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Porcelain stoneware is sintered by partial vitrification through viscous flow of a liquid phase formed at high temperature. This sintering process involves a complex evolution of both phase composition and chemistry of the liquid phase, according to the dynamic equilibrium established with the residual minerals and the new crystalline phases formed during firing. The present contribution overviews the evolution of phase composition and microstructure of seven body formulations during firing at different temperatures and dwell times. Each mixture was characterized from the chemical point of view and, once fired, by XRPD (quantitative phase composition), SEM and optical microscopy (microstructure). Moreover, the sintering behavior of the batches was investigated in isothermal conditions by optical thermo-dilatometric analysis. The results indicate that the continuous variation of the phase composition during the heating treatment affects the chemical composition of the liquid phase reflected on the densification kinetics and pyroplastic deformation of tiles. [ABSTRACT FROM AUTHOR]

The technological behavior of porcelain stoneware bodies during deposition and pressing of large slabs depends on the rheological properties of spray-dried powders and the way they affect compaction. Although the literature offers some insights into the characteristics of spray-dried powders for ceramic tiles, no data are available on bodies utilized by novel technologies for large slabs (>4 m 2 ). In order to fill this gap, a systematic approach to properties and behavior of spray-dried powders for porcelain stoneware slabs was carried out. For this purpose, twenty industrially-manufactured spray-dried powders were characterized for intrinsic features (particle size and agglomerate size distribution; shape and moisture distribution in function of agglomerate size) and rheological properties (mass flow, static and dynamic angles of repose, poured and tap density, Hausner ratio). The effect of intrinsic characteristics on the flowability of powders was appraised, as the mutual relationships between the rheological parameters, in order to unveil the best parameters to be used in ceramic tile production. Two broad classes of spray-dried powders occur with a finer and a coarser agglomerate size distribution. Results reveal that ceramic powders are free-flowing, with rheological properties fluctuating in a rather narrow range of values, which makes hard to see significant relationships among the various methods (the only strict correlation is between mass flow rate and tap density). Flowability mainly depends on the occurrence of coarser aggregates, particularly those irregular in shape, stemming from coalescence of three or more individual agglomerates. A phenomenological description of how the spray-dried powders behave in the deposition on tape and the early stage of compaction is outlined. [ABSTRACT FROM AUTHOR]

Drop on demand ink-jet printing is turning to be the leading technique in the decoration of ceramic tiles. This technology makes use in most cases of pigmented inks which are manufactured by micronizing conventional ceramic pigments in the 0.2–0.6 µm range (median diameter). Although significant improvements to optical properties are in theory put forth by reducing the pigment particle size, not all the expected advantages occur and still unanswered questions concern the color strength of micronized pigments. This is the second part of a study aimed at disclosing what happens during pigment micronizing; it is focused on phase composition and color in the submicrometric field. For this purpose, representative industrial pigments were selected: Cr–Sb-doped rutile (orange–yellow), Co–Cr–Fe–Mn–Ni spinel (black), and V-doped zircon (turquoise–blue). Such pigments were micronized in a pilot plant and characterized for particle size and morphology (SEM and HR-TEM), phase composition, crystallite size and unit cell parameters (XRD-Rietveld), optical properties (DRS) and color after application in glazes for porcelain stoneware tiles fast fired at 1200 °C (CIE L ⁎ a ⁎ b ⁎ ). Results highlight a different behavior during micronization: crystal structural and optical features are substantially changed once pigment particles turn into submicronic size. A gradually lower particle dimension is accompanied by reduction of crystallite size and increasing frequency of lattice defects (inferred from variation of unit cell parameters and optical properties) up to amorphization that may attain 75 wt%. The formation of amorphous phase takes place below a critical crystallite size (30–40 nm) which discriminates two regimes with fast and slow comminution rates. These structural changes are associated to decreasing color strength and increasing brightness through the submicrometric field. [ABSTRACT FROM AUTHOR]

The ball clays from the Donetzk basin, Ukraine are widely utilized in the production of ceramic tiles. Their commercial success stems from unrivaled technological properties, whose link to mineralogical composition and particle size is not well understood yet. This work is an in-depth investigation of the mineralogical, chemical, and physical properties with the aim to disclose the reasons of the peculiar technological behavior of these clays. Five clay samples were studied by XRF, XRD (bulk and fractions < 2 μm and < 0.2 μm), SEM, TEM, rheological characterization, particle size distribution, BET, MBI, Pfefferkorn index, Atterberg plasticity limits, and laboratory simulation of the tilemaking process. The Ukrainian clays are very fine-grained and characterized by poorly ordered kaolinite (Kaol), interstratified illite/smectite (I–Sm) and a low quartz content. The Kaol-to-I–Sm ratio is lower than in conventional ball clays and two different I–Sm types are found to be predominant in the colloidal fraction. Morphologically, clay minerals appear to be mostly subhedral lamellae that curl and fold under pressure. These characteristics explain the outstanding technological properties of Ukrainian clays and particularly their high plasticity and suitable rheological behavior. These properties depend on the peculiar conjunction of mineralogical and grain size factors that are difficult to be reproduced by clay blending or mixing design. [ABSTRACT FROM AUTHOR]

Abstract: A reappraisal of crystal structure, optical properties and technological behaviour of black spinels in the Co—Cr—Fe—Mn system was carried out to define the best compositions and disclose the role of crystal chemistry in colouring performance and pigment-glaze interactions. Twenty ternary and quaternary spinel formulations were designed using crystallochemical criteria and prepared by simulating the industrial synthesis. Powder samples were characterized through X-ray diffraction (Rietveld), optical, Raman and Mössbauer spectroscopies, and technological testing in several glazes and glassy coatings. Black spinels fall in a relatively narrow field of unit cell and inversion parameters in between “chromites” and “magnetites”. Their cation distribution is governed by Co2+ occupancy of tetrahedrally-coordinated site T and Cr3+ occupancy of octahedrally-coordinated site M, with Mn3+ hosted at M, while Mn2+ and Fe3+ are distributed over both sites. Nevertheless, Raman and Mössbauer spectra indicate a growing disorder in cation partitioning going towards the iron-rich terms. The pigment technological behaviour depends to a large extent on crystal chemistry of spinels, with no effect by grain or crystallite size. The best colouring performance corresponds to recommended compositions able to withstand corrosion and change in crystal chemistry in contact with melted glaze. Three different pathways can give rise to excellent black pigments: (a) strongly disordered spinels having an inversion parameter i∼0.2; (b) moderately disordered spinels, free of Mn3+, with i<0.1; (c) apparently ordered and normal spinels halfway from cobalt chromite and cobalt ferrite. Poor technological performances are due to different conditions leaving room to colour bleaching (low Cr amount), glaze bloating (excess of Mn3+) and limited resistance to corrosion (oversaturation of site M by Cr+Fe with formation of kenotetrahedral spinels or eskolaite—hematite solid solutions). [Copyright &y& Elsevier]

The purpose of this study is to evaluate different lightweight concretes for the first time formulated with lightweight expanded aggregates produced only with industrial waste. The motive is that a lack of knowledge still exists in the literature about how to integrate these waste-based aggregates in lightweight concrete. To achieve the desired bloating and aggregates physical properties, PC-TV screen glass and ceramic tile polishing sludge were selected as suitable raw materials. Both were characterized by mineralogical and chemical analyses and the effect of different combinations was pointed out. Hot-stage microscopy was used to determine the bloating rates and firing behavior. Lightweight expanded aggregates were obtained using both static laboratory kiln and rotating pilot kiln, by firing at maximum temperatures between 1150 and 1200 °C, to simulate the industrial production process and favor scaling up. The so obtained aggregates were characterized from the physical-mechanical point of view, highlighting an important bloating attitude and bulk density lower than 700 kg/m3 for all the test conditions. Bulk density, water absorption and mechanical properties are fully comparable to commercial counterparts. The best material was used as coarse aggregate in lightweight structural concrete and cellular concrete prepared at pilot scale (for structural application and thermal/acoustic insulation, respectively). The technical properties are consistent with standard requirements of compressive strength (>25 MPa for lightweight structural concrete) and thermal conductivity (18–24 W/m*K for cellular concrete). These results demonstrate the technological feasibility of using waste-based aggregates into lightweight concrete design, according to a circular economy vision. • Old screens and ceramic muds show to be suitable for lightweight aggregates production. • Waste-based lightweight aggregates production and characterization. • Use of waste-based lightweight aggregates in concrete manufacturing. • Characterization of concrete containing waste-based coarse aggregates. • Comparison between experimental products (aggregates and concretes) and commercial ones. [ABSTRACT FROM AUTHOR]

Abstract: A novel technique to manufacture special-purpose tiles (i.e. trim pieces, steps, skirting boards, etc.) has been recently developed on the basis of a pyroplastic shaping of porcelain stoneware tiles. This innovative process involves a second firing, peaking at temperatures close to those of sintering, whose effect was investigated by comparing industrially manufactured tiles before and after pyroplastic shaping. Characterization by XRF, XRPD, SEM and standard testing (ISO 10545) put in evidence that pyroplastic bending induced little changes in the water absorption and bulk density values, as in phase composition. Limited variations occurring to closed porosity, mechanical strength and microstructure do not significantly affect the overall technological performance of the special-purpose tiles, which is substantially the same of the original porcelain stoneware tiles. A detailed microstructural characterization was performed for the first time on porcelain stoneware tiles: coarse grains (>10μm) represent 10–15% of total volume, while fine-grained crystals, dispersed in the glassy phase, amount from 30% to 65% of the viscous matrix. The pyroplastic behaviour was found to depend in a complex way on such microstructural and compositional features, which deeply affect the effective viscosity of the matrix. [Copyright &y& Elsevier]

In the last years, the ceramic sector started to focus on the exploitation of local georesources with circular economy solutions based on waste recycling. The present work aims to assess both the technological feasibly of waste recycling in the clay bricks manufacturing and the effect of different degreasers on the same clay body in order to disclose which factors control technological behavior. Non-plastic waste materials (e.i. mining and construction wastes) were added to a plastic clay and the brickmaking process was performed at the laboratory scale. The main technological properties of the semi-finished and finished products does not suffer any significant variations, indeed with the addition of mining and construction wastes were improved the plasticity reaching optimal values for the clay bricks extrusion. A clear disadvantage is found related to the decrease of mechanical strength, both in dry and fired products, ascribable to the convergence of microstructural factors. In conclusion, the different effects on the technological properties of clay bricks are correlated to the type of waste added as degreaser and the complex interplay of particle size distribution and mineralogical composition in ceramic matrix. • The main technological properties of the products does not suffer any variations. • The addition of mining/construction wastes were improved the plasticity of clay brick. • A disadvantage found is due to a decrease of mechanical strength of the clay brick. [ABSTRACT FROM AUTHOR]

The present study aims to provide a deeper understanding of how different particle size distributions and degrees of powder compaction affect the densification process of porcelain stoneware tiles. For this purpose, three different batches underwent a laboratory simulation of the industrial tilemaking process, at growing grinding time or increasing forming pressure, with technological characterization of both unfired and fired products. Sintering behaviour was determined by hot-stage microscopy. Phase composition was determined by XRD-Rietveld allowing the estimation of the chemical composition and physical properties of the liquid phase. The results illustrate the impact of too fine (or too coarse) grain size on powder compaction, firing shrinkage, water absorption, efficiency of densification, sintering rate, stability at high temperature, and risk of anticipated overfiring (and similarly for too high or too low forming pressure). Simultaneous variation of particle size and forming pressure beyond usual standards induced changes in technological behaviour that in most cases compensate each other. Phase composition is moderately influenced by particle size and little by powder compaction. The vitreous phase mainly suffered from a decreasing degree of polymerization as the particle size became finer or the dry bulk density decreased. In case of insufficient grinding, too much residual feldspars caused improper composition and properties of the glassy phase, which resulted in lower efficiency of densification and slower sintering rate. Both the sintering kinetics and degree of densification depend on the timescale, i.e. the ratio of surface tension to viscosity (melt) and median particle size. Nevertheless, a low powder compaction can trigger a microstructural effect on sintering (improving both densification rate and efficiency) that may outweigh the timescale effect. [ABSTRACT FROM AUTHOR]

In view of the transition to circular economy, the ceramic industry is searching for suitable flux substitutes. Glassy wastes are promising candidates, even though they may significantly affect the firing behaviour and particularly high temperature permanent deformations. The goal of this study is to assess the sintering behaviour of porcelain stoneware bodies containing glassy wastes. Five different sources (bottle, PC-TV screen, lamp, frit manufacturing and porcelain stoneware grinding) provided glassy wastes that were separately added to a reference batch (20-40-60%) in substitution of feldspars. Each batch was characterized from the chemical point of view and its sintering kinetics were determined in isothermal conditions by optical thermo-dilatometric analysis. The quantitative phase composition after firing was determined by XRD-Rietveld in order to calculate chemical and physical properties of the vitreous phase at high temperature. The substitution of feldspathic fluxes determines a clear change in the firing behaviour: waste glasses lower the temperature at which the densification starts, and extend the early stage occurring before a constant rate sintering takes place. In some cases, the deviation is important and entails a lower sintering efficiency. The reasons behind this different firing behaviour are discussed in terms of composition, structure, and properties of the melt. • Glasses instead of feldspathic fluxes determine a global change of firing behaviour. • Different firing behaviors depend on the structural changes occurring to the melt. • Wastes anticipate the densification start and modify the extent of sintering stages. • In batches containing specific glassy wastes a lower sintering efficiency can occur. [ABSTRACT FROM AUTHOR]

• The LWA firing behaviour was appraised by in-situ and ex-situ experiments. • Bloating of glassy LWA is enhanced and anticipated by increasing the aggregate size. • Glass composition defines both start and extent of bloating induced by SiC addition. • A phenomenological model of bloating was proposed for glassy LWA. • The effect of glass viscosity on macro- and microstructure was pointed out. The firing behaviour of five waste glasses was studied to define key parameters able to predict bloating performances in industrial lightweight aggregates production. Bloating was investigated by hot-stage microscopy and laboratory kiln experiments, also with SiC as expanding agent. In-situ and ex-situ results were contrasted to shed light on the scale up process. Both macro- and microstructure of aggregates are related to the melt properties and crystallization phenomena. A phenomenological model for SiC-induced expansion was proposed. The glass chemical composition controls melt viscosity and kinetics of expansion, while the increase of aggregate size fosters the extent of bloating. [ABSTRACT FROM AUTHOR]

The advent of inkjet printing (IJP) as the main used decoration technique for ceramic products has changed the technological requirements for pigments and the way they are obtained. To meet IJP and durability requirements, it is necessary to micronise the colourants to achieve particles smaller than 1 μm (median particle diameter d50 ∼300 nm). The high-energy milling process induces microstructural changes that affect the colour properties. In order to understand the effect of particle size reduction on chromatic properties, an in-depth study was carried out by simulating the industrial milling process on a pilot scale. The effect of micronisation was investigated for four ceramic pigments (yellow zircon, brown spinel, pink malayaite, and green eskolaite) and one dye (blue olivine) by UV–vis–NIR optical spectroscopy (DRS), chromatic coordinates and XRPD analyses (Rietveld method). This study was carried out on the colourants both as they were and mixed with glass, reproducing the industrial firing process. The results obtained led to the definition of the main aspects responsible for the colour evolution derived from the milling process. [ABSTRACT FROM AUTHOR]

The advent of inkjet printing as digital decoration of ceramic materials has irreversibly modified the industrial decoration technology, imposing companies to change the colorant production process. The inkjet application requires micronized particles in the ultrafine particle size range (smaller than 1 μm). Particles size reduction of ceramic colorants is performed by a high-energy comminution process in wet-operated bead mills, affecting colorants properties. Since a deep knowledge of milling-induced microstructural changes is still lacking, the micronization effects on a set of five industrial ceramic colorants are thoughtfully investigated in this work by simulating the industrial process at a pilot plant. Particle size distribution and energy consumption are monitored during the comminution process. The compositional (including crystallite size and microstrain analysis of the main phases) and morphological variation of four ceramic pigments (yellow zircon, brown spinel, pink malayaite, and green eskolaite) and one dye (blue olivine) is investigated by XRPD (Rietveld method) and SEM analyses. The analytical approach combined with a physical/semiempirical modelling of the colorants elastic features versus the energy demand for particle reduction has yielded details on the nature of the micronization-induced microstructural changes in ceramic colorants. Specifically, the comminution efficiency as well as the crystalline phase stability are related to the intrinsic properties of each colorant. Brittle breakage rather than plastic deformation on comminution are also system dependent. When an euhedral to subhedral crystal habit is maintained a brittle fracture is preserved throughout the comminution progress, while the formation of flake-like particles and particle agglomeration are strong evidences of plastic deformation. The last evidence deals with the material elastic features. Materials with high bulk modulus convert the grinding energy to lattice defects that lead to particle breakage by brittle fractures, while materials with lower bulk modulus convert/dissipate part of the supplied energy in plastic deformations, drastically decreasing the comminution process efficiency. [ABSTRACT FROM AUTHOR]

This work aims to evaluate the use of bottom biomass ash as an alternative raw material in porcelain stoneware bodies. For this purpose, ash coming from a biomass thermoelectric power plant in Emilia-Romagna (Italy) was selected and its chemical, mineralogical and thermal properties determined. Data indicated its technological role as a flux, so it was introduced in a porcelain stoneware batch in partial replacement of feldspars and experimented at laboratory scale. A bottleneck, relative to the rheological behavior of the slips, was overcome by a slight deflocculant increase. The powder compacts were fired from 1000° to 1220 °C in order to follow the evolution of the technological properties, phase composition (XRPD-Rietveld) and microstructure (SEM). The introduction of ash allowed to lower the firing temperature by 20 °C, while keeping the technological properties comparable with those of the benchmark. Moreover, the mineralogical and microstructural data revealed different sintering kinetics. [ABSTRACT FROM AUTHOR]

Yb-doped and undoped Y3Al5O12 (YAG) nanometric powders were synthesized by microwave assisted liquid synthesis from nitrate precursors and calcination. The homogeneous and rapid heating of the reaction volume of this synthesis process allowed a precise control on particle nucleation and growth and the formation of nanometric primary particles with a narrow size distribution. The chemical composition of synthesized powders was checked by ICP and FTIR analysis. The morphology of the precursor and calcined powders as well as the microstructure after sintering were analysed by SEM. High temperature XRD and TG-DTA analyses, performed in the 800-1200 °C range, with two temperature ramps, made it possible for a thorough investigation of the crystallization behaviour of YAG and Yb:YAG precursors. The results show that the crystallization of Yb:YAG from the powder precursors started at 900 °C and that both the presence of Yb and a high heating rate accelerate the YAG formation through the intermediate YAH (hexagonal YAlO3 phase). The intermediate phases Y4Al2O9 (YAM) and YAlO3 (YAP) that are generally observed during the calcination step of nanometric YAG powder did not form. Pellets of pressed powders were sintered under high vacuum in clean atmosphere at 1700 °C for 16 h and a full density translucent material with grain size from 3 to 5 µm was obtained. [ABSTRACT FROM AUTHOR]

The ceramic tile industry is a dynamic sector whose technological innovation and market trends have drawn a complex picture of products and processes. Raw materials have been deeply involved in such an evolution: the flexibility of current manufacturing cycles enables the use of a very wide range of clays, whose chemical and mineralogical composition, particle size distribution and ceramic properties were reappraised. The classical reference schemes are no longer able to properly describe and predict the role of clays in tilemaking. In order to fill this gap, an industry-oriented, technological classification of clay raw materials is proposed on the basis of chemical (Fe2O3 content) and mineralogical parameters (amount of phyllosilicates and carbonates) together with particle size (fractions <2μm and >63μm) and plasticity (methylene blue index and Atterberg plastic index). It firstly discriminates light-firing and dark-firing clays according to an iron oxide threshold of 3%. Light-firing clays are distinguished by the amount of kaolinite group minerals and plasticity in "kaolins" (high-grade, low-grade, and raw kaolins, kaolinitic loams) and "plastic clays" (ball clays, pyrophyllitic clays, white bentonites); the distinction of three classes of ball clays with increasing plasticity is envisaged. Dark-firing clays are classified according to coarse-grained fraction and amount of carbonates in carbonate-rich types (marly and carbonatic clays), red loams and red clays; these latter are furtherly differentiated by the relative abundance of clay minerals. Such a classification is essential to draw the guidelines for body formulation and to explain the criteria followed in the industrial practice for each category of ceramic tiles. Key properties are discussed to design batches for porous tiles, vitrified and semi-vitrified red stoneware, and vitrified light-firing bodies. Both the clay classification and the guidelines to body formulation are intended to provide up-to-date tools to assess the ceramic potential and correct use in tilemaking of clay materials, but they cannot substitute a throroughful technological characterization, adequately simulating the industrial processing. [ABSTRACT FROM AUTHOR]

[Display omitted] • Borates, wollastonite, diopside, spodumene and phonolite were used as strong fluxes. • Strong fluxes modify phase composition and liquid phase viscosity and surface tension. • Flux changes melt properties: polymerization degree, network modifiers, Al speciation. • Strong fluxes anticipate densification start affecting sintering kinetics differently. • Microstructure and physical properties of porcelain stoneware depend on the flux type. Strong fluxes are needed to fire vitrified ceramics at temperatures significantly lower than those usually reached in industrial firing cycles. This work is aimed at understanding the role of strong fluxes in the microstructural evolution during sintering. Six fluxes (colemanite, ulexite, wollastonite, diopside, spodumene and phonolite) were individually added to a porcelain stoneware batch and processed in standard conditions. Compacts and fired bodies were characterized by optical dilatometry, XRD-Rietveld, SEM and measuring technological properties. Strong fluxes change the firing behaviour with a complex interplay of sintering kinetics, microstructural features, and phase composition. Every flux has its own repercussion on the properties of the liquid phase (chemical composition, degree of polymerization, viscosity and surface tension) which are key points to explain the observed microstructure, densification rates, and stability at high temperature. Batches with phonolite, wollastonite or diopside exhibit characteristics closer to standard porcelain stoneware, while spodumene and borates suffer from unsatisfactory microstructures and lower densification efficiency. [ABSTRACT FROM AUTHOR]

Ce-doped garnets find their use as scintillators as well as phosphors. Among synthetic garnets, gallium gadolinium aluminium garnet (GGAG) is a relatively new and interesting material, a mixed garnet that has displayed very good scintillation and luminescence properties. In the presented work Ce-doped GGAG ceramic samples were produced by reaction sintering from commercial oxide powders. The mixed powders were pressed into pellets and sintered. In order to achieve transparency of a ceramic material, it is crucial to eliminate porosity. Therefore, a number of process parameters and their influence on the microstructure and optical quality of the ceramics were studied: starting powders, sintering additives and sintering conditions. The sintering process consisted of two steps, viz. pressureless sintering in air followed by hot isostatic pressing. Luminescence properties of the sintered ceramics were studied by means of photo-luminescence and correlated to the fabrication process parameters. [ABSTRACT FROM AUTHOR]

The compaction behavior of spray-dried powders has turned into concern in porcelain stoneware manufacturing due to the increasing diffusion of large slabs. It is necessary to fill a knowledge gap between the compaction behavior with conventional presses and novel technologies. For this purpose, eighteen industrially-manufactured spray dried bodies were characterized for specific properties connected to the compaction behavior (curves of bulk density, intergranular and intragranular porosity in function of applied load, apparent yield strength). In addition, the firing behavior was investigated in order to reveal any effect of dry bulk density on firing shrinkage and bulk density of fired samples. Powder compressibility is within 50% and 55% (Carr index) and is primarily controlled by moisture. Two regimes are found: low pressure (fast density increasing by granule cave in and closure of intergranular porosity) and high pressure (slow density gain by downsizing microporosity). A peculiar mechanism is unveiled: granules squeeze in the low-P regime and further densification is achieved through microfracture around individual agglomerate. A phenomenological model is illustrated for the compaction of spray-dried powders. In conclusion, the performance of spray dried bodies during compaction is crucial to control the uniformity, in terms of porosity and bulk density, which has important repercussions on the properties of final slabs, especially differential shrinkages and deformation during firing due to density gradients. [ABSTRACT FROM AUTHOR]

Cobalt-based pigments were synthesized using clinopyroxene as crystalline host. The pigments are a Ca(Co x Mg 1- x )Si 2 O 6 solid solution, with x  = 0.05, 0.2, 0.4, 0.5, 0.6, 0.8 Co apfu , and were synthesized from melt. The pigments were characterized by single-crystal X-ray diffraction, SEM-EDS, optical microscopy, colorimetry and optical spectroscopy. The pigments were then tested in glazes and glassy coatings for ceramic tiles to determine their colouring performance. The synthesized pigments showed a pink colour with different shades, due to the absorption of Co 2+ in octahedral coordination. The colour disappeared in the Ca(Co 0.05 Mg 0.95 )Si 2 O 6 pigment after grinding. The Ca(Co x Mg 1- x )Si 2 O 6 pigments partially dissolved in glazes, acting like dyes, and bestowed a blue shade on coatings, with a colour efficiency similar to industrially used blue colorants (cobalt aluminate spinel and cobalt olivine). In the glassy coatings, pigment dissolution occurred at a lower extent, and a colour intermediate between the pyroxenes and the pigment-bearing glazes was obtained. The Co content in pyroxene dyes is lower than conventional pigments, being down to 5%wt. [ABSTRACT FROM AUTHOR]

In the present study metakaolin (MK), prepared by calcination of a natural tunisian kaolin at 750 °C, was mixed with different amounts of phosphoric acid (11–48 wt% P 2 O 5 ) and heated at various temperatures (250, 500, 750 and 1000 °C) for 4 h. The structure of materials was characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy (MEB), thermal analysis (DTA/TGA). The XRD results showed the formation of an AlPO 4 phase for the MK-H 3 PO 4 mixtures containing 11, 17 and 27 wt% P 2 O 5 independently of the heating temperature. While with 48 wt% of P 2 O 5 two major phases have been formed Al(PO 3 ) 3 at 500 °C and AlSi 2 (PO 4 ) 3 at 1000 °C. These results were confirmed by 27 Al, 31 P and 29 Si MAS-NMR techniques. [ABSTRACT FROM AUTHOR]

Several batches of porcelain stoneware were formulated by partial replacement of coarse Na and Na-K feldspars (standard batch) with quartz in two different grain sizes, micronized K-feldspar or a mixture of these components. Everyone was processed (laboratory scale) by wet and dry routes to compare: sintering curve, microstructure, phase composition and viscosity of the liquid phase at high temperature. Pyroplasticity index was determined by 3-point bending test. Results indicate that finer the quartz particles, more they dissolved in liquid phase, increasing viscosity and reducing deformation. Micronized K-feldspar increased the sintering kinetics (causing deformation). However, when K-feldspar was combined with quartz, densification rate was improved without compromising integrity of tile. About dry route, microstructural heterogeneity turned as fundamental to prevent Pyroplastic deformation. As conclusion, pyroplastic deformation occurs by different mechanisms in samples prepared by the two processing routes and bodies prepared via dry route are less likely to undergo pyroplastic deformation. [ABSTRACT FROM AUTHOR]

In recent times, there has been an increasing demand for white-firing clay materials in ceramic tile production, due to a progressive shift towards porcelain stoneware bodies. Furthermore, the recent geopolitics events have challenging the industries possibilities to satisfy their clay demand for porcelain stoneware production by well-established commercial channels. In this framework, there is a need to expand the reserves of ball clays, especially in Mediterranean countries, which have a low frequency of clay deposits suitable for the production of porcelain stoneware. For this purpose, the Tabarka clays in Northern-West of Tunisia have been studied. They have already been investigated in the literature for the production of various types of ceramic tiles (but not porcelain stoneware). The technological behaviour of bodies, containing 40% clay, was assessed by simulating the industrial tile manufacturing process on a laboratory scale. Tabarka clays (TC) were compared with high and low plasticity ball clays (which define a reference range for the raw materials currently used). TC exhibit a fine grain size distribution and plasticity approaching the most plastic ball clays currently used. The behaviour of the bodies containing TC during grinding, pressing, and drying is fully suitable, with values of the technological parameters that lie within the reference range. Using TC, a slightly higher temperature was necessary during firing to get the desired water absorption (<0.5%). This was due to the kaolinite-rich composition of TC that is more refractory than ball clays taken as reference, which contain illite and illite-smectite interstratified terms, along with kaolinite. The properties of fired bodies based on TC fulfil the technical requirements for porcelain stoneware tiles. • Two clays from the north of Tunisia were tested for porcelain stoneware tiles production. • The batches were designed and samples were prepared simulating the industrial process. • The properties of semi-finished products were comparable to the benchmark. • The higher amount of kaolinite led to limited refractoriness increase. • The new clays are suitable for porcelain stoneware tiles and slabs manufacturing. [ABSTRACT FROM AUTHOR]

Digital decoration of ceramic tiles has turned to be a prevalent and dynamic technology in the last decade. Current printers use drop-on-demand (DOD) ink-jet print heads (IJP) fed with solvent-based inks containing ceramic pigments as coloring agents. However, due to environmental constraints, water-based systems are envisaged as a green alternative for ceramic tiles decoration. Nevertheless, aqueous suspensions are difficult to be managed because physical properties of water are far away from the DOD-IJP operating window. Thus, the control on the stability, homogeneity and rheology of such water-based systems is an important factor to achieve better product performances. This study was aimed at exploring both the rheological behavior and stability of three inks based on micronized pigments dispersed in water and water-MEG solutions: (V,Zr)SiO 4 (turquoise zircon, TZ), (Cr,Sb)TiO 2 (orange rutile, OR) and (Co,Mn) (Fe,Cr) 2 O 4 (black spinel, BS). The pigments were ground to submicronic size in water using a circulation type agitator mill and by changing the main parameters affecting the particle-size distribution (rotation speed, type and amount of dispersants). The stability of aqueous pigment suspensions was studied by measuring the zeta potential as function of pH and by sedimentation tests. The zeta potential was sufficiently strong (lower than −30 mV) to render the inks stable in the 7–10 pH range. Rheological measurements of the suspensions showed a Newtonian flow behavior for zircon and rutile inks and slightly pseudoplastic for the spinel one. An attempt was also made to evaluate the inks printability with the help of dimensionless numbers based on the relevant physical and rheological properties affecting the jetting, i.e. viscosity, surface tension and density. It was observed that the ground aqueous systems fall in the printable fluid region obtained with semi-empirical models. [ABSTRACT FROM AUTHOR]

The pyroxene structure was successfully used as a crystalline host for a new Co-based pink pigment. Different compositions of Ca 1−x Co 1+x Si 2 O 6 (0 ≤ x ≤ 1) powders were prepared by using the solid state method with different temperatures and times of annealing. The characterization of the fired samples was performed by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), colorimetry and technological tests simulating the ceramic applications. A monophasic pink clinopyroxene was obtained for compositions close to the CaCoSi 2 O 6 end-member. For increasing cobalt content, an assemblage of Co-olivine, Co-åkermanite and Ca,Co-pyroxene was found. Samples containing Co-åkermanite have a deep blue color stemming from Co 2+ in tetrahedral coordination, while those with Co-olivine are violet due to Co 2+ in octahedral coordination. The best conditions for the synthesis of a CaCoSi 2 O 6 single phase were then investigated, finding that a proper annealing cycle at 1150 °C prevents the formation of metastable Co-åkermanite. The CaCoSi 2 O 6 pyroxene was tested as a ceramic pigment into glazes and glassy coatings. In most cases, it behaves like a dye, completely dissolving in the melted glaze, analogously to Co-olivine. [ABSTRACT FROM AUTHOR]

Inkjet printing is increasingly used to decorate ceramic tiles, with pigmented inks in most cases. These inks are manufactured by micronizing conventional ceramic pigments down to median diameters of 0.2–0.6 µm. Although such a size reduction theoretically offers significant advantages in terms of the optical and fluid mechanical properties of the pigment particles, still unanswered questions concern color strength and the efficiency of the milling process. The present study aimed to elucidate how micronizing influences the pigments’ particle size and shape, and the specific surface area. For this purpose, industrial pigments were selected to represent crystal structures of different density, hardness, cleavage and fracture toughness, i.e. rutile, spinel and zircon. The pigments were micronized in a pilot plant, controlling carrier, solid load, dispersant type and concentration, rotation speed, amount and size of grinding media, temperature and milling time. The pigments were characterized by particle size distribution (laser diffraction and dynamic light scattering) and morphology (SEM). The results revealed a different behavior of the pigments during micronization, with changes in particle size and shape partially consistent with the literature. The pigments’ grindability differed: zircon>rutile≥spinel. [ABSTRACT FROM AUTHOR]

The temperature dependence of Young׳s modulus of silica refractories exhibiting pore volume fractions in the range 18.6–20.9% is investigated via impulse excitation up to 800–1000 °C and four-point bending up to 500 °C. This temperature dependence exhibits a broad valley approximately below 200 °C, where Young׳s modulus values decrease to 55–62% of their room temperature magnitude. Upon further heating, Young׳s modulus increases steeply within a few degrees around 230 °C, followed by an S-shaped increase to maximum values corresponding to 174–282% of the room temperature values. Cooling branches are different from heating branches and do not form closed loops, indicating damage accumulation. Dilatation measurements, polarization microscopy, micromechanical calculations and X-ray phase analysis are used to underpin the conclusion that the observed elastic effects are not caused by phase transitions alone, but are combined effects of phase transitions and microcrack opening and partial closure. [ABSTRACT FROM AUTHOR]

Abstract: Tionite is the insoluble residue from the titania slag dissolution process for TiO2 manufacturing. It is a fine-grained sludge consisting of rutile, anatase, amorphous phase and bassanite. Chemical composition is TiO2 (ca. 50%), SiO2 (ca. 30%) and minor Al, Ca, Mg, and Fe, plus residual sulfur, implying an acidic pH of waste. Moisture is about 35% of dry weight. The potential of tionite as colouring agent in clay bricks was appraised by admixing (up to 9%) either as-produced or neutralized tionite to four industrial clay bodies. The effect on technological behaviour was assessed by laboratory simulation of the industrial brickmaking process and determining working moisture, drying sensitivity, shrinkage and bending strength, water absorption, bulk density, efflorescence, and colour. The use of tionite is technologically feasible, with little adjustment of industrial cycle, and resulting brick performances depend remarkably on the composition and properties of clay bodies. Carbonate-rich bodies seem to be affected by tionite more during drying than during firing; carbonate-poor bodies range from little changes to consistent worsening of brick performances. No relevant changes of process and product parameters were found up to 3% tionite. Additions over 5% induce significant variations, such as increase of working moisture and water absorption, decrease of bulk density and bending strength. A definite and consistent improvement of this technological behaviour is achieved by using neutralized tionite. The yearly output of tionite could be entirely recycled by approximately four average-size brickworks adding about 3% of residue (dry weight). [ABSTRACT FROM AUTHOR]

Abstract: Low-cost, naturally-occurring mixtures of feldspar and zeolite occurring in epiclastic rocks are promising substitutes for conventional quartz-feldspathic fluxes in ceramic bodies, since their fusibility and low hardness are expected to improve both grinding and sintering. Three epiclastic outcrops, with a different zeolite-to-feldspar ratio, were characterized (XRPD, fusibility) and tested in porcelain stoneware bodies; their behaviour during processing was appraised and compared with that of a reference. The addition of an epiclastic rock (20wt.%), replacing rhyolite and aplite fluxes, brought about some significant advantages, mainly represented by better grindability, lower firing temperature with improved mechanical strength and lower porosity. Disadvantages concern increased slip viscosity, worse powder compressibility, resulting in larger firing shrinkage, and a darker colour of the tiles due to relatively high amounts of iron oxide. [Copyright &y& Elsevier]