Evaluation of durability and cleanability performances of protective treatments for lapped ceramic tiles—Part 1

The aim of this work was to evaluate the durability, in real pedestrian traffic condition, of lapped porcelain stonewares with surface protective treatments able to fill up pores opened by the lapping process. In recent years, the durability of ceramic surfaces has become a key factor during the design process. The knowledge of the material durability is important to define the behavior of tiles in use. Standard procedures for indirectly assessing durability are already available (ie, abrasion tests, staining resistance, chemical resistance), however, a standard method of aging in real condition is still lacking. In this study, a durability test was performed in real pedestrian traffic condition by fixing tiles on floors to evaluate the surface characteristics after 6 and 12 months. The surface characteristics were analyzed before and after the durability tests and in terms of microstructural observations and determination of surface porosity. In particular a durability index has been estimated as ratio of pores filled by dirtiness respect to the original surface porosity (before aging). Results of durability index showed good correlation between aging duration and kind of treatments (industrially applied and/or manually applied).     

Enhancement of hybrid sol–gel coating and industrial application on polished porcelain stoneware tiles and investigation of the performance

The aim of this study was to improve stain resistance and cleanability of polished porcelain tiles by sealing the micro-pores on the tile surface with hybrid sol–gel solution. For this purpose, solutions of a hybrid system based on sol–gel technology were prepared. The obtained solution were applied to polished unglazed tiles and subsequently spread with soft tissue thereon. To achieve complete sealing, both organic and inorganic particles of different sizes were mixed with obtained sol–gel hybrid systems. Coated surfaces were investigated by means of a scanning electron microscope. The stain resistance and the cleanability of the tile surface were tested according to ISO-10545-2014 standard. It has been shown that prepared sealing material permanently prevents formation of stains such as olive oil, green staining paste, iodine solution on the surface of polished porcelain tiles. The remains of stains on the surface can easily be removed from it with a cleaning agent.     

Dependence of surface porosity on the polishing depth of porcelain stoneware tiles

Porcelain stoneware tile polishing is a process that adds value to ceramic tiles due to the high gloss achieved upon reducing surface roughness. However, surface polishing removes a fine layer of the product, revealing numerous “closed” pores initially located inside the material, which may compromise some of its properties such as stain resistance. The literature indicates a possible orientation of pores on surfaces parallel to that of the use of the product. Based on this hypothesis, the present work aimed to evaluate how the thickness of the layer removed by polishing acts upon the profile of exposed surface pores, and hence, on the stain resistance of the product. The results of this study are novel and reveal that the staining tendency of porcelain stoneware tile can be altered significantly by varying the conditions of the surface wear produced by polishing.     

Effect of soda-lime glass on sintering and technological properties of porcelain stoneware tiles

The feasibility of waste glass recycling in ceramic tile production was assessed with special reference to fully vitrified products (porcelain stoneware). Soda-lime float or container glass was introduced, in replacement of sodic feldspar, in typical porcelain stoneware bodies (up to 10 wt.%) that underwent a laboratory simulation of tilemaking process, with a technological and compositional characterization of both fired and unfired tiles. Soda-lime glass had no significant effect on semi-finished products, but it influenced remarkably the firing behaviour, increasing shrinkage and closed porosity, decreasing open porosity and bulk density, and lowering mechanical and tribological performances. Waste glass promotes a more effective melting of quartz and a partial dissolution of mullite, leading to a more abundant and less viscous liquid phase, which accelerates the sintering kinetics. In conclusion, soda-lime glass can be used in small amounts (5% or less) with tolerable modifications of technological behaviour and performances of porcelain stoneware tiles.     

Ecologic ceramic substrates for CIGS solar cells

In this work the use of porcelain stoneware tiles as alternative substrates for CIGS thin film solar cells for the development of specific applications as those related to building integration (BIPV, BAPV) are reported. Two types of porcelain stoneware have been compared: the conventional tile (STD) and the ecological tile (ECO). The ECO ceramic paste formulation has been designed adding industrial wastes (recycled glass, sludges and chamote). Chemical, technological and functional properties of the paste have been performed. The CIGS absorber has been synthesized by an easy and low-cost way of preparation using co-precipitation method. The solar cell device has been completed and fully characterized.The achieved results indicate better performances for ECO paste compared to the STD, stated in improved mechanical resistance, thickness and morphology. It is demonstrated that both tiles are suitable for solar cell applications, but ECO substrate developed higher energy conversion efficiency of 1.3%.     

Evaluation of texture distribution during the industrial polishing process of porcelain stoneware tiles

The present work addresses the distribution of texture over the surface of porcelain stoneware tiles due to kinematics imposed by industrial polishing process. The scratching process was simulated using a computational algorithm, which was based on the kinematic equations for the whole myriad of abrasive particles. Different scratching patterns were identified over the polished surface and their corresponding positions were mapped. The experimental results showed that regions in the tile centre present smaller tendencies for exhibiting preferential textures than those of the lateral ones. The final texture was slightly asymmetric and different from those simply left by the last scratches. The definition of three polishing domains was then suggested according to different phenomenological criteria. Results from both experiments and simulations made evident the influence of the kinematic parameters adopted by the industries on the polishing quality of porcelain stoneware tiles.     

Evolution with Temperature of Crystalline and Amorphous Phases in Porcelain Stoneware

Porcelain stoneware tile is a ceramic building material characterized by high technological properties, especially regarding water absorption, chemical and frost resistance, bending strength and abrasion resistance. Because mineralogy is one of the main factors affecting the mechanical properties of porcelain stoneware, a complete determination and quantification of the mineral and amorphous phases is of special importance in the study of porcelain stoneware tiles. In the present work, a reference industrial composition (50% kaolinitic clay, 40% feldspar, and 10% quartz) of porcelain stoneware tiles fired at different temperatures (400°–1400°C) was characterized by X-ray powder diffraction combined with quantitative full-phase analysis using the Rietveld method, including amorphous content. The green composition contained albite, microcline, and muscovite as fluxing agents, which start to decompose at low temperatures (400°–800°C range) and are completely dissolved above 1200°C. The mullite phase is formed from 1100° to 1230°C and at the latter temperature, quartz particles start to dissolve. Studies of mineralogical evolution have revealed that the high heating rate (45°–50°C/min) required in ceramic tile manufacture leads to significant differences in comparisons with whiteware ceramics fired at a lower heating rate (10°C/min). Thus, the formation of mullite in porcelain stoneware occurs at higher temperatures (1100°C) whereas the transformation of β-quartz to β-cristobalite does not take place. The experimental results of this study show that qualitative mineralogical analysis, based on the intensity of a particular diffraction peak for each crystalline phase, is a suitable methodology to obtain preliminary knowledge of mineralogical changes with temperature.     

The role of viscosity on microstructure development and stain resistance in porcelain stoneware tiles

Effect of composition on viscosity of porcelain stoneware tiles and role of the viscosity on microstructure development and stain resistance in porcelain stoneware tiles were investigated. The viscosity of the tiles was successfully measured by a thermomechanical analyzer. Na₂O/K₂O ratio was used as a parameter to change the viscosity. As the Na₂O/K₂O ratio increases, the viscosity decreases. This reduction in the viscosity results in improvement of microstructure (i.e., spherical pore morphology and reduced closed porosity) and usually increases the stain resistance significantly. However, too low viscosity may also cause deformation of the tile during production. Therefore, the viscosity of the tiles should be carefully controlled and it should lie between a lower limit (in this study this value is between 10^7.93 and ～10^8.35 P) and an upper limit (in this study this value is 10^8.67 P) to achieve high stain resistant porcelain tiles with no deformation.     

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.     

Development of anorthite based white porcelain stoneware tile compositions

The scope of the present study is to develop alternative white porcelain stoneware tile compositions without using zircon (ZrSiO₄) which have shown better whiteness values than standard white porcelain stoneware tile composition containing zircon. These new compositions are designed to develop anorthite crystals in the microstructure. Technological properties of compositions such as water absorption (%), firing shrinkage (%) and colour (L^⁎, a^⁎, b^⁎) were measured and compared with standard white porcelain stoneware tile. Scanning electron microscopy (SEM), X-ray diffraction (XRD) analyses were also carried out. One of the new anorthite-based compositions was manufactured as white porcelain stoneware tile substitute of standard white porcelain stoneware containing zircon.     

Color matching algorithms in ceramic tile production

In the present work, the possibility of transferring in the ceramic tiles production the know-how developed in the field of the paints by using the Kubelka–Munk theory, in the form used for opaque surface coatings, have been evaluated. Five different target colors have been chosen as target and tried to reproduce with an industrial glaze in a cycle for fine porcelain stoneware tiles. Four industrial pigments have been chosen as basic stains for the formulations. The results show a good efficiency of the color matching algorithm applied to pigments for glazes for fine porcelain stoneware tiles. All the formulations, in fact, have allowed to obtain a value of ΔE^* lower to the accepted limit.     

Chemical hardening of glazed porcelain tiles

Glazed ceramic tiles are used for wall and floor covering thanks to their high resistance to chemicals attacks, although big efforts should be done to increase their surfaces resistance to mechanical stresses. This study investigates the applicability of a well-known glass hardening process to glazed ceramic tiles following a rational design based on Design of Experiments technique. A Mixture Design has been carried out to formulate new frits compositions to enhance the ion-exchange process, starting from a commercial product. Vickers Hardness and SEM-EDS techniques have been employed to evaluate the frits and elaborate the model. Results suggest that frits for ceramic tiles are positively affected by ion-exchange process only if an appropriate combination of ions in the starting composition is present, establishing a new category of frit formulations suitable for that purpose. The results have been confirmed employing the optimized frit for the glazing of porcelain stoneware.     

Influence of zeolites on the sintering and technological properties of porcelain stoneware tiles

Low-cost zeolitic rocks are promising substitutes for feldspathic fluxes in ceramic bodies, since their fusibility, modest hardness and high cation exchange capacity (CEC) should improve grinding and sintering. Five large-scale Italian deposits of natural zeolites with different mineralogy were characterised and tested in porcelain stoneware bodies. Their behaviour during processing was appraised and compared with that of zeolite-free bodies. Zeolites increased the slip viscosity during wet grinding, causing a coarser grain size distribution and consequently some drawbacks in both unfired and fired tiles. After overcoming this hindrance by dry grinding of zeolite rocks, the technological behaviour of zeolite-bearing tiles appear to be similar to that of current porcelain stoneware, though with larger firing shrinkage and residual closed porosity.     

Effect of porosity on the elastic properties of porcelainized stoneware tiles by a multi-layered model

Porcelainized stoneware represents a leading product in the world market of ceramic tiles, thanks to its relevant bending strength (with respect to other classes of tiles) and extremely low water absorption: these properties derive from its really low content of residual porosity. Nevertheless, an accurate investigation of the cross section of a porcelainized stoneware tile reveals a non-uniform distribution of the residual pores through the thickness, which results in a spatial gradient of properties. Porcelainized stoneware, therefore, may be looked at as a functionally graded material. In the present research, commercial porcelainized stonewares were analysed in order to define the effect of the residual porosity and its spatial distribution on the mechanical properties of tiles. Polished cross sections of porcelainized stoneware tiles were investigated by optical and scanning electron microscopy in order to define the content and distribution of residual pores as a function of distance from the working surface. For each porcelainized stoneware, the local elastic properties of the ceramic matrix were measured by a depth-sensing Vickers micro-indentation technique, then the so-obtained microstructural images and elastic properties were used to model the stoneware tile mechanical properties. In particular, the cross section of each tile was described as a multi-layered system, each layer of which was considered as a composite material formed by a ceramic matrix and residual pores. The elastic properties of each layer were predicted by applying analytical equations derived from the theory of composite materials and, as a new approach, by performing microstructure-based finite element simulations. In order to validate the proposed multi-layered model and identify the most reliable predictive technique, the numerical results were compared with experimental data obtained by a resonance-based method.     

Process of pyroplastic shaping for special-purpose porcelain stoneware tiles

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 (>10 μm) 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.     

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.     

Technological behavior of porcelain stoneware bodies with Egyptian syenites

The progressive depletion of the main feldspathic flux deposits in the World is forcing the ceramic industry to search for suitable substitutes. The aim of this study is to assess the potential of some feldspar sources in the Egyptian Eastern Desert, particularly syenites from Abu Khruq, in the manufacture of ceramic tiles. Beneficiated samples obtained by laboratory-scale mineral processing were tested into porcelain stoneware batches (from 10% to 30% wt in replacement of feldspars) and compared with a reference body through a laboratory reproduction of the industrial processing. The technological behavior of unfired tiles does not suffer any significant alteration due to the use of syenites. On the other hand, syenite-bearing bodies exhibit some changes in the phase composition and the chemistry of the melt, that turns richer in alkali, especially K₂O. The consequently increased sintering rate depends mainly on the viscosity of the liquid phase formed during firing. In conclusion, syenites can be used without technological hindrances to manufacture porcelain stoneware tiles. The firing behavior of syenite-bearing batches can be reasonably adjusted by setting key parameters (eg, the feldspar amount and the Na/K ratio), but the darker color of fired bodies requires to further lower the iron oxide.     

细炻砖铜锈缺陷研究

细炻砖作为仿古砖的一个重要组成部分一直在家庭装修中被广泛应用。在细炻砖的生产过程中,铜锈在浅色版面中容易显露出来,占总缺陷的比例比较大,也降低了产品的品质。本文用XRF对原材料和坯体配方进行了定性分析,用SEM对铜锈进行了表面形貌分析,用EDS对铜锈进行了元素分析。结果表明,铜锈主要是原料中的有色金属杂质引起。通过加强浆料的过筛,降低浆料的细度可以有效减少铜锈的产生。     

The reliability of polished porcelain stoneware tiles

The flexural strength and the Young’s modulus of as fired and polished porcelain stoneware tiles were investigated. Bending tests were carried out on suitable specimens, cut from five commercial products before and after polishing. The differences found in the flexural strength data were evaluated by the analysis of variance and the determination of the Weibull parameters. The results showed that: (i) the as fired tiles and the corresponding polished tiles cannot be always considered the same material and (ii) the data scattering is usually greater for the polished products, i.e., the reliability of the polished products, in terms of the Weibull modulus m, decreases. The results, supported by microstructural observations, surface roughness and Vickers hardness measurements, were directly attributed to the severe damage induced by the first step of machining, i.e., calibrating or grinding. Young’s modulus data showed a clear dependence on the pre-existing porosity, i.e., the production process, rather than on the machining induced damage. It was also shown that the conditions and the characteristics of the polished working surfaces strongly depend on the microstructure of the as fired material.