wt% Y₂O₃–Sc₂O₃–MgO) were developed as matrix materials for fully ceramic microencapsulated fuels. The mechanical and thermal properties of the newly developed SiC ceramics with the new additive system were investigated. Powder mixtures prepared from the additives were sintered at 1850 °C under an applied pressure of 30 MPa for 2 h in an argon or nitrogen atmosphere. We observed that both samples could be sintered to ≥99.9% of the theoretical density. The SiC ceramic sintered in argon exhibited higher toughness and thermal conductivity and lower flexural strength than the sample sintered in nitrogen. The flexural strength, fracture toughness, Vickers hardness, and thermal conductivity values of the SiC ceramics sintered in nitrogen were 1077 ± 46 MPa, 4.3 ± 0.3 MPa·m^1/2, 25.4 ± 1.2 GPa, and 99 Wm⁻¹ K⁻¹ at room temperature, respectively.     

Enhanced mechanical properties of 3 Y2O3 stabilized tetragonal ZrO2 incorporating tourmaline particles

The current study reports on the improvement of mechanical properties of 3?mol% Y₂O₃ stabilized tetragonal ZrO₂ (3Y-TZP) by introduction of tourmaline through ball milling and subsequent densification by pressureless sintering at 800, 1200, 1300, 1400?°C. Findings demonstrate that no matter which sintering temperature the 3Y-TZP ceramic containing 2?wt% tourmaline reach a maximum value in flexural strength and fracture toughness as compared to other composite ceramics. As the tourmaline content is 2?wt% and the sintering temperature is 1300?°C, the flexural strength and fracture toughness of the composite ceramics are the highest, increases of 36.2% and 36.6% over plain 3Y-TZP ceramic respectively. The unique microstructure was systematically investigated through X-ray diffraction, scanning electron microscopy, energy dispersive spectrum, and flourier transform-infrared. The strengthening and toughening mechanism of tourmaline in 3Y-TZP ceramic were also discussed.     

粉煤灰发泡陶瓷的制备及性能研究

面对日益匮乏的陶瓷原料,利用固体废弃物来制备发泡陶瓷已是当今趋势。以粉煤灰为主要原料,研究铬渣掺量、碎玻璃掺量和粉磨工艺对粉煤灰发泡陶瓷的影响。结果表明,掺入适量的铬渣可改善粉煤灰发泡陶瓷的性能,小掺量的碎玻璃对粉煤灰发泡陶瓷的性能影响较小。当原料配比为m(粉煤灰)∶m(铬渣)∶m(长石)∶m(碎玻璃)=60∶10∶20∶10时,采用湿法粉磨3 h,可以制得平均孔径为0.64 mm,体积密度为368.54 kg/m³,抗压强度为8.11 MPa的发泡陶瓷。     

Preparing ceramic membranes for oil-in-water emulsions separation with oil-based drilling cutting pyrolysis residues (ODPRs) as raw material

Oil-based drilling cutting pyrolysis residues (ODPRs) are one of the solid wastes from pyrolysis of the oil-based drilling cuttings (OBDCs) that need to be recycled as raw materials to avoid the possible pollution. In this study, a facile low-cost ceramic membrane for oil-in-water emulsions separation was prepared with ODPRs incorporating with fly ash as raw material. CaCO₃ in ODPRs would decompose acting as pore-forming agent, and anorthite was formed in resultant membranes. The obtained membrane with 30 wt% ODPRs and 70 wt% fly ash fired at 1050 °C possessed apparent porosity of 38.2%, mean pore size of 0.4 μm, flexural strength of 13.1 MPa, and Darcy permeability of 1.06 × 10⁻¹³ m². Consequently, commendable filtration performance for oil-in-water emulsions was presented. In addition, the ceramic membrane showed favorable recyclability and corrosion resistance. Leaching test indicated that the membrane is safe for oil-in-water emulsion separation. Hereby, this paper confirmed the availability of ODPRs for preparing ceramic filtration membranes, and provided a new environmental conservation way to treat oil-in-water emulsions that was consistent with the sustainable development goals.     

In situ synthesis,mechanical properties,and microstructure of reactively hot pressed WB4 ceramic with Ni as a sintering additive

In this study, tungsten tetraboride (WB₄) ceramics were synthesized in situ from powder mixtures of W and amorphous B with Ni as a sintering aid by reactive hot pressing method. The as-synthesized ceramics exhibited porosity as low as 0.375% and ultra-high Vickers hardness (H_v), as much as 49.808?±?1.683?GPa (for the low load of 0.49?N). It was seen that the addition of Ni greatly improved the sinterability of WB₄ ceramic. Besides, the flexural strength and fracture toughness of WB₄ ceramic were measured for the first time to be 332.857?±?36.763?MPa and 4.136?±?0.259?MPa?m^1/2, respectively, suggesting that the ceramic has good mechanical properties. The effects of sintering temperature and holding time on the densification, Vickers hardness, and mechanical properties of WB₄ ceramics were also investigated systematically as part of our study. The results indicated that increasing the sintering temperature can obviously improve the densification and mechanical properties of the ceramics. The bulk density and Vickers hardness of WB₄ ceramic sintered at 1650?°C for 60?min under 30?MPa revealed the highest values of 6.366?g?cm^?3 and 27.948?±?0.686?GPa (for the high load of 9.8?N), respectively. The flexural strength increased to the highest value of 332.857?±?36.763?MPa for sintering temperature up to 1550?°C, but decreased slightly as the sintering temperature further increased to 1650?°C. On the other hand, the fracture toughness increased gradually with increasing temperature. It was also found that Vickers hardness showed a similar trend as the densification of the samples with increasing temperature and holding time. Besides, no obvious improvements in the densification, mechanical properties, and Vickers hardness of the samples with sintering time were observed in this study. The microstructure and fracture behaviours of the as-synthesized WB₄ ceramic were also revealed, and the toughening mechanism has been discussed.     

Thin fly ash/ ladle furnace slag geopolymer: Effect of elevated temperature exposure on flexural properties and morphological characteristics

The flexural properties and thermal performance of 10 mm-thin geopolymers made from fly ash and ladle furnace slag were evaluated before and after exposure to elevated temperatures (300 °C, 600 °C, 900 °C, 1100 °C and 1150 °C). Class F fly ash was mixed with liquid sodium silicate (Na₂SiO₃) and 12 M sodium hydroxide (NaOH) solution using aluminosilicate/activator ratio of 1:2.5 and Na₂SiO₃/NaOH ratio of 1:4 to synthesise thin fly ash (FA) geopolymers. 40 wt% of ladle furnace slag was partially replacing fly ash to produce fly ash/slag-based (FAS) geopolymers. Thermal treatment enhanced the flexural strength of thin geopolymers. In comparison to the unexposed specimen, the flexural strength of FA geopolymers at 1150 °C and FAS geopolymers 1100 °C was increased by 161.3% to 16.2 MPa and 208.9% to 24.1 MPa, respectively. A more uniform heating was achieved in thin geopolymers which favoured the phase transformation at high temperatures and contributed to the substantial increase in flexural strength. The joint effect of elevated temperature exposure and the incorporation of ladle furnace slag further improved the flexural strength of thin geopolymers. The calcium-rich slag refined the pore structure and increased the crystallinity of thin geopolymers which aided in high strength development.     

Utilization of by-products from western coal combustion in the manufacture of mineral wool and other ceramic materials

The ash by-products from combustion or gasification of western U.S. coals have chemical and mineralogical characteristics that lend themselves to utilization in ceramic materials. Laboratory and pilot-scale fabrication of four such materials has been studied. Cyclone slag from four lignite-fired power plants and a dry scrubber ash have been fabricated into mineral wool insulation in a pilot-scale cupola. Extruded and fired mixtures of fly ash, clay and ground glass have produced ceramics with extraordinary high flexural strength. Ceramic glazed wall tile that utilize fly ash in place of clay have been prepared and shown to meet most specifications for fired clay tile. Both fired and unfired dry-pressed brick containing 100% western fly ash have met ASTM specifications for fired clay brick.