Effect of MgAl2O4 nanoparticles addition on the densification and properties of MgO-CaO refractories

MgAl₂O₄nanoparticles were added to MgO–CaO refractory ceramic composites in the range of 0–8 wt%. Refractory specimens were obtained by sintering at 1650 °C for 3 h in an electric furnace. Refractory specimens were characterized by measurements of bulk density, apparent porosity, hydration resistance, cold crushing strength, crystalline phase formation, and microstructural analysis. Results show that with additions of MgAl₂O₄ nanoparticles the bulk density of the samples increased. But the apparent porosity and cold crushing strength decreased and increased, respectively with addition MgAl₂O₄ nanoparticles up to 6 wt% and for further MgAl₂O₄ nanoparticles, due to the thermal expansion mismatch, the results is reversed. Also, the hydration resistance of the samples was appreciably improved by the addition of MgAl₂O₄ nanoparticles due to its effect on decreasing the amount of free CaO in the refractory composite and promotion of densification by creating a dense microstructure.     

Densification and Properties of Fe2O3 Nanoparticles added CaO Refractories

Up to 8 wt. % of Nano-iron oxide was added to CaO refractory matrix. The crystalline phases and microstructure characteristics of specimens sintered at 1650 °C for 5 h in an electric furnace were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The physical properties are reported in terms of bulk density, apparent porosity and hydration resistance. The mechanical behavior was studied by a cold crushing strength (CCS) and flexural strength at 1200 °C test. As a result, it was found that the presence of Nano-iron oxide in the CaO refractory matrix induced 2CaO.Fe₂O₃ (C₂F), CaO.Fe₂O₃ (CF) and 3CaO.Al₂O₃ (C₃A) phase’s formation, which improved the sintering process. Nano-iron oxide also influenced the bonding structure through a direct bonding enhancement. On the Other hand, the presence of Nano-iron oxide resulting in improvement properties of CaO refractory matrix refractories such as bulk density, hydration resistance and cold crushing strength. The maximum flexural strength at 1200 °C is achieved by the samples containing 4 wt. % nano-Fe₂O₃.     

Effects of nano-alumina content on the formation of interconnected pores in porous purging plug materials

The physical properties and microstructure of porous purging plug materials added with different nano-alumina contents and firing temperatures were investigated by means of X-ray diffraction, scanning electron microscopy, air permeability, pore size distribution, mean pore size, apparent porosity, bulk density, and cold crushing strength (CCS) tests. The results showed that the addition of nano-alumina had a great effect on the physical properties and microstructure of the porous purging plug materials. With increasing nano-alumina content in the composition, the main phase was α-Al₂O₃ in all compositions and the mean pore size, apparent porosity and air permeability all increased due to the increased number of pores and pore size of the specimens which facilitated the formation of interconnected pores. When the sintering temperature was changed from 1550 °C to 1650 °C, some of the smaller pores vanished due to solid phase sintering, which reduced the apparent porosity, and some open pores connected to form interconnected pores, which promoted increased air permeability. In addition, the strength and porosity were found to follow the relationship σ = σ₀ exp (-b P). When the apparent porosity increased, the CCS decreased, and vice versa.     

Development of fireclay aluminosilicate refractory from lithomargic clay deposits

Lithomargic clay until now has not been utilised to produce refractory bodies due to its low plasticity. In this work, the development and evaluation of fireclay refractory material produced from lithomargic clay deposit has been studied by addition of clay binder. Three formulations were prepared by mixing, semi-dry moulding, drying and firing at temperatures ranging from 1200 to 1400 °C. The fired samples were investigated to determine their physical properties such as bulk density, apparent porosity, linear firing shrinkage, and cold crushing strength. The chemical and mineralogical compositions were also determined. The results show that the linear firing shrinkage values were within limits acceptable for refractory clays. The cold crushing strength increases as temperature increased to 1400 °C. Cold crushing strength increased with increasing binder content. The increase of the highly refractory phases (cristobalite and mullite) and the densification of the bricks due to the presence of fluxing agents were responsible for the high cold crushing strength values. The investigated properties indicate that lithomargic clay underlying bauxite deposits could be used to produce fire clay aluminosilicate refractories.     

An approach for matrix densification based on particle packing and its effect on lightweight Al2O3-MgO castables

For lightweight refractory containing lightweight aggregates, the properties of the matrix are decisive to its performance. In the present work, Dinger–Funk equation was adopted to calculate the theoretical packing density of a castables matrix based on Stovall linear packing model and to design its particle size distribution. Four lightweight Al₂O₃-MgO castables with different particle size distribution (represented by q-value) were prepared and examined. Results show that a suitable q-value was needed to ensure acceptable properties including sintering characteristics, strength and slag resistance, which deteriorated distinctly at high q (>=0.31). For the sample with q=0.28, the matrix showed dense and uniform mirostructure, and the properties of castable reached a favourable compromise among sintering characteristics (apparent porosity=14.8%, bulk density=3.02 g cm⁻³, permanent linear change<0.6%), strength (cold modulus of rapture=12.4 MPa, cold crashing strength=155.5 MPa), and resistance against both slag corrosion (I_c=22.4%) and penetration (I_p=11.5%). The sample with q=0.25 showed the highest strength and resistance against slag corrosion (matrix dissolved in slag), but its slag penetration resistance was lower due to the existence of cracks between aggregates and matrix.     

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.     

Preparation and characterization of porous alumina ceramics through starch consolidation casting technique

This work aims at studying the influence of thermal treatment on the microstructure, resistivity and technological properties of porous alumina ceramics prepared via starch consolidation casting (SCC) technique. Colloidal suspensions were prepared with three different contents of alumina solid loading (55, 60 and 65 mass%) and corn starch (3, 8 and 13 mass%). The sintered samples at 1400, 1500, 1600 and 1700 °C, show open porosity between 46 and 64%, depending on the starch content in the precursor suspensions and sintering temperature. The pore structures were analyzed by SEM. The effect of corn starch content on the apparent porosity, pore size distribution, linear shrinkage and electrical resistivity as well as cold crushing strength of the sintered porous alumina ceramics was also measured. These porous alumina ceramics are promising porous ceramic materials for using in a wide range of thermal, electrical and bioceramics applications as well as filters/membranes and gas burners, due to their excellent combination properties.     

Preparation and thermal shock resistance of corundum-mullite composite ceramics from andalusite

Corundum-mullite composite ceramics have high hardness, small plastic deformation and other excellent performances at high temperature. Corundum-mullite composite ceramics were fabricated from andalusite and α-Al₂O₃ by in-situ synthesis technology. Effects of mullite/corundum ratio and sintering temperatures on the water absorption, apparent porosity, bulk density, bending strength, thermal shock resistance, phase composition and microstructure of the sample were investigated. Results indicated that the in-situ synthesized mullite from andalusite combined with corundum satisfactorily, which significantly improved the thermal shock resistance as no crack formed after 30 cycles of thermal shock (1100 °C-room temperature, air cooling). Formula A4 (andalusite: 37.31 wt%, α-Al₂O₃: 62.69 wt%, TiO₂ in addition: 1 wt%, mullite: corundum=6:4 in wt%) achieved the optimum properties when sintering at 1650 °C, which were listed as follows: water absorption of 0.15%, apparent porosity of 0.42%, and bulk density of 3.21 g⋅cm⁻³, bending strength of 117.32 MPa. The phase composition of the sintered samples before and after thermal shock tests were mullite and corundum constantly. The fracture modes of the crystals were transgranular and intergranular fractures, which could endow the samples with high thermal shock resistance.     

Influence of additives on nano-SiC whisker formation in alumina silicate–SiC–C monolithic refractories

This study describes the effect of additives on increasing cold crushing strength (CCS) and bulk density (BD) of alumina silicate–SiC–C monolithic refractories. Two series of carbon-containing monolithics were prepared from Iranian chamotte (sample A) and Chinese bauxite (sample B), as 65 wt.% in each case together with, 15 wt.% SiC-containing material regenerates (crushed sagger) and 10 wt.% fine coke (a total of 90% aggregate) and 10 wt.% resole (phenol formaldehyde resin) as binder. Different values of additives (such as silicon and ferrosilicon metal) are added to the mixture and BD, apparent porosity and CCS are measured after tempering at 200 °C for 2 h and firing at 1100 °C and 1400 °C for 2 h. At low temperature of 200 °C, Si and ferrosilicon contribute to the formation of stronger cross-linking in the resit structure and provide CCS values of as high as about 65 MPa. At 1400 °C, SiC whiskers of nano-sized diameter are formed due to the presence of Si and FeSi₂ and improve CCS values of as high as about 3–4 times in sample containing 6 wt.% ferrosilicon metal as additive.     

Dry pressed ceramic tiles based on fly ash–clay body: Influence of fly ash granulometry and pentasodium triphosphate addition

Fly ash from brown coal (70 wt.%) and stoneware clay (30 wt.%) were used for the dry pressed ceramic tiles (according to EN 14411) raw materials mixture. The effects of fly ash milling and pentasodium triphosphate addition as a deflocculant and fluxing agent on the properties of green body (flexural strength, bulk density) and fired body (EN ISO 10545—water absorption, bulk density, true density, apparent porosity, flexural strength, frost resistance) were studied and explained as a function of the firing temperature (1000–1150 °C). Fly ash milling (corresponding to 5 wt.% residue of fly ash grains on 0.063 mm sieve) increased the sintering abilities of the fly ash–clay body. A similar effect was achieved by 1.3 wt.% pentasodium triphosphate (PST) addition with an increase in green body flexural strength and a decrease in water content of the granulate. Fly ash–clay bodies can be frost resistant with water absorption above 10% due to positive pore size distribution, which were examined using the high-pressure mercury porosimetry method.     

Densification behavior and properties of alumina–chrome ceramics: Effect of TiO2

Highly dense alumina–chrome bodies with low porosity are usually used as corrosion and thermal resistant refractories. Alumina–chrome refractory with molar ratio 1:1 was developed using chemical grade hydrated alumina and chromium (III) oxide by conventional sintering route. Batch materials were attrition milled, isostatically pressed and sintered in the temperature range from 1000 °C to 1700 °C with 2 h soaking at peak temperature. Phase development of the sintered materials with temperature was studied by X-ray diffraction. Sintering temperature, sintering condition and addition of sintering aid (TiO₂) have immense effect on the densification of the alumina–chrome refractory. Highly dense alumina–chrome refractory with almost nil apparent porosity was developed at 1500 °C in reducing atmosphere. Flexural strength of the sintered materials at room temperature and at 1200 °C was also measured. 1 wt% TiO₂ gives the optimum result with respect to densification and flexural strength.     

Mechanical behaviour of a low-clay translucent whiteware

A novel low-clay translucent whiteware body, using mostly non-plastic prefired materials and only a small amount of clay, was fabricated by slip casting and the effect of slip's solid content and sintering temperature on the mechanical behaviour was investigated. The degree of densification in the sintered specimens was determined by measuring the bulk density. The mechanical behaviour was determined by measuring the flexural strength and fracture toughness. Young's modulus and hardness were also measured. X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies were carried out to analyse the microstructure.The flexural strength and fracture toughness increase with both increasing the slip's solid content and the sintering temperature up to a certain level, but further increase in solid content and sintering temperature had an adverse effect on the properties. The maximum flexural strength (∼135 MPa) and fracture toughness (∼1.85 MPa m^1/2) values were attained with specimens produced from a slip having 45 vol.% solid content at a sintering temperature of 1350 °C. It was found that the amount and distribution of closed pores, their size and possible link with each other control the flexural strength and fracture toughness of the low-clay translucent whiteware.     

The physical and mechanical properties of alumina-based ultralow cement castable refractories

In this study, the effects of the type of alumina on the physical, chemical and mechanical properties of the ultralow cement castable (ULCC) refractories were investigated. Brown fused alumina, tabular alumina and rotary bauxite-based ULCC refractories were prepared by mixing each type of alumina with silicon carbide, carbon, cement, metallic silicon and microsilica. The density, porosity and cold crushing strength (CCS) of the refractory castables were measured after drying at 110 °C for 24 h and firing at 1450 °C for 5 h. The slag penetration resistance of the refractory castables was determined using slag corrosion tests. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD) were used to characterize the castables. It was found that all three refractory castables had strong slag penetration resistance and that the tabular alumina-based refractory castable had the largest specific cold crushing strength with an acceptable percent of porosity among the refractory castables.     

Sintering behavior of waste serpentine from abdasht chromite mines Abdasht chromite mines and kaolin blends

The aim of this study is investigating on sintering behavior of Abdasht waste serpentine and kaolin blends. According to this, three formulations of dry milled waste serpentine with 25%, 50% and 75% high grade kaolin were wet milled. The slurries were then dried, sieved and uniaxially pressed at 100 MPa and fired for 2 h soaking time at temperatures between 1100 and 1400 °C. Sintered samples were investigated by bulk density, apparent porosity, water absorption, linear shrinkage and phase changes with raising temperatures in order to characterize their sintering process. It was revealed that all samples were starting to melt at 1350 °C and the sintering was completed for all specimens at 1300 °C. The only phases of fully sintered samples were cordierite and enstatite. Cordierite concentration, however, increased with enhancing kaolin percentage in composition. The results of this study can introduce Abdasht waste serpentine as magnesium silicate source into the ceramic industries and may help to solve environmental problems caused by several million tones wastes in Abdasht chromite mines.     

Mechanical properties of porous ceramic scaffolds: Influence of internal dimensions

Highly porous ceramic scaffolds have been fabricated from a 70% SiO₂–30% CaO glass powder using stereolithography and the lost-mould process combined with gel-casting. After sintering at 1200 °C the glass crystallised to a structure of wollastonite and pseudowollastonite grains in a glassy matrix with a bulk porosity of 1.3%. All scaffolds had a simple cubic strut structure with an internal porosity of approximately 42% and internal pore dimensions in the range 300–600 μm. The mean crushing strength of the scaffolds is in the range 10–25 MPa with the largest pore sizes showing the weakest strengths. The variability of scaffold strengths has been characterised using Weibull statistics and each set of scaffolds showed a Weibull modulus of m≈3 independent of pore size. The equivalent strength of the struts within the porous ceramics was estimated to be in the range 40–80 MPa using the models of the Gibson and Ashby. These strengths were found to scale with specimen size consistent with the Weibull modulus obtained from compression tests. Using a Weibull analysis, these strengths are shown to be in accordance with the strength of 3-point bend specimens of the bulk glass material fabricated using identical methods. The strength and Weibull modulus of these scaffolds are comparable to those reported for other porous ceramic scaffold materials of similar porosity made by different fabrication routes.     

The influence of silica nanoparticles addition on the physical,mechanical, thermo-mechanical as well as microstructure of Mag-Dol refractory composites

The high hydration potential of CaO and MgO phases restricted the application of Mag-Dol refractory composites. In this study, the impact of nano-silica (SiO₂) addition on the physical, mechanical, thermo-mechanical as well as microstructure of Mag-Dol refractory composites is investigated. Mag-Dol compositions were prepared by using calcined dolomite and magnesite particles (micron, 0–1, 1–3, 3–5, and 5–8 mm), liquid resin, and 0, 0.5, 1, 1.5, 2, and 2.5 wt% nano SiO₂ as additives. Specimens were heated up to 1650 °C for the 3 h soaking period. Fired specimens were characterized by physical (apparent porosity, bulk density, and hydration resistance), mechanical (cold crushing strength), and thermo-mechanical (flexural strength at 1200 °C) measurements. XRD and SEM/EDS analysis were done to study phases and microstructure analysis of the fired samples, respectively. Results showed that by adding up to 2.5 wt% nano-SiO₂, due to the formation of CaO·MgO·2SiO₂ (Diopside), 2CaO·MgO·2SiO₂ (Akermanite), and CaO·MgO·SiO₂ (Monticellite) phases, physical and mechanical properties were enhanced. But the highest flexural strength value is achieved for 1 wt% nano-SiO₂ containing sample.     

Sinterability,microstructure and compressive strength of porous glass-ceramics from metallurgical silicon slag and waste glass

Porous glass-ceramics have been prepared by the direct sintering of powder mixtures of metallurgical silicon slag and waste glass. The thermal behavior of silicon slag was examined by differential thermal analysis and thermogravimetry to clarify the foaming mechanism of porous glass-ceramics. The mass loss of silicon slag below 700 °C was attributed to the oxidation of amorphous carbon from residual metallurgical coke in the silicon slag, and the mass gain above 800 °C to the passive oxidation of silicon carbide. The porosity of sintered glass-ceramics was characterized in terms of the apparent density and pore size. By simply adjusting the content of waste glass and sintering parameters (i.e. temperature, time and heating rate), the apparent density changed from 0.4 g/cm³ to 0.5 g/cm³, and the pore size from 0.7 mm to 1.4 mm. In addition to the existing crystalline phases in the silicon slag, the gehlenite phase appeared in the sintered glass-ceramics. The compressive strength of porous glass-ceramics firstly increased and then decreased with the sintering temperature, reaching a maximal value of 1.8 MPa at 750 °C. The mechanical strength was primarily influenced by the crystallinity of glass-ceramics and the interfaces between the crystalline phases and the glassy matrix. These sintered porous glass-ceramics exhibit superior properties such as light-weight, heat-insulation and sound-absorption, and could found their potential applications in the construction decoration.     

Low-cost porous mullite ceramic membrane supports fabricated from kyanite by casting and reaction sintering

Porous mullite supports are firstly fabricated by casting and reaction sintering based on kyanite with Al(OH)₃ as porogenic agent. The effects of composition and sintering temperature on phase evolution, microstructure, apparent porosity, pore size distribution, linear shrinkage, gas permeation flux and mechanical property of supports are systematically investigated. Results show that the mullitization of kyanite generates needle-like mullite crystals, which form skeleton structures and improve the apparent porosity and strength of supports. Al(OH)₃ addition not only promotes the formation of needle-like mullite but also enhances the apparent porosity of supports. Temperature promotes the development of mullite, from 1450 to 1500 °C, the amount and size of needle-like mullite crystals increase, ≥1500 °C, they reveal columnar morphology. The support prepared with kyanite+40 wt%Al(OH)₃ sintered at 1500–1550 °C exhibits high apparent porosity, good gas permeation flux, excellent mechanical performance and interlocked network structure composed of well development needle-like mullite.     

The influence of nano-silica and zircon additions on the sintering and mechanical properties of in situ formed forsterite

The influence of nano-silica and zircon additions on the sintering and mechanical properties of in situ formed forsterite fired at 1550 °C for 2 h was investigated. The results indicated that, nano-silica improved in situ formed forsterite at the firing temperature, while zircon additions enhanced the sintering of the investigated samples. XRD analysis and SEM examination observed a good crystallinity of in situ formed forsterite with nano-silica and/or zircon additions. Densification parameter (BD ∼3.22 g/cm³ and AP ∼5.82%), cold crushing strength (CCS ∼285 MPa) and micro-hardness (Hv ∼660) were enhanced with zircon additions.     

Reaction sintering of β-tricalcium phosphates and their mechanical properties

Various kinds of calcium oxides, carbonates and phosphates were used as the raw materials, and β-TCP ceramics was fabricated by reaction sintering at 1100 °C, and the sinterability, the reaction sintering behavior and mechanical properties of reaction-sintered β-TCP were investigated. Reaction-sintered bodies using CaHPO₄ + HAp consisted of single β-TCP phase, and bulk density and bending strength increased with extending sintering time. On the contrary, normal-sintered β-TCP synthesized using CaHPO₄ + HAp did not change in bulk density and bending strength with extending sintering time. Reaction-sintered body using CaHPO₄ + HAp as the raw materials showed higher bulk density and bending strength than normal sintered β-TCP.