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.     

From nanometric zirconia powder to transparent polycrystal

Coprecipitated zirconia-yttria (8 mol%) gel subjected to hydrothermal treatment at 240 °C resulted in the solid solution powder of 8 nm particle sizes and specific surface area 132.7 m²/g. Uniaxial compaction followed by cold isostatic pressing under 300 MPa resulted in samples of the extremely small and narrow pore size distribution. Such samples start to shrink at about 200 °C which is related to the desorption of water layers surrounding particles. The state of closed porosity is achieved at 1150 °C. Pore closing was performed in air or oxygen atmosphere. Hot isostatic pressing at 1150 °C for 2 h under 250 MPa argon pressure led to transparent materials. Some pores remained in the material whose preliminary pore closure was performed in air. The samples initially sintered in oxygen atmosphere show no porosity and higher light transmittance than the former ones.     

The influence of mineralogical composition on the colour of clay body

By using methods of Mössbauer spectroscopy, X-ray diffraction and colorimetrics analysis, the dependence of the colour of a ceramic body on its mineralogical composition was determined. In order to achieve this objective, the mineralogical composition and colour parameters of the Girininkai deposit (Lithuania) easily fusible clay and its mixture with the glauconite rock during firing at 600, 800, 1000, and 1025 °C were investigated in detail. By changing firing temperature from 600 to 1025 °C and duration of isothermal exposure from 0.5 to 4 h the amount of iron in hematite increases 5 times, whereas the amount of Fe³⁺ incorporated into the crystal lattice of other compounds diminishes 3–4 times if compared with natural clay. For this reason, the value of red colour coordinate increases from 18.44 to 23.65 NBS units and the ceramic body darkens and becomes redish brown. The colour can be intensified even more by adding to the forming mass finely ground glauconite.     

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.     

Preparation and characterization of glazes from combinations of different industrial wastes

The work reported here involves the preparation and characterization of ceramic glazes made from combinations of different industrial wastes. The wastes were float glass, granite and lime shale (a raw material waste from the oil shale industry in São Mateus do Sul, state of Paraná, Brazil), which were used to replace natural raw materials in a proportion of up to 50% in weight. The compositions were formulated using the Seger method and prepared by conventional ceramic processing. The stabilized suspensions were applied in commercial wall tile and porcelain stoneware tile, which were sintered at temperatures of 1080 °C and 1150 °C, respectively, using two different heating cycle. Three compositions were developed, two of which yielded opaque glazes and one a transparent glaze. Linear thermal expansion coefficients (α) of 80.10^?7 °C^?1 to 100.10^?7 °C^?1, and glaze softening temperatures of 600–700 °C were characterized by dilatometric analysis. The glaze compositions showed chemical resistance against acid and alkaline attack after 96 h, showing a mass loss of less than 0.1% in weight. The surface hardness of the glazes determined by the Mohs scale, Vickers microhardness and abrasion resistance (PEI indices) were between 6–7, 3–3.7 GPa and 3–4, respectively. These properties are compatible with those of commercial glazes for wall tiles and porcelain stoneware ceramics.     

Cordierite obtained from compositions containing kaolin waste,talc and magnesium oxide

The purpose of this study was to obtain cordierite from compositions containing kaolin waste (with different particle-size distributions), talc and magnesium oxide, for use in the production of refractory and insulating materials. The samples were characterized by means of the following techniques: chemical analysis by X-ray fluorescence, X-ray diffraction, particle size analysis and thermogravimetric and differential thermal analysis. The microstructure of the samples was analyzed using scanning electron microscopy. Rectangular test specimens (50 mm × 15 mm × 5 mm) were prepared by uniaxial pressing (13.0 MPa), dried at 110 °C/24 h and sintered at temperatures of 950, 1050, 1150, 1250 and 1350 °C. The mineralogical analysis revealed the beginning of the formation of characteristic peaks of cordierite phase at 1250 °C, and more intense peaks were identified at 1350 °C. The morphological analysis revealed rose-like and hexagonal tube-like crystals.     

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.     

Rheological properties of montmorillonitic clay suspensions: Effect of firing and interlayer cations

The effect of a prior firing of three montmorillonite clays, exhibiting different nature of interlayer cations, on the rheological behaviour of related aqueous suspensions (5 and 10 mass% of solid content) was examined. Calcinations were performed at 150 °C, 250 °C, 300 °C or 450 °C for 30 min. The rheological properties were characterized at 25 °C in the flow mode using the Herschel–Bulkley model.The alkaline interlayer cation (Na) tended to increase the yield stress of montmorillonite suspensions in comparison with earth-alkaline ones (Ca, Mg). As expected, increasing solid content led to increasing yield stress.For calcinations until 200 °C, the relevant suspensions exhibited an increasing yield stress due to a gel-like behaviour in relation with a card-house-like structure. Furthermore, calcination above 300 °C favoured the decrease of the corresponding yield stress. This behaviour seemed to be related to the modification of the surface properties of the clay platelets, more precisely to the beginning of clay dehydroxylation.     

Red mud as a substitute coloring agent for the hematite pigment

Red mud (RM), a highly alkaline iron oxide rich sludge obtained during the production of alumina, was treated to work as a coloring agent for ceramic glazes. The approach aims the valorization of this residue, adding environmental and economic value. RM was sintered at different temperatures (1100 °C, 1150 °C, 1200 °C, 1250 °C, 1300 °C and 1350 °C) to assess changes in its mineralogical composition. The obtained powders were characterized by XRD, UV–Vis and CIELab. The samples were added to glazes (transparent, opaque and matte) and their color stability analyzed by CIELab. RM sintered at 1300 °C was compared with commercial hematite producing lower coloring power but yielding better color stability on glazes for temperatures of 1100 °C ± 15 °C, thus presenting RM_1300C as a suitable substitute for the hematite pigment.     

Microwave dielectric properties of Pb2MoO5 ceramic with ultra-low sintering temperature

Ultra-low firing microwave dielectric ceramic Pb₂MoO₅ with monoclinic structure was prepared via a conventional solid state reaction method. The sintering temperature ranged from 530 °C to 650 °C. The relative densities of the ceramic samples were about 97% when the sintering temperature was greater than 570 °C. The best microwave dielectric properties were obtained in the ceramic sintered at 610 °C for 2 h with a permittivity ∼19.1, a Q × f value about 21,960 GHz (at 7.461 GHz) and a temperature coefficient value of −60 ppm/°C. From the X-ray diffraction, backscattered electron image results of the co-fired samples with 30 wt% silver and aluminum additive, the Pb₂MoO₅ ceramics were found not to react with Ag and Al at 610 °C for 4 h. The microwave dielectric properties and ultra-low sintering temperature of Pb₂MoO₅ ceramic make it a promising candidate for low temperature co-fired ceramic applications.     

The effect of boron waste in phase and microstructural development of a terracotta body during firing

In the present study, a dewatering sieve waste (TSW) of Etibor Kırka Borax company (Turkey) was employed in different amounts in order to develop an experimental terracotta floor tile body composition in combination with a feldspathic waste provided from a local sanitaryware plant and a ball clay. Several formulations were prepared and shaped by dry pressing under laboratory conditions. The obtained samples were fired at selected peak temperatures (1050, 1100 and 1150 °C) to establish their optimum firing temperatures. Some technological properties of the resultant products, namely linear firing shrinkage, water absorption and breaking strength were determined as a function of increasing TSW content in place of the sanitaryware waste at these temperatures. The phase content of the starting raw materials and that of the fired compositions was determined by XRD. The relevant polished surfaces of selected fired samples were also examined using SEM. According to the results, increased presence of TSW compared to the standard mixture of clay and the sanitaryware waste, as a co-fluxing material, in the experimental terracotta body considerably accelerated the vitrification process. The overall results indicated a prospect for using the TSW as a raw material in mixtures with both clay and sanitaryware waste for the production of a terracotta floor tile body.     

Comparative study of the sonication effect on the thermal behaviour of 1:1 and 2:1 aluminium phyllosilicate clays

The aim of this work is to compare the effect of sonication on the thermal behaviour of kaolinite and pyrophyllite. The results show that sonication produces a significant particle size reduction of both clays. Besides, this treatment modifies the thermal behaviour of these samples. Nevertheless, the change in particle size and in the modification of the thermal behaviour is different for both clays. Sonication facilitates the dehydroxylation of both clays, although, the temperature shift of the dehydroxylation is more significant for pyrophyllite than for kaolinite. Additionally, the exothermic effect at 987 °C for kaolinite is not affected by sonication while that at 1210 °C for phyrophyllite is shifted to lower temperatures.     

Compressive strength and microstructure of alkali-activated mortars with high ceramic waste content

The present work investigated alkali-activated mortars with high ceramic waste contents. Tile ceramic waste (TCW) was used as both a recycled aggregate (TCWA) and a precursor (TCWP) to obtain a binding matrix by the alkali-activation process. Mortars with natural siliceous (quartz) and calcareous (limestone) aggregates, and with other ceramic waste materials (red clay brick RCB and ceramic sanitaryware CSW waste), were also prepared for comparison purposes. Given the lower density and higher water absorption values of the ceramic aggregates, compared to the natural ones, it was necessary to adapt the preparation process of the recycled mortars by presaturating the aggregate with water before mixing with the TCWP alkali-activated paste. Aggregate type considerably determined the mechanical behaviour of the samples cured at 65 °C for 3 days. The mortars prepared with the siliceous aggregate presented poor mechanical properties, even when cured at 65 °C. The behaviour of the limestone aggregate mortars depended heavily on the applied curing temperature and, although they presented the best mechanical properties of all those cured at room temperature, their compressive strength reached a maximum when cured at 65 °C, and then decreased. The mechanical properties of the mortars prepared with TCWA progressively increased with curing time (53 MPa at 65 °C for 28 days). An optimum 50 wt% proportion was observed for the limestone/TCWA mortars (≈43 MPa, 3 days at 65 °C), whereas the mechanical properties of that prepared with siliceous particles (10 MPa) progressively increased with the TCWA content, up to 100 wt% substitution (23 MPa). Limestone particles interacted with the binding matrix, and played an interesting beneficial role at the 20 °C curing temperature, with a slight reduction when cured long term (28 days) at 65 °C. The results demonstrated a potential added value for these ceramic waste materials.     

Investigation on cristobalite crystallization in silica-based ceramic cores for investment casting

In this work, cristobalite crystallization and its effects on mechanical and chemical behaviour of injection moulded silica-based ceramic cores were investigated. In order to simulate casting process condition, the sintered samples at 1220 °C were also heated up to 1430 °C. Flexural strength test was carried out on both sintered and heat treated samples. Chemical resistance of the cores was evaluated by leaching the samples inside 43 wt% KOH solution at its boiling point. Phase evolution and microstructure were investigated by thermal analyses (DTA and DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical microscopy (OM). Results showed that cristobalite was crystallized on the surface of fused silica grains at about 1380 °C. Flexural strength of the sintered cores was decreased after simulated casting heat treatment due to cristobalite phase transformation. The formed cristobalite on the surface of fused silica grains dramatically decreased the leachability of ceramic cores.     

Low-temperature sintering of porous silicon carbide ceramic support with SDBS as sintering aid

The sintering temperature of porous silicon carbide ceramic support (PSCS) is typically higher than 1500 °C. In this paper, sodium dodecyl benzene sulfonate (SDBS) was used as a sintering additive to fabricate PSCS with high gas permeance and high bending strength at a sintering temperature less than 1200 °C. The PSCS was prepared by the dry pressing method followed by in-situ reaction. The effects of SDBS loading on the porosity, bending strength, gas permeation performance, and microstructure of the PSCS were investigated. The results showed that without SDBS, the required sintering temperature was as high as 1550 °C and resulted in a bending strength of 6.5 MPa but the sintering temperature decreased to 1150 °C with 8% SDBS and the bending strength increased to 16 MPa. The main reason was that SDBS decomposed into Na₂O which reacted with SiO₂ and ZrO₂ to form strong bonding connections. The prepared PSCS with SDBS also showed good gas permeance of 900 m³/(m² h kPa), higher than the 750 m³/(m²·h·kPa) without SDBS. This work describes the effective use of SDBS as a ceramic additive to reduce sintering temperature, while achieving high gas permeation and bending strength. The use of the low cost and commercially available SDBS produces an excellent ceramic filter with much lower energy consumption, and could also be implemented in other ceramic systems.     

Thermo-mechanical characterization of a silica-alumina refractory concrete based on calcined algerian kaolin

The aim of this work is to study the thermo-mechanical behaviour (bending and compressive tests, creep and thermal shock resistance) of a refractory concrete based on local kaolin grogs and aluminous cement. Strength tests revealed a behaviour that is almost linear elastic for temperatures up to 800 °C and visco-plastic at 900 °C. A crack bridging strengthening process was observed at 800 °C. The creep tests were carried out at different temperatures between 1000 and 1150 °C using stresses in the range (0.75–2.76 MPa). The stress exponent was about 1.255. Microscopic observations suggested an intergranular creep mechanism.A water quenching test was used for estimating the thermal shock resistance of the material. The tested samples supported 80 cycles of standardized cyclic thermal shock without failure. Ultrasonic measurements were applied in order to evaluate the of ultrasonic velocity changes after these thermal shock tests. Strength degradation of the samples was evaluated using two models based on ultrasonic velocity changes during test and compared with the experimental values.     

Densification behaviour and two stage master sintering curve in lithium sodium niobate ceramics

The Master Sintering Curve (MSC) has received much attention in recent years due to its ability to predict sintering behaviour of a given powder and green body process regardless of its thermal history. In this paper MSC, based on the combined stage sintering model is constructed for one of the most important lead-free piezoelectric viz. lithium sodium niobate, Na_1-xLi_xNbO₃ (x=0.12, LNN-12), ceramic using shrinkage data. The present study has been carried out to understand and control the densification behaviour during pressureless sintering. Two distinct stages of densification have been observed en route to the upper limit to sintering temperature. The activation energies of densification for the two temperature ranges viz. 800–1150 °C and 1150–1300°C were found to be 365 kJ/mol and 2530 kJ/mol, respectively, through the construction of MSC. The MSC should be useful in predicting the densification behaviour and the final density and for designing a reproducible fabrication schedule for the LNN-12 ceramics.     

The impact of thermal history on the microstructure and properties of Dy-α-Sialon

In order to investigate the effect of sintering schedule on the microstructure and the property, Dy-α-Sialon ceramics were sintered by hot pressing (HP) method via three intended schedules: (1) heated the sample to 1720 °C, then rapidly cooled down to room temperature (RT), (2) heated the sample to 1720 °C, then directly cooled down to 1650 °C and hold the temperature for 60 min before cooled down to RT, and (3) heated the sample to 1720 °C and hold there for 60 min then cooled down to RT. A series of positions on the as-sintered ceramic disc along the center to the edge were chosen to study their microstructure, optical transmittance and mechanical properties. The non-uniformity of microstructure and properties along the radial direction of the as-sintered samples was justified to be caused by the varied thermal schedules. This may develop methods to fabricate gradient ceramics.     

Evolution of microstructure,mineralogy and properties during firing of clay-based ceramics with borates

The effect of sieve boron waste (SBW) and borate-containing Evansite^® on the thermal behaviour, microstructure and properties of a clay-based body was investigated. SBW and Evansite^® were introduced in quantities that correspond to 0.6 wt.% B₂O₃ addition in the dry body for both cases. Cylindrical samples were extruded and fired at three different peak temperatures 900, 950 and 1000 °C. The reference body, R, and the body with SBW, RB, demonstrate a comparable dilatometric behaviour whereas the densification for the body with Evansite^®, RV, initiated 50 °C approximately lower and resulted in higher firing shrinkage. After firing at 900 °C, the physico-mechanical properties as well as the microstructure are comparable. Nonetheless, åkermanite is formed in RB, whereas hercynite and mullite, the latter at 1000 °C, are formed in RV. For firing at 1000 °C, the role of borates is intensified. Water absorption is reduced by 16.1% and 18.0%, whereas bending strength increased by 27.6% and 40.8%, for RB and RV respectively, compared to the reference formulation. This is attributed predominantly to the enhanced vitrification that took place in the boron-containing bodies.     

Effect of firing temperature and atmosphere on ceramics made of NW Peloponnese clay sediments. Part I: Reaction paths,crystalline phases,microstructure and colour

Archaeometric investigation on ancient ceramic collected from excavations in NW Peloponnese demonstrated that the ancient potters used the local Plio-Pleistocene clay sedimentary deposits for a large historical period. Three representative raw materials of these local sediments were chosen for experimental work aiming to evaluate their firing behaviour in a propane-fired kiln, with a different atmosphere and temperature. The determination of mineralogy and microstructure was carried out by XRD and SEM-EDS analysis. For ceramics fired at 850 and 950 °C, no significant mineralogical and microstructural differences were observed between the oxidising and reducing atmosphere. The main pyrometamorphic phases are fassaite, gehlenite, anorthite and wollastonite. On the contrary, at 1050 °C in reducing atmosphere, gehlenite and wollastonite are diminished whereas the content of anorthite, fassaite and amorphous phase is higher. The higher vitrification is attributed to Fe²⁺ that participates either in the formation of eutectic phases or in low melting crystalline phases.