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.     

Preparation and thermal shock resistance of anorthite solar thermal energy storage ceramics from magnesium slag

Anorthite solar thermal energy storage ceramics were fabricated from magnesium slag solid waste by pressureless sintering. The effects of CaO/SiO₂ ratio and sintering temperature on the physical, chemical, and thermophysical properties of ceramics were explored. X-ray diffraction results demonstrated that thermal shock process contributed to the formation of anorthite, and increasing CaO/SiO₂ ratio promoted the transformation of anorthite (CAS₂) into melilite (C₂AS). Some micro-cracks were found according to SEM analysis, forming by the mismatch of thermal expansion coefficients among phases. The combined effects of the low thermal expansion coefficient of anorthite and micro-crack toughing endowed the ceramic with good thermal shock resistance. Optimum comprehensive performances were observed in the sample with a CaO/SiO₂ ratio of 0.58 sintered at 1160°C, of which the specific thermal storage capacity was 0.63 J·g^-1·°C^-1(room temperature). The bending strength increased by 0.22% after 30 thermal shock times (room temperature-800°C, wind cooling). Therefore, the anorthite ceramics exhibited great potential for solar thermal energy storage.     

Fundamental study on the preparation of insulating ceramics via the phase reconstruction of phosphate tailings

Insulating ceramics with anorthite and diopside as the main crystal phases were manufactured using phosphate tailings, coal gangue, and quartz as raw materials, which were fired at 1160–1190 °C for 2 h. The structure and micromorphology of the samples were investigated using X-ray diffraction, field-emission scanning electron microscopy, and X-ray photoelectron spectroscopy. Dense ceramic samples exhibited an apparent porosity of less than 0.3%, densities of 2.64–2.86 g/cm³, bending strength of 127.71–172.73 MPa, dielectric constants of 6.98–7.79, and dielectric losses of 0.0086–0.0024 at 20 °C and frequency of 1 MHz. The effects of impurity elements on ceramic properties are determined. The excellent electrical properties can be attributed to the solid solution of iron in the crystalline phases, such as diopside, which reduce leakage conductance in ceramics and alleviate the influence of relaxation and space charge polarization on the electrical properties of ceramics. This study provides a new strategy for preparing high-value insulating ceramics from solid waste.     

Improving the flexural strength of porcelain by residual stress in anorthite coating

Flexural strength is an important parameter of domestic ceramics to meet the criteria for mechanized washing, filling, and sealing process in the automatic production line. In this work, the anorthite coating was prepared using calcite, silica, and alumina as raw materials. Taking the temperature with the highest matrix strength as the optimal temperature, the influences of chemical formulation on the phase composition and the coefficient of thermal expansion (CTE) of coatings were investigated. The enhancement effect of the coating with different formulations and surface number and thickness was compared. As a result, the flexural strength of the ordinary domestic ceramic body was improved due to the residual compressive stress in the coating caused by a mismatch of CTE between the coating and the matrix. At the optimal sintering temperature (1280°C), the coating with 20 mass% alumina addition has the best strengthening effect on the body, in which the thickness of the anorthite coating is approximately 50 µm and the flexural strength has increased by 64.6%, from 88 ± 4 to 144 ± 6 MPa.     

Evolution mechanism of pore structure in sintered coal gangue ceramsites

Preparation of high-strength densified ceramsites and porous water-retaining ceramsites to replace part of the original gravel aggregates and natural moisturizing materials, respectively, is an effective way for the resource utilization of coal gangue (CG). The pore structure of ceramsites affects their strength, density, porosity, etc. In this paper, CG was used as the sole raw material to prepare ceramsites at different temperatures (600–1220 °C). By using methods of thermogravimetry-differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), scanning electron microscope (SEM), element analysis (EA), heating microscope (HM), and density tester (DT), the evolution mechanism of pore structure in sintered coal gangue ceramsites (CGCs) was discussed. The results showed that there were two key temperatures: 950 °C and 1160 °C. Below 950 °C, the pore structure of CGCs was in a stable state, and the true, apparent, and closed porosity remained around 45%, 32%, and 12%, respectively. At 950–1160 °C, the true and apparent porosity decreased, and the closed porosity and compressive strength increased. Above 1160 °C, its performance deteriorated. During the heating process, the loss of carbon-containing components, phase transition, high-temperature diffusion and densification would change the pore structure. The appropriate temperature range for the preparation of porous water-retaining CGCs was about 950 °C, while that for preparing high-strength densified CGCs was about 1000–1160 °C. It was proved that qualified CGC products with different prospective applications could be prepared in different temperature regimes.     

Preparation and characteristics of porous anorthite ceramics with high porosity and high-temperature strength

High-temperature properties including compressive strength, thermal shock behavior, and thermal conductivity of porous anorthite ceramics with high specific strength were tested and analyzed. The results showed that the prepared materials merit high-temperature compressive strength, thermal stability, and conductivity. With the appropriate fabrication parameters, even though containing 0.33 g/cm³ bulk density and 88.2% porosity, its compressive strength could reach 2.03 MPa at 1000°C, 147% of that at room temperature; the residual strength ratio kept as 114.7% after a thermal shock at 1200°C. The X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) showed that anorthite grains refinement and intergranular voids filling by liquid phase were main factors for the high strength. From room temperature to 1200°C, its thermal conductivity only varied from 0.085 to 0.258 W·(m·K)⁻¹. High porosity, a large number of nanoregions in anorthite grains and amorphous phase in grain boundary were main reasons for low thermal conductivity.     

Preparation of anorthite ceramics using SPS

A mixture of kaolin (92 wt.% of kaolinite) and laboratory-grade CaCO₃ was used for anorthite preparation using the spark plasma sintering (SPS) technique. The powder was heated up to the maximum temperature (850 °C–1100 °C with a step of 50 °C, heating rate 50 °C/min) using the SPS device. The as-prepared samples were compared with a reference sample produced by dry pressing and conventional firing. Rietveld refinement performed on the X-ray diffraction data revealed that before anorthite formation, gehlenite and Ca-feldspar appeared, which gradually transformed into anorthite with increasing temperature. The sample prepared by SPS at 1100 °C contained 90 wt.% of anorthite, while the anorthite content in the reference sample reached only 47 wt.%. The porosity of the samples prepared by SPS reached significantly lower values compared to that of the reference sample. Therefore, SPS can be considered as a promising technique in the preparation of anorthite ceramics at lower temperatures.     

Fabrication and characterization of anorthite-based ceramic using mineral raw materials

The purpose of this study is to design a novel single crystalline phase ceramic based on anorthite whose properties fulfill the tableware market requirements such as high appearance quality, strength and thermal shock resistance. To obtain the single phase anorthite ceramic, ball clay, quartz, calcite, feldspar and alumina were used as raw materials. The single phase anorthite ceramic was fabricated by slip casting and sintering at 1230 °C for 1 h. It has a high flexural strength of 103 MPa, which is higher than that of the conventional porcelain. The single phase anorthite ceramic had relatively low (4.9 × 10^?6 K^?1) thermal expansion coefficient which can be matched with applicable glaze easily. Furthermore, the single phase anorthite ceramic had high degree of whiteness (L* = 94) and excellent translucency behavior which could achieve a high-quality decorative effect.     

Effect of Ni diffusion into BaZr0.1Ce0.7Y0.1Yb0.1O3−δ electrolyte during high temperature co-sintering in anode-supported solid oxide fuel cells

Diffusion behavior of Ni during high temperature co-sintering was quantitatively investigated for anode-supported solid oxide fuel cells (SOFCs) that had BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3?δ (BZCYYb) proton-conducting electrolyte and NiO-BZCYYb anode. Although diffused Ni in such SOFCs effectively acts as a sintering aid to densify the BZCYYb electrolyte layer, it often negatively affects the electrolyte conductivity. In the present study, field emission electron probe microanalysis (with wavelength dispersive X-ray spectroscopy) clearly revealed that Ni diffused into the BZCYYb electrolyte layer, and that the amount of diffused Ni increased with increasing co-sintering temperature. In particular, relatively high Ni concentration within the electrolyte layer was observed near the electrolyte/anode interface, e.g., approximately 1.5 and 2.8 wt% at co-sintering temperature of 1300 and 1400 °C, respectively. Electrochemical measurements showed that, compared with the lower co-sintering temperatures (1300–1350 °C), the highest co-sintering temperature (1400 °C) led to the highest ohmic resistance because of lower electrolyte conductivity. These results suggest that high co-sintering temperature causes excessive Ni diffusion into the BZCYYb electrolyte layer, thus degrading the intrinsic electrolyte conductivity and consequently degrading the SOFC performance.     

Sintering of anorthite ceramic body based on interstratified illite-smectite clay

The effects of the different properties of plastic raw materials for whitewares based on different clay minerals (kaolinite vs interstratified illite/smectite [I/S]) on the sintering process (according to vacuum water absorption) was studied for anorthite porcelain body creation after firing in the range of 950–1500 °C to achieve optimal sintering temperature. The mineralogical composition of the fired bodies was observed from the firing temperature of 950 °C to the sintering temperature. Calcium carbonate decreases the sintering temperature and creates a new mineralogical phase in the fired body – anorthite – which decreases the linear thermal expansion coefficient and increases the strength (modulus of rupture) of the sintered bodies. All the described effects take place at lower firing temperatures when the I/S plastic raw material is compared with kaolin.     

Recycling of lime mud and fly ash for fabrication of anorthite ceramic at low sintering temperature

Lime mud is a kind of waste generated during causticization reaction of an alkali recycling process in paper industry. Lime mud and fly ash were reused as raw materials to fabricate anorthite ceramics through solid state reactions. Both sintering temperature and lime mud content influenced the crystalline phases in the prepared ceramics. Anorthite was the major phase in all samples (samples L36, L40, L50 and L60) and it was prominent in sample L36 (containing 36 wt% lime mud). The results also showed that anorthite ceramic can be synthesized at low sintering temperature (1100 °C). Gehlenite and wollastonite were formed in the samples possessing higher calcium (above 40 wt% lime mud) or at lower sintering temperatures. Bulk density, water absorption and compressive strength were measured. These ceramics were of light weight and had high water absorption. Recycling of lime mud and fly ash as raw materials of anorthite ceramic is a feasible approach to solve the solid wastes.     

Preparation of porous cordierite ceramic by gel-casting method

Porous cordierite ceramics were synthesised by gel-casting method, using talcum powder, kaolin and alumina as raw materials. Organic monomers and cross-linker were used as additives. The phase composition and microstructure were investigated by X-ray diffraction and scanning electron microscope. The open porosity, compressive strength and thermal expansion coefficient were tested by the Archimedes method, universal testing machine and thermal expansion instrument, respectively. The results indicate that sintering temperature and holding time have a great influence on the cordierite properties. We obtain the good performance of porous cordierite ceramic sintering at 1350°C for 3?h. The cordierite phase content in the sample is higher and the crystallinity is better. At this point, the porosity is 58.53%, the compressive strength is 22.44?MPa and thermal expansion coefficient reaches 1.69?×?10^?6?°C^?1.     

Fabrication of high performance macroporous tubular silicon carbide gas filters by extrusion method

Hot gas filtration requires high performance tubular filters, but low permeability, low strength, and high sintering temperature of silicon carbide (SiC) filters limit their use. In this work, a high permeability tubular SiC support was fabricated with high strength at a sintering temperature of 1200?°C, when 100?µm SiC particles were used as aggregate, sodium dodecyl benzene sulfonate (SDBS) was used as sintering aid and organic additives were used as binders. Plasticity of the mixed particles was optimized by adjusting the ratios of methylcellulose, paraffin, and glycerol. The porosity, pore diameter, gas permeation coefficient, and bending strength of the SiC ceramic support reached 45.0%, 34.2?µm, 4.6?×?10^–12 m², and 22.8?MPa, respectively. Furthermore, compared to the cold isostatic pressure (CIP) technique, the extrusion method led to sharper peak of the pore diameter distribution, achieved higher bending strength, and had a more homogeneous microstructure.     

The effect of boron containing frits on the anorthite formation temperature in kaolin–wollastonite mixtures

In order to investigate the effect of boron containing frits on anorthite formation temperature in kaolin–wollastonite mixture, four different frit compositions containing boron were prepared according to Seger formulas. One of these compositions also contained lead. Four different batches composed of 40 mass% kaolin, 40 mass% wollastonite and 20 mass% frit were prepared. The linear dilatometric (LD) curves of the batches were determined and subsequently the firing schedule (FS) curves were obtained from the LD curves. Cylindrical pellets prepared from each of the batches were fired in an especially designed furnace up to respectively 950, 975, 1000, 1025, 1050 °C. The firing period including the cooling process was adjusted to 210 min. The variation of the bulk densities of the products as a function of temperature were examined. X-ray diffraction (XRD) patterns of the products were also determined and it was observed that the minimum anorthite formation temperature was 1000 °C. Since it was known that with batches not containing any frit, the minimum anorthite formation temperature was 1100 °C, it was understood that leaded or unleaded boron containing frits decreased the anorthite formation temperatures around 100 °C.     

The Effect of Phase Composition on the Mechanical Properties of LTCC Material

For the fabrication of complex, micro‐electromechanical systems (MEMS) devices based on low‐temperature co‐fired ceramic (LTCC), higher firing temperatures and longer times than those proposed by the LTCC producer are needed. These changes to the thermal budget may influence the material properties and consequently its functional properties. The effect of the firing conditions on the LTCC DuPont 951 and thus on the phase composition, that is, the alumina/anorthite ratio and porosity, on the mechanical properties is presented. The samples fired at low temperatures (800°C) had a high porosity (7%), which significantly contributed to the low elastic modulus (100 GPa) and the low mechanical strength of the LTCC (140 MPa). The samples fired at 850°C, which had only 1% of porosity, resulted in an elastic modulus of 122 GPa and a flexural strength of 224 MPa. A further increase in the temperature contributed to a slight decrease in the elastic modulus, while no significant difference in the flexural strength could be observed. The enhancement of the flexural strength with an increasing firing temperature was mainly related to a decrease in the porosity and to a lesser extent to the different ratio of the alumina/anorthite phases. The effect of firing time on the phase composition at selected temperatures (i.e., 100 h at 700 and 800°C) is also discussed.     

Elaboration and characterization of tubular macroporous ceramic support for membranes from kaolin and dolomite

A low cost macroporous support for ceramic membranes was prepared by in situ reaction sintering from local natural mineral kaolin with dolomite as sintering inhibitor. The characterization focused on the phase evolution, microstructure, pore structure, mechanical strength and water permeability at various compositions and sintering temperatures. The sintering of kaolin was improved with 5 wt% dolomite, but clearly inhibited with ≥10 wt% dolomite. For the 20 wt% dolomite samples, the crystalline phases were mainly composed of mullite, cordierite and anorthite after sintering between 1,150 and 1,300 °C. Moreover, both mean pore size and mechanical strength increased with increasing sintering temperature from 1,100 to 1,300 °C, but the water permeability and porosity decreased. The 1,250 °C sintered macroporous support with 20 wt% dolomite exhibited good performances such as porosity 44.6%, mean pore size 4.7 μm, bending strength 47.6 MPa, water permeability 10.76 m³ m⁻² h⁻¹ bar⁻¹, as well as good chemical resistance. This work provides opportunities to develop cost-effective ceramic supports with controllable pore size, porosity, and high strength for high performance membranes.     

The effect of high temperature on the residual mechanical performance of concrete made with recycled ceramic coarse aggregates

This paper presents an experimental study on the residual mechanical properties of concrete with recycled ceramic coarse aggregate (RCCA) after exposure to elevated temperatures. Four concrete mixes were produced: a control concrete and three concrete mixes with replacement ratios of 20, 50 and 100% of natural aggregate (NA) by RCCA. The specimens were subjected to temperatures of 200, 400 and 600°C, for a period of 60 min. After cooling down to room temperature, the following concrete properties were evaluated: (i) compressive strength; (ii) splitting tensile strength; (iii) modulus of elasticity; (iv) ultrasonic pulse velocity (UPV); and (v) water absorption by immersion. At ambient temperature, as expected, the replacement of NA by RCCA resulted in a performance reduction of concrete. After exposure to elevated temperature, in general, the results obtained indicated an improvement of the residual relative mechanical properties of the mixes with RCCA, particularly after exposure to 400 and 600°C. However, exposure to the highest temperature (600°C) tended to cause spalling in concrete mixes containing RCCA. Significant linear correlations were observed between the residual compressive strength of all concrete mixes and both the UPV and the water absorption by immersion. Copyright © 2014 John Wiley & Sons, Ltd.     

Influence of fine ceramic aggregates on the residual properties of concrete subjected to elevated temperature

The residual properties of concrete subjected to elevated temperature are of importance to assess the stability of the structure. This paper investigates the performance of concrete containing white ware ceramic sand exposed to elevated temperature. Concrete mixes containing 0%, 50%, and 100% ceramic sand were prepared. The specimen were exposed to elevated temperatures of 200°C, 500°C, and 800°C for a duration of 60 minutes. Their residual mechanical properties (compressive strength, split tensile strength), ultra sonic pulse velocity, and mass change for different cooling regimes were investigated and compared among specimen. The results showed that incorporation of ceramic sand in concrete mixes improved the resistance against elevated temperature of hardened concrete.     

Preparation and characterization of porous anorthite ceramics from red mud and fly ash

The porous anorthite ceramics with high porosity, good mechanical strength and low heat conductivity were prepared using red mud and fly ash as raw materials via the pore forming method. The effects of sintering temperature and fly ash on phase evolution, densification, compressive strength, thermal conductivity and microstructure of the ceramic materials were investigated. The results showed that the compressive strength of the porous ceramics had an obvious improvement with the increase in fly ash, and the densification and heat conductivity decreased firstly and then increased. In particular, specimen S2 containing 30 wt% red mud and 40 wt% fly ash sintered at 1150°C had the better performances. It had the water absorption of 18.18%, open porosity of 38.52%, bulk density of 1.29 g/cm³, compressive strength of 42.46 MPa, and heat conductivity of 1.24 W/m·K. X-ray diffraction analysis indicated that mullite, anorthite, α-quartz, and diopside ferrian were the dominant phases in the specimens. Scanning electron microscopy micrographs illustrated that plenty of open pores with strip shape and closed pores with axiolitic shape existed in the specimens. Furthermore, the existence of mullite could prevent crack propagation to enhance the energy of inter-granular fracture. It endowed the porous anorthite ceramics with high porosity, good compressive strength, and low heat conductivity.     

Effect of CaO/SiO2 ratio on phase transformation and properties of anorthite-based ceramics from coal fly ash and steel slag

Anorthite-based ceramics were produced entirely from coal fly ash and steel slag. The effect of the CaO/SiO₂ ratio (0.12–0.8) on the phase transitions was examined by adding steel slag to coal fly ash in the range of 10–50 wt%, and a temperature range of 900–1200 °C. The influence of CaO/SiO₂ and sintering temperatures on the technological properties were assessed by response surface methodology (RSM) and correlated with the phase changes. The results revealed that anorthite was the main phase for the CaO/SiO₂ ratio ranging from 0.12 to 0.56, while at 1200 °C, a ratio of 0.8 involved a high content of gehlenite. RSM showed that the CaO/SiO₂ ratio was the main influencing factor on the density, while the variation of apparent porosity and compressive strength were more affected by sintering temperature. The crystallisation of the anorthite phase significantly enhanced the properties of the obtained ceramics, whereas the appearance of gehlenite reduced the mechanical strength. The optimum conditions to fabricate anorthite-based ceramics with suitable properties were found to be a CaO/SiO₂ ratio of 0.46 and a temperature of 1188 °C. The optimised anorthite-based ceramic exhibited a low thermal conductivity (0.39 W/m.K) and a dielectric constant of 6.03 at 1 MHz, along with a compressive strength of 41 MPa, which makes this sample a potential candidate for insulator applications.