Influence of lithium oxide as auxiliary flux on the properties of triaxial porcelain bodies

The influence of Li₂O-content on the properties of standard triaxial porcelains was investigated. Two series of porcelain formulations were produced. The first one comprised seven model formulations with increasing amount of Li₂CO₃ (1–7 wt.%), with respect to the standard triaxial porcelain formulation. The experimental results showed that desirable properties for tableware porcelains can be attained if the Li₂O-content does not exceed ∼1.5 wt.%. In the light of this conclusion, the second series of formulations aimed at producing new porcelains using Li-bearing natural rocks. Under an industrial perspective, the most important finding is that these compositions matured at temperatures 100–120 °C lower than the standard triaxial porcelain formulation and exhibited remarkable resistance at over-firing conditions. The role of increasing Li₂O-content at the different stages of firing is interpreted in the light of its influence on densification, the evolution of crystalline phases and microstructure.     

Sugarcane Bagasse ash substituent feldspar for the production of porcelain electrical insulators

Sugarcane bagasse ash (SCBA) industrial waste was used to partially substitute feldspar in the production of porcelain electrical insulators from a mixture of locally available Bombawuha clay (BC), Chancho sand (CS), and mixture of Wolkite (WF) and Arerti feldspar (AF) (50:50%). The raw materials were characterized for their chemical composition, mineralogy, thermal behavior, and plasticity. The porcelain electrical insulator formulations containing various proportions of SCBA (0 wt%, 10 wt%, 15 wt%, and 20 wt%) were fired at different firing temperatures of 1200 °C, 1250 °C, and 1300 °C for 2.5 h. The fired bodies were evaluated for water absorption, apparent porosity, bulk density, flexural strength, dielectric strength, and microstructure. The results showed that SCBA had 65.06% silica (SiO₂) and had higher alkaline and iron oxide (Fe₂O₃) contents than the natural feldspars. Bombawuha clay contained kaolinite as a major mineral with a middle range plasticity index (PI = 11.2%) and the met the required purity necessary for mullite phase formation. The feldspars had a low alkali content of sodium oxide plus potassium oxide (Na₂O + K₂O) of < 6 wt %. The prepared porcelain insulator containing 10% SCBA, 50% BC, 30% mixed feldspars, and 10% CS, and fired at 1250 °C for 2.5 h exhibited a water absorption of 0.35%, flexural strength of 42.50 MPa, and a dielectric strength of 6.59 kV/mm, which satisfies the obligatory properties for quality porcelain insulators. Further, the partial replacement of feldspar by SCBA up to 10 w% lowered the firing temperature by 50 °C.     

Numerical and experimental study of ion exchange in porcelain tiles

Recently ion exchange, also known as chemical tempering, has been applied to strengthening of porcelain tiles based on the substitution of ions present in the material by larger ones. This paper investigates the chemical tempering in industrial porcelain tiles by the variation of process parameters such as temperature, immersion time, and chemical composition of the porcelain tile. Furthermore, a numerical simulation of the ionic diffusion process was applied. Using a design of experiments approach, the results show that the temperature and the chemical composition primarily affected the flexural strength of the tile. The largest increment obtained was 37% resulting in a porcelain tile with flexural strength of 73 MPa after chemical tempering. Through numerical simulation, it was possible to estimate a diffusion coefficient of potassium ions equal to 1.25 × 10⁻¹⁴ m²·s⁻¹ into the porcelain tile microstructure. This value is about 10 times higher than the diffusion coefficient in glasses.     

Hydro deformation in ceramic tiles at the pre-firing stage

The production of ceramic tiles with larger sizes and reduced thickness has increased the challenge of producing high-quality ceramic tiles in short single-firing cycles. For porcelain tiles, the pressing step is of upmost importance for the microstructure of the green bodies. The particle size distribution, mineral composition of the pastes and porosity before firing define the water flow during the decoration process. Hydro deformation is the curvature of unfired ceramic tiles caused by water absorption during the decoration step before firing. In this work, the hydro deformation is studied in function of tile thickness, compaction, and clay composition according to a 2^K factorial design. Two compositions of porcelain tiles (glazed and polished) were pressed at two thicknesses (3–6 mm) and pressing pressures (35.5–49.8 MPa) forming ceramic tiles with 55 × 110 mm² of surface area. Chemical (XRF), mineralogical (XRD), thermogravimetric (TG), specific surface area (BET), granulometric, bulk density, and porosity analyses were performed for the green tiles of both compositions. To simulate the hydro deformation during the decoration step, the curvature (mm) of the tiles was studied within a 0–180 min interval. The water absorption rate through the surface (g.m⁻²·s⁻¹) of the tiles in an interval of 0–180 s was studied as a function of thickness, pressure and porcelain tile composition. As a result, the thickness of the tiles can change the curvatures from concave to convex. Pressing conditions and composition of the tiles can change the water absorption rates. Porcelain tiles with higher content of clay minerals develop convex curvatures. For tiles with lower content of clay minerals, concave curvatures were developed.     

Recycling bagasse and rice hulls ash as a pore-forming agent in the fabrication of cordierite–spinel porous ceramics

Bagasse and rice hulls ash are both waste materials. In recent years, in order to meet environmental protection, these materials have been recycled in the production of porous ceramics. A solid-state reaction mechanism of calcined alumina and talc was used to prepare cordierite–spinel porous ceramics. Talc was added from 30 to 60 wt.% at the expense of alumina and sintered at 1400°C for 2 h. The effect of bagasse and rice hulls ash (as a pore forming agent) on the densification parameters, cold crushing strength (CCS), and pore size distribution was also studied. The phase composition (X-ray diffraction) and microstructure (scanning electron microscopy) of sintered samples were investigated. The results showed that the main phases present in the samples are cordierite, corundum, spinel, and sapphirine. In the sample with a higher amount of talc additions (60 wt.%), only the formation of the cordierite and spinel phases was observed. The bulk density of the samples and the apparent porosity ranged from 1.77 to 2.26 g/cm³ and from 28.6% to 48.21%, respectively. The CCS of the samples ranges from 13.9 to 36.3 MPa. The microstructures of the sintered samples were observed for the formation of cordierite phase, alumina phase, and spinel phase in an excellent crystallization and phase arrangement.     

Composition and structure of the shells of aluminosilicate microspheres in fly ash formed on the combustion of Ekibastuz coal

A systematic study of the chemical and phase composition and structure of the shells of narrow fractions of nonmagnetic cenospheres separated from a concentrate of fly ash cenospheres from the combustion of pulverized SS (weakly caking coal) coal from the Ekibastuz Basin was carried out. It was found that the separated narrow fractions were characterized by a high Al₂O₃ content of 33–38 wt %. The phase composition included 57–73 wt % vitreous phase, 25–40 wt % mullite, and 1.2–2.5 wt % quartz. An increase in the average diameter, thickness, and porosity of the glass-crystalline shell of globules with the concentration of aluminum was observed. Globules of the following two types were identified in the obtained fractions: spherical globules with an annular structure and foamy globules with a network structure formed from different illite and kaolinite mineral precursors, respectively.     

Composition and ceramic properties of ball clays for porcelain stoneware tiles manufacture in Poland

The paper presents the results of investigations of mineralogical, grain and chemical composition, and ceramic properties of 18 white-firing ball clays from five producing regions in eastern and central Europe. Relationships between the bending strength of the clay after drying and its grain size and mineralogical composition were studied. High contents of illite and illite/smectite minerals in the clay, and low crystallinity of kaolinite and illite, strongly influenced plasticity but also improved sinterability, as appropriate phase transitions of clay minerals began at lower temperatures. The clays were also highly reactive towards other components of the ceramic batches for porcelain stoneware tile production. The ball clays from the Donetsk region in Eastern Ukraine are recommended for the porcelain stoneware tile production in Poland, while the Polish, German and Czech clays may be used only as supplementary components.     

High-alumina porcelain with the addition of a Li2O-bearing fluxing agent

We have studied the influence of increasing the amount of β-spodumene (LiAlSi₂O₆), as a Li₂O-containing flux, on the phase composition, the microstructure evolution and the physical properties of high-alumina porcelain. Quartz reacts with β-spodumene in the temperature range 1150–1250 °C, forming lithium aluminium silicates with a larger amount of SiO₂. The presence of lithium minerals contributes to a lower CTE for the fired bodies. At 1300 °C an improved flexural strength is achieved with compositions containing 1.0 or 1.2 wt.% of Li₂O, as a result of a more uniform microstructure. With increasing amounts of Li₂O the overfiring effect is greatly enhanced. The most favourable characteristics from an industrial perspective with regard to flexural strength and deformation during firing were attained by using a high-alumina porcelain composition containing 1.0 wt.% Li₂O.     

Effect of calcination temperature on use of high-boron-content waste for low-temperature wall tile production

The effects of the calcination temperature on raw-colemanite-waste properties and calcined waste content on wall tile production were investigated. Waste containing 11.24% B₂O₃ calcined between 500 and 800°C was added to wall tile granules in various ratios (0–100 wt.%) to produce a low-temperature-sintered wall tile by adding the maximum content of boron waste, as determined through optimal calcination. The low-temperature (850–1000°C) sinterability of the samples and the effect of the calcined colemanite-waste content on the wall tile properties were investigated. The samples were characterised using X-ray fluorescence, X-ray powder diffraction, differential thermal analysis, thermogravimetric analysis, Fourier-transform infrared spectroscopy, scanning electron microscopy, and colourimetry. The waste calcined at 800°C exhibited a substantially different phase distribution, bond structure, morphology, and colour. The wall tile produced using 40 wt.% colemanite waste calcined at 800°C and subsequently sintered at 950°C exhibited the optimal properties. The linear firing shrinkage, water absorption, and flexural strength of the optimised wall tile were 0.88%, 16.04%, and 36.07 MPa, respectively. The optimised wall tile exhibited major albite, quartz, and diopside phases and 64% higher strength. The sample calcined at 800°C showed that high colemanite-waste content could be incorporated into ceramic bodies.     

Thermal evaluation of the use of porous ceramic plates on ventilated façades – Part I: Effect of composition and firing temperature on porosity and bending strength

The effect of composition and firing temperature on porosity and bending strength of porous ceramic plates for using in ventilated façades were investigated. Two byproducts, basalt and lime mud, were used to obtain porous ceramic plates in accordance with the circular economy concept. Basalt, which is a fine powder-based byproduct generated from the processing of basaltic rock, was used to replace feldspar as the flux mineral in a ceramic composition. Lime mud, a byproduct of the pulp and paper mill process and containing a high content of calcium carbonate (CaCO₃), was incorporated in the ceramic composition to generate pores. The specimens were prepared using three different amounts of lime mud (20, 30, and 40 wt%) and three firing temperatures (900°C, 1000°C, and 1100°C), and their porosity and bending strength were determined. The highest performing specimen (5.1 ± 1.3 MPa bending strength and 42.6 ± 0.5% porosity) was obtained by using 40 wt% of lime mud and a firing temperature of 1100°C with potential for further thermal tests in a ventilated façade in comparison with a commercial porcelain ceramic tile as reference material.     

Effect of talc and titania on the microstructure and mechanical properties of alumina ceramics

Alumina (Al₂O₃) ceramics were prepared with nucleating agent of titania (TiO₂) and fluxing agent of talc by the normal pressure sintering method. The effects of TiO₂ and talc on phase composition, microstructure, and mechanical properties of Al₂O₃ ceramics, as well the strengthening and toughening mechanism were investigated. It was found that the introduction of TiO₂ could be favorable for mullite formation and the improved densification of ceramics, resulting in improved mechanical properties of ceramics as well. Ceramics possess the best comprehensive mechanical performance with fracture toughness of 3.69 MPa·m^1/2 and flexural strength of 255.9 MPa when 4 wt.% TiO₂ and 15 wt.% talc were added. Mineral bridges and nanocrystallites were found in the glass‐ceramics, which caused the deflection of cracks so that it strengthened the grain boundary and improved the mechanical properties of ceramics.     

Phase equilibria of the propylene glycol/CO2 and propylene glycol/ethanol/CO2 systems

The high-pressure vapour–liquid phase equilibria (P–T–x–y) of the binary mixture propylene glycol/CO₂ have been experimentally investigated at temperatures of (398.2, 423.2 and 453.2) K over the pressure range from (2.5 to 55.0) MPa using a static-analytic method. Furthermore, the high-pressure vapour–liquid phase equilibria (P–T–x–y) of the ternary mixture propylene glycol/CO₂/ethanol at constant temperatures of (398.2, 423.2 and 453.2) K and at constant pressure of 15.0 MPa have been determined using a static-analytic method. Initial concentrations of components in propylene glycol (PG)/ethanol (EtOH) mixture vary from 10 up to 90 wt.%. In general, for binary system it was observed that the solubility of CO₂ in the heavy propylene glycol reach phase increases with increasing pressure at constant temperature. On the contrary, the composition of gaseous phase is not influenced by the pressure or the temperature. On average the solubility of PG in light phase of CO₂ amounts to 30 wt.%. The system behaviour at temperature of 398.2 K was investigated up to 70.0 MPa and a single-phase region was not observed. Above the pressure 60.0 MPa a single-phase region of the system was observed for the temperature of 423.2 K. For the temperature of 453.2 K the single-phase was observed above the pressure of 48.0 MPa. For ternary system it was observed that the composition of heavy phase is slightly influenced by the temperature when the mass fraction of EtOH in initial mixture is higher than 50 wt.%. If the mass fraction of PG in initial mixture is higher than 50 wt.%, the composition of heavy phase is not influenced by the temperature anymore. The composition of the PG, EtOH and CO₂ in light phase remains more or less unchanged and it is not influenced by the conditions.     

Boron mining and enrichment waste: A promising raw material for porcelain tile production

Boron mining and enrichment waste (BW) from boric acid (H₃BO₃) production is a by-product of the boron industry. BW exhibits B₂O₃ contents of 16-31 wt%, and therefore, could be used to effectively lower the sintering temperature of ceramics without increasing their thermal expansion coefficient. Herein, we introduced 3-10 wt% of BW to a formulation used for commercial porcelain tile production, and achieved a sintering temperature decrease of 38°C (to 1195°C). The resulting porcelain tiles exhibited a strength of 44.80 MPa and water absorption percentage of 0.01%, and therefore, met TS ISO EN 10545 requirements. Thus, this study paves the way for the use and valorization of BW in the production of porcelain tiles and could inspire the search for other opportunities to utilize BW in ceramic production.     

Preparation study for the low thermal expansion spodumene/mullite composites

In this work, spodumene/mullite ceramics with low thermal expansion were successfully prepared from spodumene, quartz, talc, and clay. The effects of spodumene content and sintering temperature on the mechanical properties of spodumene/mullite ceramics were investigated. The formed phases were then detected by X-ray diffraction analysis and the microstructures of the sintered bodies were determined by scanning electron microscopy. The interaction effects of the spodumene content and sintering temperature on the apparent porosity and bulk density were studied by response surface methodology. The results demonstrate that an appropriate sintering temperature and spodumene content can promote densification, improve the mechanical properties, and reduce the coefficient of thermal expansion (CTE) of spodumene/mullite ceramics. At the spodumene content of 40 wt.%, the sintering temperature of 1270°C, and the holding time of 90 min, the bending strength was 60.45 MPa, the CTE was 1.73 × 10^–6/°C (α_{[25–650°C]} < 2 × 10^–6/°C), the bulk density was 2.28 g cm^-3, and the apparent porosity was 0.43%. Therefore, this study was of guiding significance for reducing the production cost of spodumene low thermal expansion ceramics and improving product quality.     

Influence of mineralogical composition of applied ball clays on properties of porcelain tiles

Relations between quality of ball clays applied in raw materials batches for manufacture of porcelain tiles, and physical properties and microstructure of obtained tiles, were investigated. Studied clays constituted 35% of the batch, while the other components were unchangeable. Stoch index, new IR 3620/3700 index and XRIR index (Stoch index multiplied by IR 3620/3700 index), were proposed to take into account crystallinity of kaolinite and contents of illite and smectites in studied clays. Relationship between XRIR index value of clay, and water absorption and abrasion resistance of the obtained ceramic material, was found. Water absorption of the tile is also correlated with grain size distribution and bending strength of used clay. Optimal ball clay for porcelain tile production should exhibit: Stoch index - min. 4.3; IR 3620/3700 index - min. 1.2; XRIR index - min. 4.8; grain size median - max. 0.27 μm; bending strength after drying - min. 8.0 MPa.     

Comparisons of pulverized coal combustion in air and in mixtures of O2/CO2

Pulverized coal combustion in air and the mixtures of O₂/CO₂ has been experimentally investigated in a 20 kW down-fired combustor (190 mm id×3 m). Detailed comparisons of gas temperature profiles, gas composition profiles, char burnouts, conversions of coal–N to NO_x and coal–S to SO₂ and CO emissions have been made between coal combustion in air and coal combustion in various O₂/CO₂ mixtures. The effectiveness of air/oxidant staging on reducing NO_x emissions has also been investigated for coal combustion in air and O₂/CO₂ mixtures. The results show that simply replacing the N₂ in the combustion air with CO₂ will result in a significant decrease of combustion gas temperatures. However, coal combustion in 30% O₂/70% CO₂ can produce matching gas temperature profiles to those of coal combustion in air while having a lower coal–N to NO_x conversion, a better char burnout and a lower CO emission. The results also confirm that air/oxidant staging is very effective in reducing NO_x emissions for coal combustion in both air and a 30% O₂/70% CO₂ mixture. SO₂ emissions are proved to be almost independent of the combustion media investigated.     

Optimization of the technological properties of porcelain tile bodies containing rice straw ash using the design of experiments methodology

This study examines the effects of replacing fluxing and filler materials with rice straw ash (RSA) in manufacturing porcelain stoneware tile, using the design of experiments (DOE) methodology. The results of the characterization were used to obtain statistically significant, valid regression equations, relating the technological properties of the dried and fired test pieces to the raw materials content in the unfired mixtures. The regression models were analysed in relation to the X-ray diffraction and scanning electron microscopy results and used to determine the most appropriate combinations of traditional raw materials and RSA to produce porcelain stoneware tiles with specific technological properties. The studied range of tile body compositions: clay (40 wt%), feldspar (20–50 wt%), feldspathic sand (5–20 wt%), and RSA (0–25 wt%) was shown to be appropriate for porcelain stoneware tile manufacture.     

Effect of the green porous texture on porcelain tile properties

Porcelain tile is a product characterised by low water absorption (usually less than 0.1%) and excellent mechanical properties. To enhance tile aesthetic qualities, much of the porcelain tile production is polished to provide a high-gloss surface finish, in which certain closed pores in the tile body become visible. This apparent porosity of the polished tile, which had been closed porosity before polishing, sometimes lowers the product's stain resistance.Test pieces were formed from a porcelain tile composition prepared under different milling conditions, pressing variables being kept constant, and the pore size distribution of these pieces was determined. The effect of the porous texture of the green pieces on the evolution of porosity during sintering and on the residual porosity of the densified body was analysed. It was verified that the porous texture of the fired piece was conditioned by the porosity and size of the largest pores in the green piece. The effect of residual porosity on stain resistance was determined by two cleaning methods. The presence of large pores in the green body, stemming from insufficient milling of the raw materials mixture, led to tiles with greater residual porosity and worse stain resistance.     

Eco-friendly preparation and structural characterization of calcium silicates derived from eggshell and silica gel

In this work, the preparation of calcium–silicate-based composites consisting of natural waste from calcium source as eggshell and silica gel from a desiccator as a silicon source both presenting alternative materials for cheap preparation of eco-friendly calcium–silicate bioceramics has been investigated. The effect of the CaO/SiO₂ ratio on microstructural properties has also been studied. The pseudowollastonite formation has been observed in the case of 40 wt.% CaO and 60 wt.% SiO₂ with lowest porosity and highest density 2.6 g/cm³. In the case of 50 wt.% CaO and 50 wt.% SiO₂, the phase transformation from pseudowollastonite to wollastonite was observed. Increasing the calcium content caused higher apparent porosity with 19%. It was shown that the development of novel ceramics from reused waste, eggshell, or silica can be an optimal solution for the low-cost preparation of calcium silicates with potential applications in medicine or cement, food industry.     

Effect of TiO2 additions on densification and mechanical properties of new mulltifunction resistant porcelains using economic raw materials

The aim of this work is to determine the effect of TiO₂ on sintering and mechanical proprieties of new multifunction resistant (MFR) porcelain prepared from local abundant raw materials. Based on a preliminary work, the new selected composition was 30 wt% kaolins (20 wt% kaolin halloysite type + 10 wt% kaolin Tamazart), 45 wt% k-feldspar and 25 wt% quartz and containing different contents of TiO₂ (3, 5 and 8 wt%). The sintering temperatures of mixtures were between 1140 and 1260 °C. Subsequently, the obtained phases in the elaborated samples were investigated by X-ray diffraction and Fourier transform infrared spectroscopy analyses, Raman spectroscopy and SEM analysis. The optimum sintering conditions gave a higher bulk density (2.47 g.cm⁻³) and excellent mechanical properties: The three point flexural strength (3PFS), Vickers micro-hardness (VMH) and apparent porosity (P_A) of porcelains sintered at 1160 °C were 238 MPa, 12.3 GPa and 2%, respectively. This obtained 3PFS value is drastically higher than that achieved for conventional porcelains (ranged between 60 and 80 MPa). Moreover, these two best 3PFS (238 MPa) and VMH (12.3 GPa) values achieved for this new MFR porcelains were considerably higher when compared to those values (3PFS=218 MPa and VMH=6.5 GPa) obtained by others for porcelain −30% ZrO₂ composite, even though their mixtures were hot pressed in vacuum at 970 °C for 2 min. Besides, the maximum value achieved for the new MFR porcelains is nearby that of the flexural strength of porcelain containing 5 wt% TiO₂ and 30 wt % alumina (about 240 MPa). In other words, the presence of 30 wt % alumina in their product well confirm the benefic effect of the used raw materials (saving 30 wt % alumina) on porcelain strengthening.