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.
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•Technological evolution in the ceramic tile sector and effect on product typologies.•Chemical, mineralogical and granulometric features of ceramic clays worldwide.•Ceramic properties and application of clay materials in tilemaking are reappraised.•A new, industry-oriented classification of clays for ceramic tiles is proposed.•Design criteria and guidelines for batch formulation of each type of ceramic tile.
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•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.
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•Sourcing of feldspathic raw materials is reviewed, including viable flux substitutes.•A large set of data allows a critical insight into market dynamics of ceramic ...fluxes.•Technological value of different fluxes is referred to ceramic use and batch design.•Ongoing trends disclose possible criticalities in the supply of key raw materials.•Actions are envisaged to mitigate the supply risk and improve the resource efficiency.
Feldspathic fluxes are fundamental ingredients of many ceramic products, but little is known about the sources actually utilized by industry and the evolution over time. This paper reviews the sourcing of feldspathic raw materials over a long timespan (1971–2016) including viable substitutes (particularly recycled materials and end-of-life products). A large set of data was collected, critically assessed and elaborated to get an original insight into market dynamics. In addition, the technological value, based on composition and technological properties of different fluxes, is referred to ceramic use and batch design. A detailed analysis of ongoing trends, mainly driven by the technological innovation in the ceramic tile industry, disclosed possible criticalities in the medium-term supply of key raw materials, principally sodium feldspar from albitites. To mitigate this risk, three actions are envisaged to improve the resource efficiency and sustainability of feldspathic fluxes: 1) full valorization of primary deposits, 2) increasing recourse to alternative sources, 3) relaxing the technological constraints in ceramic applications.
Self-cleaning applications using TiO₂ coatings on various supporting media have been attracting increasing interest in recent years. This work discusses the issue of self-cleaning textile production ...on an industrial scale. A method for producing self-cleaning textiles starting from a commercial colloidal nanosuspension (nanosol) of TiO₂ is described. Three different treatments were developed for purifying and neutralizing the commercial TiO₂ nanosol: washing by ultrafiltration; purifying with an anion exchange resin; and neutralizing in an aqueous solution of ammonium bicarbonate. The different purified TiO₂ nanosols were characterized in terms of particle size distribution (using dynamic light scattering), electrical conductivity, and ζ potential (using electrophoretic light scattering). The TiO₂-coated textiles' functional properties were judged on their photodegradation of rhodamine B (RhB), used as a stain model. The photocatalytic performance of the differently treated TiO₂-coated textiles was compared, revealing the advantages of purification with an anion exchange resin. The study demonstrated the feasibility of applying commercial TiO₂ nanosol directly on textile surfaces, overcoming problems of existing methods that limit the industrial scalability of the process.
Five porcelain and porcelain stoneware bodies were investigated to compare sintering mechanisms and kinetics, phase and microstructure evolution, and high temperature stability. All batches were ...designed with the same raw materials and processing conditions, and characterized by optical dilatometry, XRF, XRPD-Rietveld, FEG-SEM and technological properties. Porcelain and porcelain stoneware behave distinctly during sintering, with the convolution of completely different phase evolution and melt composition/structure. The firing behavior of porcelain is essentially controlled by microstructural features. Changes in mullitization create conditions for a relatively fast densification rate at lower temperature (depolymerized melt, lower solid load) then to contrast deformations at high temperature (enhanced effective viscosity by increasing solid load, mullite aspect ratio, and melt polymerization). In porcelain stoneware, the sintering behavior is basically governed by physical and chemical properties of the melt, which depend on the stability of quartz and mullite at high temperature. A buffering effect ensures adequate effective viscosity to counteract deformation, either by preserving a sufficient skeleton or by increasing melt viscosity if quartz is melted. When a large amount of soda-lime glass is used, no buffering effect occurs with melting of feldspars, as both solid load and melt viscosity decrease. In this batch, the persistence of a feldspathic skeleton plays a key role to control pyroplasticity.
Kitchen and bathroom countertop is a demanding application, where high aesthetic standards must combine with durability, ease of maintenance, and resistance to heat, stain, scratch and chipping. The ...hard materials and composites used for high-end countertops usually contain crystalline silica phases that can be inhaled by workers during drilling and cutting operations. The occurrence of silicosis and other respiratory diseases in machining workers makes it important to know exactly how much crystalline silica is present in countertop materials. This paper collects over 300 quantitative determinations of quartz and cristobalite in porcelain stoneware products and compare these contents with other countertop materials. The sum of crystalline silica phases in porcelain stoneware is on average 21 ± 5 % by weight (mostly quartz). This content is lower than granite (∼30 %) and much lower than engineered stone (∼90 %). Possible ways to reduce the amount of crystalline silica phases in ceramic slabs are overviewed and critically discussed.
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The shear viscosity and the glass-vapor surface tension at high temperature are crucial to understand the viscous flow sintering kinetics of porcelain stoneware. Moreover, the pyroplastic deformation ...depends on the viscosity of the whole body, which is made up of a suspension of crystals dispersed in the melt. The existing fundamental theoretical background, along with semi-empirical constitutive laws for viscous flow sintering and glass densification, can be exploited through different approaches to estimate the physical properties at high temperatures starting from amount and chemical composition of the melt. In this work, a comprehensive attempt to predict the properties of the liquid phase is proposed by means of a detailed overview of existing models for viscosity and surface tension of glasses and melts at high temperature. The chemical composition of the vitreous phase and its physical properties at high temperature are estimated through an experimental approach based on the qualitative and quantitative chemical and phase analyses (by Rietveld refinement of X-ray powder diffraction patterns) of different porcelain-like materials. Repercussions on the firing behavior of ceramic bodies, are discussed. Comparative examples are provided for porcelain stoneware tiles, vitreous china and porcelain bodies, disclosing differences in composition and properties but a common sintering mechanism.
The literature on V-doped zircon pigments offers a puzzling situation with unanswered questions about the origin of turquoise colour. In this study, new diffraction (XRPD) and spectroscopic (EPR, ...EAS) data were obtained on four industrial pigments, whose vanadium content is much higher than samples from previous investigations. Both EPR features and XRPD results indicate the occurrence of tetrahedrally-coordinated V4+ at the zircon interstitial site 16g. EPR spectra at 5 K exhibit a structured multiplet attributed to isolated V4+ ions and an exchange-narrowed line resulting by clustering of V4+ ions. The intensity of optical bands is stronger in samples affected by V4+ clusters. However, the turquoise colour stems from the window of transmitted green-bluish light, which depends on both the tail of the main optical band (2B ⟵ 2A) and the onset of charge transfer phenomena. Small variations in these two bands readily turn into a chromatic shift from light blue to green.
Current trends in the ceramic industry see an increasing interest on porcelain stoneware, by which is possible to produce large tiles and slabs. These products are manufactured by spray-dried ...powders, which have excellent technological properties but require high energy and water consumption. Dry-processed granulates allow to reduce such a consumption, but suffer from a different technological behavior. A solution to improve the environmental sustainability of ceramic tile-making is mixing powders prepared by the wet and the dry routes. The technological features of industrially manufactured mixed powders was investigated (rheological properties, compaction response and firing behavior) in order to point out advantages and possible pitfalls. Dry-processed and spray-dried granulates exhibit different distributions of size, shape and moisture that reflect on distinct rheological and compaction behaviors. Mixed powders show a more or less accentuated deviation from linearity between the end terms. This fact makes possible to add up to ∼50% dry granulates by keeping the technological properties close to current industrial bodies. Sparings of water, energy and CO2 emission for the milling and granulation stages are estimated for a mix 50% spray-dried and 50% dry-processed powders.
Las tendencias actuales en la industria cerámica ven un interés creciente en el gres porcelánico, mediante el cual es posible producir grandes baldosas y losas. Estos productos se fabrican mediante polvos atomizados, que tienen excelentes propiedades tecnológicas, pero requieren de un alto consumo de energía y agua. Los granulados procesados en seco permiten reducir dicho consumo, pero presentan un comportamiento tecnológico diferente. Una solución para mejorar la sostenibilidad medioambiental de la fabricación de baldosas cerámicas es mezclar polvos preparados por vía húmeda y seca. Se investigaron las características tecnológicas de los polvos mixtos fabricados industrialmente (propiedades reológicas, respuesta de compactación y comportamiento en cocción), con el fin de señalar las ventajas y posibles inconvenientes. Los granulados procesados en seco y atomizados exhiben diferentes distribuciones de tamaño, forma y humedad que se reflejan en distintos comportamientos reológicos y de compactación. Los polvos mixtos muestran una desviación más o menos acentuada de la linealidad entre los extremos. Este hecho permite añadir hasta ∼50% de granulado seco manteniendo las propiedades tecnológicas próximas a las piezas industriales actuales. Los ahorros de agua, energía y de emisiones de CO2 para las etapas de molienda y granulación se estiman para una mezcla de 50% de polvo atomizado y 50% de polvo granulado vía seca.
Frost resistance is an important requirement for clay roof tiles and the most difficult to keep under control during industrial production. Durability under freeze/thaw cycles is related with the ...capillary pore system, and current models predict the lower the open porosity and the coarser the pore size, the better the frost resistance. This implies, however, high firing temperature and rather refractory clay bodies, which are in contrast with energy and resource efficiency. Therefore, a different approach is needed to improve frost resistance of roof tiles without stressing the concept of low porosity and coarse pore size. A case-study is presented to demonstrate how excellent frost resistance can be achieved by adjusting raw materials formulation and firing schedule. Raw materials and bodies were characterized by XRF, ASTM C958 and technological properties. Both laboratory tests were performed: the products were characterized by water absorption (ASTM C67), open porosity and pore size distribution (Hg porosimetry), phase composition (XRD-Rietveld), mechanical strength (EN 538) and frost resistance (EN 539-2) up to 400 freeze/thaw cycles. Durability of roof tiles was drastically improved acting on both pore size distribution (eliminating pores <0.1 μm) and increasing mechanical strength (>13 MPa). Phase composition plays a complex role with opposite effects of new formed phases versus amorphous phase and residual clay minerals. A satisfactory target (no damages after 250 freeze/thaw cycles) was achieved firing at 900 °C (7% pores <0.1 μm) while the best performance (no damages after 400 cycles) required firing at 950 °C (<1% pores <0.1 μm). Excellent frost resistance (well beyond the standard 50 cycles) can be obtained through proper design of firing schedule and body composition, even for temperatures below 950 °C.
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