X-RAY INVESTIGATION OF THE EFFECT OF HEAT ON CHINA CLAYS1

X-ray diffraction patterns were obtained from an English china clay and a Georgia sedimentary kaolin, both raw and fired to various temperatures. The chief crystalline constituent of the raw clays was found to be kaolinite. Upon dehydration the kaolinite lattice was destroyed and the clays gave no diffraction pattern. Mullite was formed in both clays at 950°C and the amount increased with increase of firing temperature. In addition to the mullite, free alumina was present in the Georgia clay from 950 to 1100°C and cristobalite at temperatures above 1200°C.     

Formation of high-temperature crystalline phases in ceramic from illite clay and dolomite

The formation of ceramic with the prevailing cordierite/mullite in the crystalline phase is greatly dependent on the raw materials used. In the present work the cordierite and mullite (or their solid—solutions accordingly with spinels and corundum) formation processes from compositions of natural raw materials (illite clay, dolomite) with synthetic additives (MgO, Al(OH)₃, K₂CO₃) are studied. It is shown that the illite clay acts as a low viscosity liquid medium as well as a precursor for cordierite and mullite crystallisation in ceramics.The phase development using XRD—analysis and the SEM is studied. Particles size distribution of staiting powder and some properties of ceramic obtained are also demonstrated. It is shown that formation of the above mentioned phases starts at temperatures 1050–1080 °C and remarkably increases at 1200 °C for mullite and at 1300 °C for cordierite.     

Influence of raw material type and of the overall chemical composition on phase formation and sintered microstructure of mullite aggregates

Dense mullite aggregates with varied (47–70%) alumina contents have been prepared by a conventional dry-powder pressing technique followed by heat treatments at temperatures in the range of 1450–1725 °C. Different types of clays, beach sand sillimanite (BSS) and a high purity aluminium hydroxide were used as starting materials. Mullites derived from BSS consisted of equi-axed grains whereas those obtained from clay containing precursor mixtures exhibited elongated grains. The bulk density (BD), apparent porosity (AP) and water absorption (WA) capacity of sintered mullites were found to be strongly influenced by the pre-mullitization step of the precursors and in a less extent by the type of raw material, its hydration degree and the impurity contents of Fe₂O₃, CaO and Na₂O. Mullite aggregates obtained from the three different types of aluminosilicate raw materials (i.e., ball clay, china clay and beach sand sillimanite) through a double-stage heat treatment process exhibited better sintered properties in terms of bulk density, apparent porosity, water absorption capacity and higher mullite contents in comparison to those obtained following a single-stage firing process.     

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.     

Glass-ceramic glazes for future generation floor tiles

Glaze in the CaO–MgO–Al₂O₃–SiO₂ system was heated at 950–1190 °C for 2 h and characterized. X-ray diffraction showed that only trace amount of mullite was formed in the glass-ceramic glaze heated at 950 °C. Both mullite and α-cordierite were formed in the glass-ceramic glaze heated at 1050 °C as primary and secondary phases. Glass-ceramic glazes heated at 1120 °C and 1190 °C contained α-cordierite and mullite as major and minor phases. Rietveld analysis revealed that the amount of α-cordierite increased and mullite decreased with increasing heating temperature. Field emission scanning electron microscopy showed presence of mullite crystals dispersed within residual glassy phase in the glass-ceramic glazes heated at 950 °C and 1050 °C. In the microstructures of glass-ceramic glazes heated at 1120 °C and 1190 °C α-cordierite crystals were mainly appeared. Energy Dispersive X-ray analysis corroborated X-ray diffraction results. Vickers microhardness measurement demonstrated highest hardness (8.38 ± 0.07 GPa) of the glass-ceramic glaze heated at 1190 °C.     

Synthesis of high-performance mullite ceramics based on associated rare-earth kaolin

The objective of this work was to prepare high-purity, high-strength mullite ceramics from low-cost, associated rare-earth kaolin (AREK). A reaction sintering process using calcined AREK and γ-Al₂O₃ powders was used to synthesize high-performance mullite ceramics. Mineralogical, morphological, and chemical characteristics of AREK were given. The effects of associated REEs in kaolin and sintering temperature on the microstructural evolution, phase transformation, and physical properties of mullite were studied. The results showed that the mullite contents were 98.8%, the maximum aspect ratio was 8.22 μm, the relative density was 93.04%, and the micro-Vickers hardness and flexural strength were 10.63 GPa and 184.24 MPa, after sintering at 1500°C for 4 h. For comparison, calcined without rare-earth kaolin was also employed as a raw material to synthesize mullite ceramics, and the mullite content prepared by sintering the two kaolin clays at 1320–1480°C for 4 h was quite similar. However, mullite prepared using AREK forms secondary mullite in the temperature range of 1480–1500°C with a significantly higher mullite content, and therefore, the advantages of preparing mullite based on AREK as the raw material are high purity, low mullitization temperature, and high strength.     

Utilization of recycled paper processing residues and clay of different sources for the production of porous anorthite ceramics

Production of porous anorthite ceramics from mixtures of paper processing residues and three different clays are investigated. Suitability of three different clays such as enriched clay, commercial clay and fireclay for manufacturing of anorthite based lightweight refractory bricks was studied. Porous character to the ceramic was provided by addition of paper processing residues (PPR). Samples with 30–40 wt% PPR fired at 1200–1400 °C contained anorthite (CaO·Al₂O₃·2SiO₂) as major phase and some minor secondary phases such as mullite (3Al₂O₃·2SiO₂) or gehlenite (2CaO·Al₂O₃·SiO₂), depending on the calcite to clay ratio. Anorthite formation for all clay types was quite successful in samples with 30–40 wt% of paper residues fired at 1300 °C. A higher firing temperature of 1400 °C was needed for the fireclay added samples to produce a well sintered product with large pores. Gehlenite phase occurred mostly at lower temperatures and in samples containing higher amount of calcium (50 wt% PPR). Compressive strength of compacted and fired pellets consisting of mainly anorthite ranged from 8 to 43 MPa.     

Recycling and Utilization of some Waste Clays for Production of Sintered Ceramic Bodies

The recycling of industrial waste clays for production of an interesting ceramic product is the main goal of the present research work. Ceramic bodies were prepared using Feeders or Cyclons waste clays, sand and feldspar. 0.0, 15, 20, and 25 wt.% of sand were added at the expanse of kaolin (75-50 wt.%). Constant mass percent (25 wt.%) of feldspar was added for all ceramic compositions. The designed batches were sintered at 1200–1400 °C. Physical properties were determined by water displacement method. Phase composition and microstructure were investigated by x-ray diffraction and scanning electron microscope, respectively. The compressive strength was also determined. The results indicated that the ceramic bodies prepared from Cyclons’ waste clay exhibited higher physical and mechanical properties than that prepared from Feeders’ clay after sintering at 1400 °C. The addition of sand enhances the porosity, water absorption, bulk density and mechanical strength after sintering at 1400 °C due to the formation of mullite network and glassy phases.

     

Firing transformations of Chilean clays for the manufacture of ceramic tile bodies

This contribution is focused on the study of the mineralogical changes occurring in the ceramic body after heating ceramic clays. Chile has an important local ceramic industry. Five deposits of clays with industrial applications were studied. The clays came from San Vicente de Tagua-Tagua (SVTT), Litueche (L), Las Compañías-Río Elqui (LC), La Herradura-Coquimbo (LH) and Monte Patria-Coquimbo (MP). The samples were heated to 830, 975, 1080 and 1160 °C keeping at the maximum temperature for 35 min. The bending strength of each ceramic body was determined at 1100 °C. Mineralogical analysis of the fired samples was carried out by X-ray diffraction. The SVTT contained quartz, spinel, cristobalite, microcline, albite, anorthite, hematite and enstatite; the LC clays quartz, mullite, spinel, microcline, albite, anorthite, hematite, diopside, enstatite, illite/muscovite and talc; the LH clays quartz, cristobalite, microcline, albite, anorthite, hematite, diopside, illite and augite; the MP clays quartz, cristobalite, microcline, albite, anorthite, hematite, diopside, gehlenite, enstatite and wollastonite and the L clays quartz, microcline and mullite. The persistence of illite at at least 900 °C was observed for LC and LH. SVTT and LH showed the required specifications for earthenware. The L clays were refractory clays with very low bending strength.     

Mullite development on firing in porcelain stoneware bodies

Microstructural evolution on heating was investigated in a reference industrial composition (50% kaolinitic clay, 40% feldspar and 10% quartz) of porcelain stoneware, fast fired at different temperatures (500–1400 °C). The evolution of mullite crystals, regarding shape and size progress, was examined by scanning electron microscopy (SEM). The proportion of Type I mullite crystals decreases with firing temperature and simultaneously, the size of crystals increases, reaching the maximum value of aspect ratio (3:1) at 1400 °C. Type II and Type III secondary mullite needles increase with temperature in both number and length, which leads to an increase in the aspect ratio from 5:1 to ～20:1 in Type II crystals and from ～33:1 to 50:1 in Type III mullite needles. Finally, clusters of Type III mullite fibres are observed in porcelain stoneware samples fast fired in the 1250–1280 °C interval.     

Effects of different potassium salts on the formation of mullite as the only crystal phase in kaolinite

The phase transition in kaolinite from 1050 °C to 1600 °C without and with different potassium salts (KF, KNO₃ and K₂SO₄) as mineralizers and the changes of the composition and morphologies of mullite formed in kaolinite have been investigated. The adding of enough potassium has been found to inhibit the formation of cristobalite in kaolinite. The fluorine is found to be beneficial to increase the reaction activity of aluminium from metakaolinite and so the potassium fluoride has promoted more pseudotetragonal mullite formed at 1100 °C than the other two mineralizers. The influence of potassium salts on the composition and microstructure of mullite formed from kaolinite with increasing the heating temperature has been scrutinized in detail. The formation condition for the only crystal phase of mullite from kaolinite has been shown.     

Adsorption of pigments and free fatty acids from shea butter on activated Cameroonian clays

Adsorption of pigments and free fatty acids from shea butter on acid‐activated Cameroonian local clays was investigated. The adsorption of the pigments was followed by the reduction of the absorbance of the shea butter at 295 nm. The kinetic study revealed that both the temperature at which the experiment was performed and the degree of activation of the clays influence the time of contact required to reach adsorption equilibrium of the pigments and the amount of pigments adsorbed. Thus, at 90 °C, the time required was 30 min for the clays activated with 0.5 M (A0.5M) and 1.0 M solutions (A1M) of sulphuric acid, and 45 min for the clay activated with a 2.0 M solution (A2M) of sulphuric acid. At 80 °C, the adsorption equilibrium was reached after 45 min for clays A0.5M and A1M, and 60 min for clay A2M. At 65 °C, the contact time to reach adsorption equilibrium was longer than 75 min for all the adsorbents used. At 90 °C, the amount of pigments adsorbed at equilibrium by clay A2M was about two thirds of that adsorbed by clay A0.5M, and one half of the amount adsorbed by clay A1M. For each adsorbent, the amount of pigment adsorbed decreased with temperature. A1M was the most efficient local clay for the adsorption of shea butter pigments and compared favourably with the industrial clay used as reference. The same evolution was observed with the adsorption of free fatty acids on different clays. The contact time needed for the elimination of peroxides was 40 min for all the clays used. The most efficient adsorbents for the adsorption of the pigments and free fatty acids were also the ones that gave the best results in the elimination of peroxides. The energy of activation for the adsorption of the pigments, as determined by the adsorption kinetic model of Brimberg, were 61 ± 9 kJ/mol, 73 ± 11 kJ/mol, 67 ± 10 kJ/mol, and 47 ± 7 kJ/mol for the industrial adsorbent (EN) and clays A0.5M, A1M and A2M, respectively. These values of activation energies indicate that the shea butter pigments are chemisorbed on acid‐activated clays. It was found that the adsorption isotherms follow the Freundlich equation.     

Crystalline phases and physical properties of modified stoneware body with glaze sludge

Ceramic stoneware body has been modified with ceramic and glass industrial wastes by replacing 25–100% as flux in the formula. The effects of solid wastes added to the bodies were studied after firing in the temperature range 950–1280 °C. The physical properties of linear shrinkage, bulk density, apparent porosity, water absorption and 3-point bending strength were determined. A composition which related to the general stoneware properties was found when using soda-lime cullet and glaze sludge. It had a firing range lowered to 1050–1100 °C. SEM results demonstrated the sintered microstructure increased in density with increase in solid waste in the modified body. XRD results after firing showed the crystalline phases comprised of mullite, albite calcian and quartz. Thermal expansion was measured in the range 6.53–6.67 × 10^?6 K^?1 at 30–500 °C. The modified bodies were capable of forming prototype products by slip casting and jiggering.     

Effect of fluorine-containing additive on the synthesis and sintering of compositions from natural raw materials in the “cordierite –mullite” system

The influence of the small addition of topaz on the processes of mineral formation in the “mullite–cordierite” system with a variable ratio of cordierite and mullite has been investigated. For this purpose, a series of experiments with different compositions of the initial mixtures, that is, with predominance of cordierite over mullite (KM mixtures) and predominance of mullite over cordierite (MK mixtures), has been performed. The addition of topaz in the form of topaz concentrate has been introduced in the amount of 1% by the weight of the investigated mixtures. The samples have been fired at various temperatures, that is, samples from KM mixtures (at a temperature of 1100–1300?°C) and samples from MK mixtures (at a temperature of 1400–1550?°C. The activating effect of topaz on mineral formation and sintering processes is determined by complex influence of fluoride products of topaz thermal decomposition. These reactively gaseous fluorides partially activate the solid-phase mass transfer processes and then reduce the viscosity of high-temperature melt formed during firing that intensifies the processes of ceramic matrix consolidation.Porous polycrystalline cordierite–mullitе–corundum ceramics from the KM mixtures, which contains 70%–87% cordierite, 3%–12% mullite, and 4%–11% corundum with water absorption of 3.5%–8% and bulk density of 2.10%–2.12?g/cm³ at the firing temperature of 1300?°C, has been developed. Mullite ceramics with notably corundum content (not more than 8%) from the MK mixtures with various density degrees at the firing temperature of 1500–1550?°C, porous ceramics with water absorption of 3%–11%, and dense ceramics with water absorption of less than 1% have been produced.     

Creep in Interlaminar Shear of a Nextel™720/aluminosilicate Composite at 1100°C in Air and in Steam

Creep in interlaminar shear of an oxide–oxide ceramic composite was evaluated at 1100°C in air and in steam. Composite consists of a porous aluminosilicate matrix reinforced with mullite/alumina (Nextel^?720) fibers, has no interface between fibers and matrix, and relies on the porous matrix for flaw tolerance. The interlaminar shear strength was 7.6 MPa. Creep behavior was examined for shear stresses of 2–6 MPa. Creep run‐out of 100 h was not achieved. Larger creep strains and higher creep strain rates were produced in steam. However, steam had a beneficial effect on creep lifetimes. Composite microstructure, damage, and failure mechanisms were investigated.     

Impact of spark plasma sintering (SPS) on mullite formation in porcelains

The effect of the spark plasma sintering (SPS) process on mullite formation in porcelains was studied using X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy. SPS affected the kinetics and morphology of formed mullite. After sintering at 1100°C, unlike conventional sintering, SPS promoted the formation of mullite due to the combination of vacuum and applied pressure. Mullite crystal growth was altered by the atmosphere (vacuum), dwell time (0‐15 minutes), and temperature (1000‐1200°C). The applied pressure caused the mullite needles to orient perpendicular to the direction of the applied load. Depending on SPS dwell time, the mullite formed after sintering at 1100°C also had different crystal structure (tetragonal for short dwell time of 0‐5 minutes and orthorhombic for a long dwell time of 10‐15 minutes). Dissolution of mullite was observed at 1100°C by extending the dwell time by up to 15 minutes and the dissolved mullite reprecipitated on the small needles (~40 nm) and coarsened via Oswald ripening resulting in larger mullite needles (~60 nm).     

Transparent mullite ceramic from single-phase gel by Spark Plasma Sintering

Monophasic mullite precursors with composition of 3Al₂O₃·2SiO₂ (3:2) were synthesized and then were sintered by Spark Plasma Sintering (SPS) to form transparent mullite ceramics. The precursor powders were calcined at 1100 °C for 2 h. The sintering was carried out by heating the sample to 1450 °C, holding for 10 min. The sintered body obtained a relative bulk density of above 97.5% and an infrared transmittance of 75–82% in wavelength of 2.5–4.3 μm without any additive. When the precursor powders were calcined at below 1100 °C, it was unfavorable for completely eliminating the residual OH, H₂O and organic compound. However, when calcined temperature was too high, it was unfavorable either for full densification due to the absence of viscous flow of amorphous phase. At the same calcined temperature, the transmittance of sintered body was decreased with the increase of the sintering temperature above 1450 °C owing to the elongated grain growth.     

Effect of some properties of aluminosilicate refractories on their glass resistance

Conclusions The glass resistance of aluminosilicate blocks with similar porosity values depends mainly on the quantitative ratios of crystalline and glassy phases, their chemical composition, and the grain size of the mullite. The higher the crystalline phase concentration in the refractory (mullite and mullite+ corundum) the fewer the impurities (oxides of iron, titanium, calcium, sodium, potassium, etc.), and the coarser the grains of mullite, then the lower their corrosion by molten glass.An increase in the glass resistance of aluminosilicate blocks can be achieved by using pure clays and kaolins, and also by using high temperatures for firing, longer soaking periods, and also mineralizing additives.Translated from Ogneupory, No.6, pp. 40–44, June, 1970.     

The change of phase composition in kaolinite- and illite-rich clay-based ceramic bodies

Raw material for ceramics consists mainly of kaolinite, illite, quartz and feldspar. Three representative clays, a high kaolinitic (HB), illitic- and quartz-rich (KW) and naturally mixed kaolinite–illite (P1) clays of Westerwald area, were chosen for this study [Kromer, H., 1980. Tertiary clays in the Westerwald area. Geol. Jb. D. Rhei D Hanover, 69–84]. The largest and oldest clay mining area of Germany is in the Westerwald area. These clays were mixed with each other and also with K, and Ca–Na feldspar. The high temperature phases of the mixed bodies were of three groups: crystalline phase, amorphous/glassy phase and porosity.The aim of this study was to determine: (1) the effect of kaolinite–illite–quartz ratios, (2) the effect of heat treatment and (3) the effect of feldspar on the fired mineralogy of the fired products. The crystalline phases are cristobalite, mullite, quartz, hematite and anatase. The bodies consist of crystalline phases such as quartz, mullite and cristobalite in a composite structure where crystals and pores are often embedded in amorphous/glassy phase. The formation of mullite and cristobalite is very distinctive in kaolinitic clay, and the structure is dominated by the spiky primary mullite. In the illite/sericite-rich mixtures, the high K content causes a large amount of melt superimposed on the mullite formation. The cristobalite formation is completely suppressed in illite/sericite-rich bodies. After dehydroxylation, metakaolinite and illite/sericite anhydride structures are formed. The persistence of illite/sericite anhydride peaks above 950 °C in KW clay indicates the presence of sericite/muscovite mineral. The disappearance temperatures and firing behaviour of K and Ca–Na feldspar observed within the XRD patterns of mixed bodies are different. K feldspar lines disappear earlier in HB bodies than in KW bodies, but Ca–Na feldspar shows a reverse behaviour. While Ca–Na feldspar peak intensity gradually decreases but persists at 1150–1250 °C, K feldspar suddenly disappears at 1150 °C.     

Microstructural and mechanical characterization of a mullite fiber

The effect of thermal exposure on a mullite fiber was analyzed. This type of mullite fiber, consisting of γ-Al₂O₃ and amorphous SiO₂, was developed for high-temperature applications. Heat treatments at temperatures ranging from 900 °C to 1500 °C for 1h were performed in air. Investigations showed that the tensile strength of the initial fiber was about 1.60 GPa. And the elastic modulus was about 133.51 GPa. The bundles’ strength decreased at 900 °C slightly after thermal treatment, then increased and got a maximum at 1100 °C with 1.65 GPa. At above 1100 °C, the strength degraded sharply due to the mullite phase transformation and the exaggerated grain growth. At 1300 °C, the phase reaction almost finished with a tensile strength of 0.86 GPa. And the strength retention was only 47.50%. When the heat-treated temperature got to 1500 °C, the density of surface defects in the fiber surged, making it too fragile and weak to go through the tensile tests.