Castable Samples Research Articles

Dependence of the properties of castable samples from low­cement silicon carbide castable before and after heat treatment on the action of alkalis at 1300 °C

The main properties and alkali resistance of silicon carbide castable samples heat­treated at temperatures of 110 and 1000 °C were investigated in comparison with the chamotte product, which is used for the same purposes in lining thermal units in contact with an aggressive environment.
 Samples of carbide­silicon castable after heat treatment at temperatures of 110 and 1000 °C were characterized by the following physical and chemical properties, respectively: wt. %: SiC — 84.60 and 79.70, Al2O3 — 13.65 and 13.60, CaO — 1.35 and 1.33; cold crushing strength — 42.2 and 67.5 N/mm2.
 The thermal conductivity of a silicon carbide castable sample heat­treated at a temperature of 1000 °C was determined to be 8.20 W/(m·K) and its thermal shock resistance (950 °C — water) was determined to be > 20 heat cycles.
 The studies of alkali resistance (to a mixture of alkalis Na2CO3 and K2CO3 in a ratio of 25:75) at a temperature of 1300 of carbide­silicon castable samples heat­treated at temperatures of 110 and 1000 °C in comparison with a sample of chamotte refractory, it was found that castable samples heat­treated at temperatures of 110 and 1000 °C are characterized by quite close alkali resistance (weight increase of 1.42 % and 1.5 %, respectively) and significantly higher (by ~ 4.3—4.5 times) alkali resistance than chamotte samples (weight increase of 6.5 %).
 It was established that castable samples after heat treatment at a temperature of 1000 °C after interaction with alkalis were characterized by a 1.6 times higher cold crushing strength compared to a castable sample heat­treated at a temperature of 110 °C.
 The study’s results made it possible to evaluate the performance of the silicon carbide castable in comparison with chamotte refractory during its operation in thermal units at consumers.

Preparation and characterization of low-cost refractories for aluminum-melting-furnace linings from ferrotitanium slag

Management and utilization of solid waste are necessary for sustainable development in the metallurgical industry. This study focuses on the use of ferrotitanium slag (FTS) as a low-cost source of hibonite in the refractory lining of an aluminum-melting furnace. FTS enhances the resistance of castable samples to cryolite melt penetration but reduces their bulk density and cold mechanical strength. To explain this enhancement mechanism, we present a model in which the Al2O3 and SiO2 components in the castable samples react with the cryolite melt to form a high-viscosity barrier layer against the molten cryolite. Moreover, the well-developed calcium hexaaluminate crystals in FTS, which show excellent resistance against Na+ and F−, prevent the penetration of cryolite melt through the FTS aggregate area, and reduce the penetration speed. By promoting high-efficiency resource utilization, this study contributes to the development of a circular economy in the metallurgical industry.

Restructuring-diffusion mechanism of calcium alumino-titanate in CaAl12O19–MgAl2O4–Al2O3 castables

Calcium alumino-titanate (CAT), a secondary material obtained from ferrotitanium slag, was used as a hibonite source to prepare CaAl 12 O 19 –MgAl 2 O 4 –Al 2 O 3 castables. The restructuring effect of CAT aggregate was compared by replacing tabular alumina aggregates with CAT aggregates of different particle sizes. The effects of CAT particle size on cold mechanical strength and thermal shock resistance of CaAl 12 O 19 –MgAl 2 O 4 –Al 2 O 3 castables were studied. The results showed that CAT aggregates with particle size of 5–3 or 3–1 mm led to more internal cracks or pores and reduced the cold mechanical strength of the castable samples fired at 1600 °C for 3 h. The use of CAT aggregates with particle size of 1–0 mm led to the formation of a well-bonded CAT aggregate and matrix, improving the cold mechanical strength and thermal shock resistance of the castable samples fired at 1600 °C for 3 h. The enhancement mechanism of fine CAT aggregates in this process was proposed based on the sintering of the matrix–aggregate interface with the formation of Ca(Al, Mg, Ti) 12 O 19 .

Properties dependence of castable samples from low-cement silicon carbide castable on the firing temperature and the gas atmosphere

The influence of firing temperature of 1000, 1100, 1200, 1300, 1400, 1580 °C and atmosphere (air and reducing) on the main properties of castable samples from low-cement silicon carbide castable grade of “NKBS” with a combined additive has been investigated.
 It was shown that, in air with an increase in the firing temperature in the range 1000—1580 °C, the content of silicon carbide in the castable samples decreases. Up to temperatures of 1200—1300 °C, the content of silicon carbide in the castable samples practically does not change, and oxidation slows down due to the formation of a viscous melt of a glassy phase of an alumina silicate composition on the surface of silicon carbide grains. An increase in the firing temperature above 1300 °C leads to a more intense oxidation of silicon carbide grains to SiO2, which causes a change in the porosity, density and strength of the samples. As a result of the carried out studies, it was found that, the using temperature in the air of the above mentioned low-cement silicon carbide castable should not exceed 1300 °C.
 It was found that, after firing in a reducing atmosphere in the temperature range of 1000—1580 °C in the samples of silicon carbide castable, the content of silicon carbide remains almost the same, and the density and strength increase, which ensures the efficiency of its use for lining thermal units operating in a reducing atmosphere up to temperatures of 1580 °C.

Dependence of the structural and rheological properties of a refractory vibrocasting low-cement alumina-chromia castable and the properties of samples from it on the type of dispersant additives

The effect of a new dispersant additive (on a polycarbonate basis) in comparison with a known additive based on polyethylene glycol on the structural and rheological properties of low-cement alumina-chromia castables with a moisture content of 4.5 % (flowability during vibration of freshly prepared castables and the kinetics of changes in the strength of their structure depending on holding time) and on the main properties of castables has been studied.
 Studies of the freshly prepared castables flowability during vibration have established that the additive based on polyethylene glycol has a 1.3 times greater thinning ability compared to the additive on a polycarbonate basis, but the latter is also suitable for the manufacture of vibrocasting aluminachromia castable. It has been shown that the shelf life of the mass with the addition based on polyethylene glycol is 5.5—6.0 hours, and the mass with the addition on the polycarbonate base is 3.0—3.5 hours. During the indicated time, these masses are still mobile and suitable for vibration casting.
 The formation mechanism of a coagulation-condensation-crystallization structure in the mass with the dispersant additive based on polyethylene glycol has been established. The final mechanism of structure formation in the mass with the dispersant additive on a polycarbonate basis is to be studied.
 It was shown that with the introduction of additives studied in this work into the low-cement alumina-chromia castable, castable samples with the additive based on polyethylene glycol were characterized by significantly higher property indices than castable samples with the additive on a polycarbonate base, however, the latter samples also have property indices that meet the specified requirements of the technical specifications. The properties of prepared dried unfired samples with the additive based on polyethylene glycol and on a polycarbonate base, respectively: apparent density — 3.12 and 2.93 g/cm3; cold crushing strength after hydration hardening for 7 days — 64 and 56 N/mm2.
 The properties of samples after firing at a temperature of 1580 °C: apparent density — 3.06 and 2.91 g/cm3; open porosity — 19.8 and 21.9 %; cold crushing strength — 196 and 70 N/mm2, respectively, with the additive based on polyethylene glycol and on a polycarbonate basis. But the properties of samples with the additive on a polycarbonate base are quite high, and it can be used, if necessary, as an alternative additive.
 As a result of the carried out research, the raw material base for the production of coarse-grained alumina-chromia castable and castable products from it has been expanded.

Investigation on the influence of quartz-containing additives type on the properties of castable samples from a low-cement alumina-silica castable before and after thermal treatment

Ensuring compliance of the physical and mechanical properties of castable products with differentiated operating requirements is achieved by optimizing the grain size composition of the fireclay aggregate, using a microfiller with an increased content of aluminum oxide and reactive additives in the matrix component, choosing effective deflocculants and plasticizers additives, etc.
 The influence of aggregate type, its grain size composition, quartzcontaining additives (quartz-containing additive FS or a mixture of combined grinding of ɑ-form alumina and quartz-containing additive S), amount of cement grade of “VGTs-73” on water demand, flowability of freshly prepared mass and properties of castable samples from low-cement alumina-silica castable after thermal treatment at temperatures of 110, 800 and 1400 °C. It was found that, with a partial replacement of fireclay in the castable with brown alumina, an addition introduction of ɑ-form alumina and a quartzcontaining component, an increase in the amount of high-alumina cement grade of “VGTs-73” from 5 to 8 %, the water demand decreases and the castable flowability increases, the strength of castable samples increases before and after thermal treatment.
 As a result of the carried out research, a technology for the manufacture of low-cement alumina-silica castable with Al2O3 content 52—56 % has been developed and mastered in JSC “URIR named after A. S. Berezhnoy”.

Feasibility of alkali-activated materials as a binder for refractory castables

This study aims to investigate the feasibility of alkaliactivated slag as a sole binder for refractory castables. The prepared castable samples were subjected to different firing temperatures at 850,1100 and 1300 °C. The mineralogical compositions of the fired castables were investigated using X -ray diffraction (XRD). The microstructure was examined using scanning electron microscope (SEM). Also, sintering parameters, mechanical properties as well as refractory properties in terms of permanent linear change (PLC), refractoriness under load (RUL) and thermal shock resistance (TSR) were tested. A variation in sintering parameters and mechanical properties is observed by changing firing temperatures. The geopolymer-based castables show a significant PLC only at 1300 °C, maximum expansion of 0,56 % under load and raising temperature and finally a good TSR up to 15 cycles. Phase analysis of fired castables confirmed, besides the main phases in aggregates, the formation of low melting phases at 850 and 1100 °C. Hibonite and anorthite were obviously observed with increasing the firing temperature. A needle-shaped structure is noticed embedded in glassy matrix at 110 °C, but plat-like structure of hibonite is observed at higher temperature. Overall, the results revealed that alkaliactivated slag cement is a promising binder for refractory castables at high temperature.

Effect of Hollow Corundum Microspheres Additive on Physical and Mechanical Properties and Thermal Shock Resistance Behavior of Bauxite Based Refractory Castable.

This paper analyses the effect of hollow corundum microspheres (HCM) on both physical-mechanical properties (density, ultrasonic pulse velocity, modulus of elasticity, and compressive strength) and thermal shock resistance behavior of refractory medium cement castable with bauxite aggregate. Moreover, the scanning electron microscopy (SEM) results of HCM and refractory castable samples are presented in the paper. It was found that the replacement of bauxite of 0–0.1 mm fraction by HCM (2.5%, 5%, and 10% by weight of dry mix) had no significant effect on the density and compressive strength of castable, while the modulus of elasticity decreased by 15%. Ultrasonic pulse velocity (Vup) values and the visual analysis of the samples after thermal cycling showed that a small amount of HCM in composition of refractory castable could reduce the formation and propagation of cracks and thus increase its thermal shock resistance.

Partial replacement of a traditional raw material by blast furnace slag in developing a sustainable conventional refractory castable of improved physical-mechanical properties

The exponential growth of industrial waste is a problem that has forced the development of clean technologies for their appropriate management. About 0.25–0.30 tons of slag is generated per ton of crude steel or pig iron in blast furnaces. In this research, blast furnace slag was recycled to progressively replace the fine fraction of flint clay at 0, 5, 10, 15, and 20 wt% in laboratory-scale conventional refractory castable samples fired at 120, 850, 1050, and 1400 °C. The physical properties were measured by linear shrinkage, bulk density, apparent porosity, and water absorption. The mechanical behavior was evaluated by cold crushing strength and cold modulus of rupture. Microstructural and mineralogical characteristics were analyzed by scanning electron microscopy and X-ray diffraction, respectively. The incorporation of 10 wt% of blast furnace slag allowed the development of a slag-containing conventional refractory castable with improved properties compared to those of the reference castable, such as a bulk density of 2.61 g/cm3, percentages of apparent porosity, and water absorption of 10.64% and 4.08%, respectively, and mechanical resistance of 94.5 MPa. A denser microstructure via ceramic body’s porosity reduction is reached by the anorthite crystallization from a silica-rich liquid phase with CaO contents. This densification mechanism improves mechanical resistance by about 74%. The anorthite phase and the physical and mechanical characteristics exhibited by the sustainable refractory castable are attractive and suitable for its possible application in the aluminum industry as smelting furnace lining.

The restructuring-sintering mechanism of calcium alumino-titanate in tundish permanent lining castables

Calcium alumino-titanate (CAT) is widely used as a refractory raw material in China. It is obtained by the recycling of non-ferrous metallurgical solid waste and is often used in high-temperature oxidizing environments. The main crystal phase of CAT is the Ca(Al,Ti)12O19, which is unstable under high temperature oxidizing environments and preferentially forms a new phase system. In this study, CAT raw materials were prefired in air to facilitate the reconstructing process from Ca(Al,Ti)12O19 to CaAl12O19 in air. Herein, the effects of the prefiring of CAT raw materials on the mechanical properties and thermal conductivity of tundish permanent lining castables were studied and mechanism of restructuring-sintering of CAT aggregates in this process was proposed based on it. The results showed that the use of prefired CAT raw materials reduced the bulk density and mechanical strength of the castable samples after baking at 110 °C for 24 h. After firing at 1400 °C for 3 h, these samples showed greater volume shrinkage and higher mechanical strength than those prepared using CAT raw materials. The use of prefired CAT raw materials reduced the formation of anorthite in the castable matrix, thereby reducing the pores formed by liquid–phase transfer in the matrix; however, it resulted in castables with higher thermal conductivity than those prepared using unfired CAT raw materials.

Analysis on mechanism of explosive spalling resistance of aluminum powder on ladle corundum-based refractory castable

The effect of the particle size (50–325 mesh) and content (0–0.1 wt%) of metallic aluminum powder on the explosive spalling resistance of corundum-based refractory castables were investigated in this study based on air permeability, pore size distribution, heat release, fracture energy and microstructural analyses. The experimental results show that the addition of Al powder significantly improves their explosive spalling resistance. As observed, the explosive spalling resistance of the castables was mainly influenced by the permeability mechanism and the fracture energy. Permeable paths were generated in the microstructure of the sample during the overlapping period between the curing process and the H2 gas forming one, derived from the Al–H2O reaction. The highest permeability level was obtained when 0.075 wt % of Al powder with size of 100 mesh was incorporated into the corundum-based refractory castables. At the same time, the fracture energy of the castable samples was increased accordingly.

Improvements of properties of alumina-silica refractory castables by alumina-coated aluminum powders

Castables have been widely used in repairing furnace linings owing to their numerous verified benefits. Al2O3 coated Al powders (ACAPs) were prepared to prevent their hydration during castable preparing. The effect of ACAPs on the properties of cement-bonded Al2O3–SiO2 refractory castables under simulated coke drying quenching (CDQ) conditions was investigated. The results show AlN plate in situ formed at 800 °C and grew into whiskers and fibers at 1000 °C because metal Al melted and expanded which resulted to the cracks of Al2O3 shell, Al liquid flew out and directly reacted with N2. The strength and thermal shock resistance of castable samples were greatly improved by adding ACAPs while keeping minor expansion. The toughness of castables was greatly enhanced after fired at 1000 °C owe to AlN whiskers and fibers in situ synthesized in the castable matrix. The optimum amount of ACAPs is 9 wt % which provides higher strength and excellent toughness.

Waste rice husk ash derived sol: A potential binder in high alumina refractory castables as a replacement of hydraulic binder

This work provides a new source of binder system, i.e., waste rice husk ash (RHA) derived sol for unshaped refractories. A facile (alkali extraction) route is used to synthesize the sol from the waste RHA. The sol consists of single phase amorphous silica with average particle size around 22 nm. The estimated cost of 30 wt% solid containing RHA derived sol per litre is around 9.35 $. The temperature of mullite (3Al2O3·2SiO2) formation from waste derived sol and reactive alumina containing system is 1250 °C or higher. The sol (30 wt% SiO2) and the dry sol (99.32 wt% SiO2) is used to formulate the high alumina castable samples. The sintering is performed at 1400 °C, 1500 °C and 1550 °C. The different physico-mechanical characterizations and blast furnace slag (BFS) corrosion test at 1500 °C are comprehensively investigated with the prepared castables. The properties of 5 wt% silica (3 wt% from sol and 2 wt% dry sol) containing castable specimens are also equated with the commercially available different silica containing high alumina castable systems. The obtain results are good agreements with the source data. These promising characteristic confirms the possibility of using waste RHA derived sol for manufacturing of cement free high alumina castable and leads to the economic and ecological benefits in the refractory industries.

Molten salt synthesis and characterization of SiC whiskers containing coating on graphite for application in Al2O3-SiC-C castables

The wettability of graphite was significantly ameliorated by applying a molten liquid phase method using silicon powder as a raw material to form a SiC whisker coating on a graphite surface. The effect of replacing the ball pitch with coated graphite (CG) on the microstructure, apparent porosity, physical properties, oxidation resistance, and slag corrosion resistance of Al2O3–SiC–C (ASC) refractory castables was investigated by a scanning electron microscope, an energy-dispersive X-ray spectroscope, a transmission electron microscope, and other dedicated equipment. The results show that a uniform and continuous SiC whisker coating was formed on the surface of the graphite, and the contact angle with water changed from 89° to only 6.5°. The apparent porosity of the ASC specimen with 2 wt.% of CG decreased by 12.7%, and the cold modulus of rupture increased from 7.08 to 13.30 MPa with increasing CG content from 0 to 2 wt.%. Further, the sample containing 2 wt.% of CG exhibited the largest force and displacement values. Owing to the presence of SiC whiskers on the surface, the ASC castable samples with 2 wt.% of CG have the best antioxidation performance, and those with 1 wt.% of CG have the best anti-slag performance.

Microwave and conventional treatment of low-cement high-alumina castables with different water to cement ratios; Part I. Drying

The aim of this study was to determine the effect of the microwave and conventional drying method on the strength, porosity and composition of low-cement alumina castables with various water to cement ratios (wcr). High-alumina low-cement castable samples were prepared with different w/c ratios: 0.64, 0.75, 0.82 and 1.13. Changes in wcr were effected through volumetric replacement of cement with 0–0,045-mm tabular alumina having a comparable particle size. Water content in all the composition was constant (4,5%). After curing, the samples were dried conventionally in a laboratory electric drier or in a laboratory microwave drier. After drying open porosity and modulus of rupture were determined. The pore size distribution, pore median and tortuosity of the samples were measured by the mercury porosimetry method. Phase composition was determined using X-ray diffraction. The Rietveld method was used for quantitative analysis. It was found that at low wcr (0.62) the main hydrate formed in the castable was C3AH6, which caused a release of a smaller amount of water during the drying process, mainly pore water, resulting in lower open porosity and lower pore size than in the castables with a high wcr (1.13). At a low wcr, the strenght of castable was higher due to a higher amount of hydrates, low porosity and small pore size. On the other hand, at a high wcr, the strength of castable was lower owing to a higher amount of water released in the drying process, which led to loosening the structure of castable. With an increased water-to-cement ratio the degree of CA2 hydration decreased. The temperature rise due to cement hydration probably influenced the kinetics of this process.

Design of Experiments (DOE) applied to high-alumina calcium aluminate cement-bonded castables

Using statistical methods (such as Design of Experiments – DOE) is an important issue due to the increasing demand to reduce costs and optimize properties of different processes and products. In order to assess the feasibility of these techniques in the refractory field, the Response Surface Method (RSM) and Tukey test were applied to optimize the modulus of rupture of high-alumina calcium aluminate cement (CAC)-bonded castables after curing and drying, considering three continuous factors (the amount of water, cement and dispersant) and a categorical one (the drying process). In addition to the cold mechanical strength, the flowability and apparent porosity of all the castable samples were also measured. The Tukey test pointed out the composition with the highest strength after curing and after drying. Based on the attained RSM models, the first one (considering all four factors) indicated the composition with the highest modulus of rupture after curing, but it could not properly predict the castables' properties after drying. The other two RSM models analysed different processing conditions (post curing and post drying, separately) highlighting that the complexity of the drying stage related to the hydration of the cement phases invalidated the models used for the post drying condition. The major difficulty in using statistical techniques comes from the transient and non-equilibrium conditions experienced by refractory ceramic castables during their processing and working conditions.

Microstructure evolution during the heating process and its effect on the elastic properties of CAC-bonded alumina castables

In this paper, the alumina-based refractory castables were prepared using calcium aluminate cement (CAC) as binder. The phase composition and microstructural evolution of the castable samples during the heating process were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis (DTA), and thermal expansion analysis. The relationship between the microstructural characteristics of castables and the properties of elastic modulus was investigated. The results showed that, after the castables were cured at room temperature for 24h and dried at 110°C for 24h, the main phases present after hydration were Al2O3, C3AH6, and AH3. C3AH6 and AH3 would be transformed into C12A7 and AlO(OH) or Al2O3, as temperature increased from 110°C to 800°C, the microcrack caused by decomposition of the hydrates will led to the lower values of both the elastic modulus and strength of the castable samples. From 900°C to 1000°C, CA was recrystallized through the reaction between C12A7 and A, resulting in an increase in elastic modulus and strength. Between 1100°C and 1200°C, CA2 would be formed, which resulted in a volume expansion of the material and a slight increase in elastic modulus. Finally, the CA6 phase was formed as the temperature increased to 1400°C. The enhancement of the strength and elastic modulus of the castable samples could be attributed to their strong interlocking bonds between platelet CA6 and matrix particles at high temperature.

Application of polycarboxylate in low cement castables

The performance of low cement castable with polycarboxylate was studied by comparing the effects of polycarboxylate superplasticisers KS-JS70 and LMS-P and sodium tripolyphosphate on castable samples. The fluidity, bulk density, apparent porosity, compressive strength and bending strength of the castable samples at room temperature were determined, and the scanning electron microscopy images of the samples under different temperatures were analysed. Results showed that the use of polycarboxylate superplasticisers KS-JS70 and LMS-P reduced the water consumption, improved the fluidity, increased the compressive bending strength at room temperature and enhanced the physical properties of the castable samples. The slurry rheology and ζ-potential of powder particles were measured to analyse the mechanism of polycarboxylate in low cement castables.

Research on Appropriate Additives of Recycling Ladle Lining Materials for Al<sub>2</sub>O<sub>3</sub>-SiC-C Iron Runner Castable

In this experiment, preparation of samples were based on original ratio of bauxite matrix Al2O3-SiC-C castable for iron runner, and partial bauxites of size 8-5 mm, 5-3 mm, 3-0 mm and <0.074 mm were replaced by the same size granular recycling Al2O3-MgO-C materials from spent ladle bricks, respectively. Si3N4 and B4C were introduced as appropriate additives. Effects of their amount on the performance of Al2O3-SiC-C castable for iron runner were investigated by testing on physical properties and erosion resistance of the samples after sintering. The results showed that the introduction of Si3N4 and B4C led to high temperature flexural strength of the recycled castable samples decline, while the right amount could improve the room temperature flexural and pressure strength along with the erosion resistance to blast furnace slag. When introduced 3% of Si3N4and 0.3% of B4C respectively, the comprehensive performance of the samples were the best.

Dryout temperature–vapor pressure profile of polymeric fiber containing refractory castables

Different types of polymeric fibers are added to refractory castables' formulations as drying additives to make dewatering processes easier and reduce the risks of explosive spalling of these low permeability materials. Many works describe how these fibers control the permeability increase in castables before and after their melting-decomposition. The water vapor pressure profile developed inside the structure and how it is modified by the fiber presence has not yet been explored systemically. In the present paper, thin K-type thermocouples were inserted (at different depths) in polymeric fibers (olefin copolymer, polypropylene and aramidic ones) containing castable samples. Their temperature profile and mass loss were recorded during the first heat-up. After combining internal heating and mass loss rate, vapor pressure levels calculated using Antoine's equation, hot air permeametry (HAP) and work of fracture (γWOF) results, novel insights into the mechanisms by which polymeric fibers avoid explosive spalling during castables dewatering were attained.