Influence of structural and phase conversions on the thermomechanical properties of chamotte concrete based on high-alumina cement

Conclusions The dehydration of the calcium hydroaluminates and aluminum hydrates when the concrete is heated causes the binder to lose strength and results in the formation of a porous structure.The structure of the cement block depends on the fineness of the cement. During heating at 1100°C, two processes develop in the binder, viz., solid-phase diffusion sintering accompanied by shrinkage, and coalescence of the pores.The reaction of the cement with the chamotte results in the formation of low-temperature minerals in the form of calcium aluminosilicates, the consequence being that liquid-phase sintering continues accompanied by considerable shrinkage.Above 1200°C and especially above 1300°C a large amount of liquid phase is formed in the concrete and dissolves the crystalline minerals (calcium aluminosilicates, mullite, cristobalite, and corundum).The peak temperature at which the chamotte concrete based on high-alumina cement can be used is 1350°C.Translated from Ogneupory, No. 1, pp. 52–57, January, 1977.     

Evaluating the behavior of some types of ceramics during testing in a stream of high-temperature gas

Conclusions We established the capacity of a range of ceramics to withstand high rates of thermal stress (frequent heating). Thus, the spalling-resistant quartz ceramics and aluminum titanate with low values can withstand repeated frequent heating to temperatures above 1400°C, practically close to their fusing temperatures. Nonspalling-resistant materials based on corundum and mullite chamotte, having high values, withstand frequent heating to much lower temperatures (up to 1200°C). Incorporating into the corundum zircon, aluminum titanate with a reduced value increases the resistance to frequent heating at higher temperatures (1500°C and above). The increase in the content of Al₂O₃ in the mullite chamotte increases its capacity for frequent heating.Translated from Ogneupory, No. 6, pp. 21–24, June, 1987.     

Reaction between zinc oxide and chamotte refractories

Conclusions As a result of chemical reaction between chamotte and zinc oxide, zinc aluminate ZnAl₂O₄ and zinc silicate Zn₂SiO₄ are formed.The mullite is not chemically resistant under the action of zinc oxide. Even at 900°C mullite reacts with zinc oxide, forming ZnAl₂O₄ and SiO₂.In accordance with the quantity of melt formed in the refractory under the action of zinc oxide, high-alumina products should be more resistant to the action of vapors and oxides of zinc at elevated temperatures than chamotte and semiacid refractories.Translated from Ogneupory, No. 7, pp. 36–39, July, 1971.     

Resistance of steel-teeming stoppers

Conclusions In order to make spalling-resistant stoppers which can be used to teem metal satisfactorily, the charge at the refractory shop of the Nizhnly Tagil Metallurgical Combine should include 7–10% chamotte coarser than 2 mm and 45–50% finer than 0.54 mm. The temperature should be raised from 30 to 150° at the rate of 5–7°/hr, and from 800 to 1200° at the rate of 20–25°/hr. The holding time at 1280–1300° should be 10 hours.When laying down new specifications for standards for steel-teeming stoppers, it is essential to stipulate a porosity of 15–20% at refractoriness-under-load of 2 kg/cm² of not less than 1320 or not more than 1380°     

Dependence of the properties of chamotte on the firing regime of the briquette

Conclusions The physicomechanical properties and phase composition of chamotte depends on the composition of the gas medium, the firing temperature, and the cooling regime. In the briquettes fired at 1500°C in an oxidizing medium with the cooling temperature lowered from 1500 to 1200°C, the apparent density increases and the porosity decreases. In the briquettes fired in a reducing medium with the cooling temperature lowered from 1500 to 1300°C, there is a reduction in the apparent density and an increase in the porosity but with a further reduction of the cooling temperature to 1200°C, these properties change in the opposite direction.The apparent density of chamotte and its concentration of mullite is increased with a reduction in the cooling rate. The apparent density of the chamotte from the Polozhsk kaolin under analogous cooling conditions increases more rapidly than that from the Novoselitsk kaolin chamotte.The temperature interval in which the cooling rate effectively increases the apparent density of the chamotte fired in an oxidizing medium is 1500–1300°C. In a reducing medium this interval is shifted 100°C toward lower temperatures.The experiments have shown that an increase in the cooling rate of the fired briquettes at temperatures below 1200°C has only a very slight effect on the apparent density and porosity of the Novoselitsk kaolin chamotte. When the chamotte has been obtained from Polozhsk kaolin, a reduction in the cooling rate at temperatures below 1200°C, and particularly in a reducing medium, has a quite perceptible effect on the increase in apparent density and reduction of porosity of the material.Translated from Ogneupory, No. 3, pp. 22–26, March, 1982.     

Control of aluminum phosphate coating on mullite fibers by surface modification with polyethylenimine

Aluminum phosphate (AlPO₄) is a promising oxidation-resistant and weak interface for ceramic-matrix composites. In this research, AlPO₄ coating was deposited on mullite fibers by an improved liquid-phase method based on electrostatic attraction. A cationic polyelectrolyte, polyethylenimine (PEI), was used for surface modification of mullite fibers. The formation process, phase evolution and microstructure of the coating were studied. The zeta potential of AlPO₄ particles, PEI-adsorbed AlPO₄ particles, and PEI-adsorbed mullite particles was characterized to find the proper pH value for improving electrostatic attraction. The obtained AlPO₄ coating was porous and continuous, whose thickness could be controlled by multiple coating cycles. The relatively low calcination temperature (600 or 1000 °C) was a useful heat treatment method to develop bonding between coating and fiber as well as reduce the fiber strength degradation. The phase transformations of AlPO₄ have little volume change, and cristobalite AlPO₄ is thermal compatible with mullite. Therefore, the coating structure was preserved after calcining at 1200 °C. The technique is also applicable for other fibers contained mullite phase to fabricate high-performance AlPO₄ coated mullite/mullite composites.     

Resistance of blast furnace refractories to the action of aggressive reactants under conditions of an oxidizing gaseous atmosphere

Conclusions Investigations have been made of the resistance of ShPD-41, ShPD-39, and ShUD-37 chamotte refractories to the action of K₂CO₃, Fe₂O₃, blast furnace dust, and initial and final blast furnace slags under conditions of an oxidizing atmosphere. The investigation results showed that iron oxides and slag break down these refractories at 1400–1500°C. Dense ShPD-41 refractory is more resistant to the action of the reactants.The most resistant to the action of slags and iron oxides at 1400–1500°C are silicon carbide refractories with binders of silicon nitride and oxynitride.Translated from Ogneupory, No. 7/8, pp. 24–27, July–August, 1992.     

Sintering mullite — corundum briquette in oxidizing and reducing conditions

Conclusions Rapid sintering of mullite-corundum briquette occurs in the range 1400–1600°. The rate of sintering of mullite-corundum briquette in reducing atmospheres at 1200–1400° is higher than in oxidizing conditions, owing to the formation in these conditions of a large quantity of liquid phase. At temperatures above 1400° the process of reduction of the SiO₂ is accelerated, and volatile SiO is formed preventing sintering of the briquette.Translated from Ogneupory, No. 3, pp. 24–27, March, 1973.     

Silicon carbide refractories with a complex bond of Si2On,SiC, Si3N4

Conclusions The physical and chemical processes taking place during the firing of products obtained from silicon carbide and silicon largely depend on the gaseous medium.When the articles are fired at 1400° C in a nitrogen atmosphere not containing oxygen or containing only small quantities of it (less than 0.05%) a bond is formed of silicon nitride.During the firing of articles at 1400° C and nitrogen atmosphere containing about 2% oxygen, almost no nitride or oxynitride silicon is formed. Silica is formed in the form of cristobalite and a glassy film, and also we get a large amount of the silicon remainning in the free state.When the articles are fired at 1550° C in a coke filling in a flame furnace, a bond of complex composition forms, which consists of cubic silicon carbide, oxynitride (Si₂ON₂), silicon nitride and cristobalite.Translated from Ogneupory, No. 5, pp. 50–56, May, 1967.     

Effect of nickel nitrate on phase changes in mullite and mullite — Corundum refractories when heated in oxidizing and reducing conditions

Conclusions During heating in oxidizing atmospheres of mullite-corundum specimens with an addition of nickel nitrate at 500–900°C, NiO is formed, and an increase in temperature from 900 to 1500°C is characterized by the development of nickel spinel, NiAl₂O₄, the quantity of which increases with an increase in the content of the Ni(NO₃)₂ · 6H₂O, and the firing temperature.The high temperature diffractometric method showed that in the system 3Al₂O₃ · 2SiO₂-NiO heating in oxidizing conditions, and commencing at 1000°C, NiO decomposes the mullite with the formation of aluminonickel spinel, whose quantity rapidly increases with further temperature rise. At 1500°C the mullite is preserved in small quantities.The nickel spinel, formed as a result of heating to 1300°C the mullite-corundum refractory containing 25% Ni(NO₃)₂ · 6H₂O during firing in the silicon carbide mixture and in the hydrogen atmosphere at 1400°C for 30 min, in the main decomposes with the separation of metallic nickel and corundum. However, x-ray patterns confirm that in these conditions the nickel spinel is not completely decomposed.Translated from Ogneupory, No.5, pp.32–36, May, 1972.     

Deformation characteristics of silicon carbide and chamotte refractories

Conclusions It was established that silicon-carbide refractories with bonds of silicon nitride and oxynitride, and also self-bonded silicon carbide articles possess 2–3 times higher elasticity moduli compared with chamotte (firebrick) refractories.The anisotropy of the elasticity modulus determined in the longitudinal and transverse directions of the refractories is markedly higher (anisotropy coefficient n=2.28–2.49) than for chamotte refractories (n=1.22).In high-temperature conditions (1600–1650°C) the compressive strength of SiC refractories is about 28 N/mm², and the tensile strength 2.1 N/mm², i.e., the compressive strength is about 10 times higher than the tensile.At room temperature the strength of SiC refractories is double that of chamotte, while at 1200–1400°C this difference increases to 10 times.Silicon carbide self-bonded articles at high temperatures possess the highest strength properties, which confirms the effectiveness of using them in service under the action of abrasive forces.A new method was developed for determining the deformation and strength characteristics of refractories on the UITS-0.5/2.5 test machine.A method was developed for obtaining tension and compression diagrams from the results of tests for pure and longitudinal-transverse bending.Translated from Ogneupory, No. 7, pp. 8–13, July, 1989.     

Sintering and recrystallization of high-purity magnesia-alumina spinel during hot pressing

Conclusions A study was made of the densification and recrystallization of magnesia — alumina spinel of high dispersion during hot pressing in the temperature range 1200–1600°C, a pressure of 60–300 cm², and a holding time of 10–30 min.The relative density of 92–98% theoretical was achieved at 1300–1400°C, a pressure of 300 kg/cm², and a soaking of 10 min. The spinel of theoretical density with a finely crystalline homogeneous structure was obtained by hot pressing at 1450–1600°C.Intensive recrystallization of the spinels during hot pressing occurs in the temperature range ensuring maximum rate of densification (1450–1600°C).The rate of recrystallization of the spinel grains during hot pressing at 1400°C and a pressure of 300 kg/cm² and soakings of 10 and 30 min, has the order of 10^–6 cm/sec and the maximum for the grains of diameter from 2 to 6.Report read at a symposium on pure oxide sintering, Khar'kov, 1968.Deceased.Translated from Ogneupory, No.6, pp. 32–36, June, 1970.     

Changes in the properties of aluminosilicate refractories under prolonged reducing conditions

Conclusions Studies of corundum and aluminosilicate refractories of dense and granular structures in an atmosphere of hydrogen and dissociated ammonia at 1200, 1500, and 1700°C in periods of 175 and 50 h showed that the resistance of the products increases with an increase in the alumina concentration and density. The maximum resistance is exhibited by corundum products. In the aluminosilicate refractories there is some additional sintering of the material with the separation of mullite and glass. Simultaneously on the surface of the specimens we detected deeper mineralogical changes, accompanied by the decomposition of the mullite, with the formation of corundum, silicon monoxide, and glass.The changes in the phase composition are accompanied by a change in the structure, and an increase in the creep. Considering that a reduction in the temperature of 100°C causes a reduction in the creep by approximately a half [26], it can be recommended that corundum refractories should be used (under a load of 2 kg/cm²) in a reducing atmosphere at temperatures of up to 1550–1600°C, sillimanite up to 1450–1500°C, kaolin and chamotte (high-grog) up to 1300°C, with a reduction in the load and an increase in the density, the temperature of application for the products examined, especially corundum, can be increased.Translated from Ogneupory, No.5, pp.26–32, May, 1972.     

Differentiated use of refractories in blast-furnace hot blast stoves

Conclusions In the service of blast furnace hot blast stoves the lining and checkerwork fail in individual zones as the result of deformation of the refractories, formation of cracks, and chemical corrosion of the lining under the action of low-melting dust at high temperatures.It was shown that the deformation of aluminosilicate refractories depends not only upon temperature, load, and hold but also to a large degree upon the physicochemical properties of the parts, especially upon the composition and quantity of impurities.On the basis of an analysis of the service of refractories in hot blast stoves recommendations were developed on differentiated use of refractories in the different zones of the lining and checkerwork.For lining of the walls and the checkerwork of high-temperature hot blast stoves operating with a blast temperature up to 1300°C (temperature under the crown 1500–1550°C) the use of the following forms of refractories (GOST 20901-75) is recommended: 1550–1200°C temperature zone — type DV dinas parts; 1200–1100°C temperature zone — type MKV-72 mullite-corundum parts; 1100–900°C temperature zone — type ShV-42 chamotte parts; 900–700°C temperature zone — type ShV-37 chamotte parts; 700–400°C temperature zone — type ShV-28 chamotte parts.To line the lower portion of the combustion chamber mullite-corundum parts of types MK-80 to Technical Specification 14-8-405-82 or MKV-72 must be used.Lining of the crown, the inner wall of the combustion chamber, the hot blast lines, and the gas burners must be done with mortars corresponding to the types of refractories used (dinas and VT-1) with the addition of 10–12% orthophosphoric acid.Type MKRP-340 mullite-silica fiber parts to GOST 23619-79 and other fiber materials are recommended for use as the compensation gap filler up to temperatures of not higher than 1150°C, and asbestos-vermiculite plates to GOST 13450-68 in the up to 600°C temperature zone.The shell of the hot blast stove in the high temperature zone must be covered with thermal-insulation gunited concrete.Translated from Ogneupory, No. 7, pp. 44–50, July, 1986.     

AlPO4-coated mullite/alumina fiber reinforced reaction-bonded mullite composites

A precursor for reaction-bonded mullite (RBM) is formulated by premixing Al₂O₃, Si, mullite seeds and mixed-rare-earth-oxides (MREO). An ethanol suspension thereof is stabilized with polyethyleneimine protonated by acetic acid. The solid in the suspension is infiltrated into unidirectional mullite/alumina fiber-preforms by electrophoretic infiltration deposition to produce fiber-reinforced, RBM green bodies. Crack-free composites with ≤25% porosity were achieved after pressureless sintering at 1300 °C. Pre-coating the fibers with AlPO₄ as a weak intervening layer facilitates significant fiber pullout on composite fracture and confers superior damage tolerance. The bend strength is 170 MPa at 25 °C ≤ T ≤ 1100 °C. At 1200 °C, the composite fails in shear due to MREO-based, glassy phase formation. However, the AlPO₄ coating acts as a weak layer even after thermal aging at 1300 °C for 100 h.     

Mullite formation from the pyrolysis of aluminium-loaded polymethylsiloxanes: The influence of aluminium powder characteristics

Polymethylsiloxane (PMS) filled with a range of aluminium powders of different size and morphology have been used to produce precursor mixtures to form mullite bodies. The size and shape of the Al powder is shown to have a strong influence on the temperature and mechanism of mullite formation, on the final microstructure and phase composition of the product. The reaction proceeds by decomposition of the PMS producing amorphous SiO₂. Al oxidation occurs both by reaction with the atmosphere and by reduction of the amorphous SiO₂ to produce α-Al₂O₃. Crystallisation of cristobalite was also observed prior to mullitisation. It is these components of the microstructure that react to produce mullite. The onset of mullite formation occurs at different temperatures, depending on the initial Al powder size and morphology. Large, flake morphology Al powders produced the greatest quantity of mullite and showed the lowest temperatures for mullite formation. XRD analysis identified 3:2 mullite in samples using large Al particles after heating to 1400 °C and at 1700 °C in samples using small Al powders.     

Study of corundum phosphate-bonded concrete

Conclusions The concentration of orthophosphoric acid affects the properties of corundum concretes: with an increase in the concentration of the acid in the concretes during drying a larger quantity of AlPO₄ is formed.Translated from Ogneupory, No. 8, pp. 43–44, August, 1970.     

Structure of the BaO – Al2O3 – SiO2 System (A Review)

The results of theoretical calculations and experimental studies of the ternary system BaO – Al₂O₃ – SiO₂ in the subsolidus range at temperatures of 1200 – 1400°C are considered. Complete splitting of this system into elementary polytypes is performed, a topological graph of the relationship between these polytypes is shown, and geometrical characteristics of the binary and ternary compounds comprising this system are supplied.     

Ceramic cements and ceramic concretes of quartz-chamotte composition

Conclusions On the basis of quartz sand and scrap chamotte refractories (28.0% Al₂O₃, 67.0% SiO₂) we obtained highly concentrated bonding suspensions (cement slips) of mixed composition enabling us to form castings with a porosity of 12.2–18.3% characterized by bending strengths of up to 4.5 MPa and compressive strengths of up to 32.4 MPa.During the use of quartz-chamotte suspensions as bond and of chamotte scrap (grog) as the filler we obtained ceramic concretes with an original compressive strength of 18.4–25.6 MPa, and a porosity of 14.6–19.2%.We studied the change in the physicomechanical properties of the bonds (cements) and the ceramic concretes during heat treatment in the range 100–1450°C.The chamotte ceramic concretes based on quartz-chamotte bonds of the optimal compositions possess an increased spalling resistance (16 water-heat cycles from 1300°C during testing of specimen cubes with edges of 50 mm).Translated from Ogneupory, No. 5, pp. 5–9, May, 1986     

Thermomechanical properties of ceramics in the systems Al2O3-TiO2 and Al2O3-TiO2-mullite

Conclusions We studied certain properties of ceramics in the systems Al₂O₃-TiO₂ and Al₂O₃-TiO₂-mullite, obtained by the use of the double-stage synthesis of aluminum titanate.We established the nature of the change in the high-temperature strength in relation to the ratio of Al₂O₃ and aluminum titanate. The maximum high-temperature strength (bending) at 1200°C is possessed by ceramic with a corundum matrix and a volume proportion of aluminum titanate equal to 40–45%.It is established that the addition of CaO + SiO₂ made in amounts of up to 1.0–1.5% contributes to the partial breakdown of the aluminum titanate in the compositions Al₂O₃-TiO₂ and the production of a ceramic with a bending strength of 160–190 N/mm² at 20–200°C, thermal-shock resistance 650–800°C, and thermal conductivity of 1.9–2.1 W/(m·K).We studied the effect of the mullite concentration on the properties of the ceramic in the system Al₂O₃-TiO₂-mullite. The introduction of mullite in amounts of not more than 50%, containing up to 3% of impurities, contributes to an increase in the ceramic's strength in the range 20–1300°C and in the thermal shock resistance.Translated from Ogneupory No. 2, pp. 22–26, February, 1988.