Source: http://jmrt.com.br/en-incorporation-quartzite-waste-in-mixtures-avance-S223878541830735X
Timestamp: 2019-04-18 10:36:17+00:00

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Table 1. Experimental composition of sanitary ware (% by weight).
Table 2. Chemical composition and loss on ignition (LOI) for raw materials utilized (% by weight).
Table 3. Chemical compositions of prepared ceramic mixtures (% by weight).
Table 4. Particle size distribution.
Table 5. Physico-mechanical characterization of the specimens.
Quartzite is a mineral that contains oxides that are present in the principal raw materials utilized in traditional ceramic production. However, during its extraction and processing, large amounts of waste are generated. This study, therefore, sought to investigate the incorporation of quartzite waste into the ceramic mass, replacing up to 25% of the feldspar by weight. Both, the waste and the conventional raw materials (clay, feldspar, quartz, and kaolin) were characterized. The compositions were formulated based on common industrial products. Suspensions were prepared to evaluate pH and deflocculant content. The specimens were produced using the slip casting process, dried, and fired at 1200°C. The ceramic masses containing up to 15% waste had yield strength values greater than 35MPa, water absorption in the 0.5% range and shrinkage from firing within the range recommended for sanitary ware manufacture. It was concluded that the quartzite waste can partially replace, up to 15wt%, the feldspar in ceramics for sanitary ware production.
Quartzite is a metamorphic mineral consisting mainly of quartz grains and used as ornamental stone [1,2]. The corresponding industry processes large quantities of raw stone to create a wide variety of finished products, thereby generating considerable amounts of waste that are highly polluting. This is exacerbated when these wastes are improperly disposed, specially when discarded into the environment (rivers, lakes, and soil) . Today, in Brazil, the challenge facing the ornamental stone industry, regarding proper disposal of wastes generated during extraction and processing, is the more restrictive requirements imposed by the environmental legislation.
During processing of the quartzite blocks, two types of waste are generally produced, the first being powder from the sawing of the blocks to transform them into slabs, and the second being chips that come from cutting the slabs. Quartzite waste exhibits a non-plastic behavior and, like most traditional ceramic raw materials, is composed mainly of silica (SiO2) and alumina (Al2O3), followed by alkali oxides (Na2O, K2O) [4,5]. Therefore, this type of industrial waste can potentially be incorporated into masses destined for the production of ceramic artifacts. Some studies have been carried out to test the use of quartzite waste as an alternative raw material for the production of red ceramic [4,6]. However, little is known about its potential application for white ceramics, which requires higher firing temperatures. Therefore, due to the scarcity of studies in the literature and in order to find an environmentally adequate destination for quartzite waste, this study analyses the use of this material in the production of sanitary ware, such as ceramic bathroom fixtures.
Sanitary ware is classified as a white ceramic product, consisting of clays having low amounts of iron oxide, which provides a light color to the product following the sintering process. It is traditionally made from raw materials such as clay, quartz, and feldspar. The latter is the most important fluxing agent for densification, promoting the formation of a liquid phase that reacts with other constituents. This phase gradually permeates the microstructure, leading to its densification and producing a ceramic material with low porosity and a high amorphous content . Fluxing material plays an important role in the fundamental reactions of traditional ceramics, influencing the equilibrium between crystalline (particularly mullite) and amorphous phases [8,9]. The use of various recycled materials of different origins as alternative fluxes has been widely discussed in the literature. Previous studies [6,10–14] have proved the viability of using industrial waste as a replacement for feldspar in triaxial model (clay-quartz-flux) of ceramic formulation. These studies reported on ceramic production characteristics meeting technical requirements, with emphasis on the environmental aspects of waste management, contributing to the preservation of natural sources and non-renewable mineral resources.
On the other side, the ceramic industry has been noted for its potential to absorb some wastes, mainly from mineral extraction. This is due to the similarity of their physicochemical characteristics with those of conventionally used raw minerals [12,15–17]. Given the huge amounts of non-renewable mineral resources consumed by the ceramic industry, this potential becomes even more important .
Valuing of industrial waste and classifying it as an alternative raw material could present several advantages over the use of primary natural resources, namely: the reduction in volume of natural raw materials extracted (preservation of resources), lower energy consumption during subsequent processing (reduced costs), and lower pollutant emission levels (improved public health and safety) .
The reuse of quartzite waste not only responds to ecological needs by avoiding inappropriate disposal of material, but also provides economic benefits. Indeed, its reuse may contribute with an additional value to the final product, with a reduction in costs. In this context, the objective of this study is to evaluate the use of quartzite waste as a substitute of feldspar in ceramic for sanitary ware production. This was based on the similarities in their physicochemical characteristics and low iron content, emphasizing the effect on technological properties.
Raw materials used were: a clay, kaolin, feldspar and quartz mixture, supplied by a sanitary ware company, and quartzite waste from a processing facility located in the city of Várzea, PB. The dispersant used was sodium silicate (Pernambuco Chemical S/A).
Chemical composition of the raw materials was determined through X-ray fluorescence (Shimadzu, EDX-720). The granulometry of the quartzite waste and feldspar was determined through laser scattering in a Cilas 1064 LD granulometer, obtaining mean particle sizes of 23 and 27μm, respectively. The mineralogical characterization was carried out using a Shimadzu XRD 6000, with Cu-Kα radiation (40kV/30mA), a goniometer velocity of 2°/min and step of 0.02° scanning from 5° to 60°.
The compositions (Table 1) were formulated based on an industrial ceramic mass, referred to as MP, with feldspar partially replaced by waste at amounts of 10%, 15%, 20%, and 25% by weight, referred to as M10, M15, M20, and M25, respectively.
Experimental composition of sanitary ware (% by weight).
The mixtures were characterized through X-Ray Fluorescence (Shimadzu, EDX-720) and particle size determination (Cilas, 1064 LD).
The slurries were prepared with a 70% in solids content. The density of the suspensions was determined using a densitometer with volumetric capacity of 100ml (Servitech, 774). The influence of the dispersant concentration on the viscosity of the suspensions was analyzed using the deflocculation curve up to the minimum of apparent viscosity using a Brookfield viscometer model LTV, spindle number 2, at a shear rate of 20 RPM for 1min. The pH was also evaluated using a digital pH meter (AKSO, AK90).
The dimensional changes suffered by the material during firing and densification of the mixtures (after shape forming and drying the test specimens) were measured by a dilatometer, Setsys model 16/18 from SETARAM Instrumentation.
The test specimens, with dimensions of 6.0mm×2.0mm×0.5mm, were prepared through the slip casting process and then dried for 48h, remaining 24h at room temperature and 24h in an oven at 100°C. Following this, they were fired in an electric furnace using a defined firing cycle based on the industry standard sintering parameters: 2°C/min up to 600°C, 3°C/min up to 900°C, 4°C/min up to 1000°C, 5°C/min up to 1200°C, remaining at the maximum temperature for 40min. The samples were then cooled at a rate of 6°C/min down to 850°C, 2°C/min down to 600°C, 1°C/min down to 500°C, and 2°C/min down to room temperature.
The technological properties analyzed were: linear shrinkage during firing (LR), water absorption (WA), apparent density, Vickers hardness (VH), and the flexural strength at three points (BTS). The apparent density of the specimens was determined using the Archimedes method. Water absorption was quantified according to the ASTM C373 standard .
Linear shrinkage was obtained from measurements of sample length before and after firing. Hardness tests were conducted using a LECO Hardness Tester, equipped with a Vickers diamond penetrator and a 0.5kgf test load. The bending strength of the sintered specimens was determined using an Instron universal machine with a load cell of 5kN and bridge displacement rate of 2mm/min, according to ASTM C674 . Ten samples test were used for each composition mixture. Statistical analysis was performed using the t-test for means analysis and the F-test for variance comparison.
The mineralogical and microstructural characterizations were performed using XRD (PANalytical X’Pert Pro MPD) and SEM (Hitachi, TM 1000). The mineralogical analysis was performed in a diffractometer equipped with an X’Celerator detector, Cu-Kα radiation at 40kV and 40mA, 2è from 5° to 70°, and a counting time of 10s. For the morphological characterization, the sintered samples were polished and attacked with a hydrofluoric acid (HF) solution for 1min.
The chemical composition of the raw materials is shown in Table 2. It was observed that the waste and the feldspar contain similar amounts of oxides, justifying its use as a partial replacement for feldspar in the ceramic mass.
Chemical composition and loss on ignition (LOI) for raw materials utilized (% by weight).
Through particle size analysis it was observed that mean diameters of the quartzite waste and feldspar were of 23 and 27μm, respectively.
Diffraction pattern of the feldspar shows characteristic peaks of microcline (84: 0708), albite (JCPDS: 84-0752), quartz (JCPDS: 46-1045), and muscovite mica (JCPDS: 83: 1803). Microcline (84: 0708), quartz (JCPDS: 46-1045), and muscovite mica (JCPDS: 83: 1803) were present in the waste.
The chemical composition of the ceramic slurries is shown in Table 3.
Chemical compositions of prepared ceramic mixtures (% by weight).
It can be seen that the addition of the waste did not interfere with the chemical composition of the ceramic masses, when compared among each other. The silica content varied between approximately 65.6% and 66.8%, while the alumina content varied between 26.1% and 27.4%. The mixtures also contained significant amounts of fluxes (Fe2O3, K2O, CaO, and MgO). Some researchers , who have studied the substitution of feldspar in sanitary ware dish by soda-lime-silica glass wastes, detected silica and alumina contents similar to those found in this study.
Ferric oxide (Fe2O3) content was between 1.5 to 2.0%, suitable to manufacture white ceramic products [4,21,22].
The mean particle size (DM) and the volumetric fraction of the masses are shown in Table 4.
It can be observed that the addition of quartzite waste increased the amount of fine particles, causing a reduction in the average particle size of masses with content higher than 15% in quartzite, probably due to the waste having a finer grain size than the feldspar. The granulometry has a strong influence on the ceramic processing. The masses containing 20% and 25% of waste have similar granulometric behavior, with a mean diameter near 6μm and similar particle size fractions.
Fig. 1(a–e) shows the relationship of apparent viscosity and pH on the deflocculation curve as a function of the dispersant content.
Deflocculation and pH curves as a function of the silicate content: (a) MP, (b) M10, (c) M15, (d) M20, (e) M25.
It can be observed that, as the sodium silicate content increases, the viscosity decrease to a minimum, i.e., a flocculation-free system, and above this point, the addition of sodium silicate causes an increase in viscosity. According to Papo et al. , above the adsorption saturation limit, that is, the minimum viscosity value, additional dispersant leads to excess Na+ cations in the medium, which are not adsorbed by the dispersed particles and cause destabilization, flocculation, and an increase in viscosity. An excessive amount of Na+ cations reduces the thickness of the diffusion layer and therefore the zeta potential value decreases, inducing flocculation of the suspension [24,25]. It is therefore possible to conclude that, when sodium silicate concentrations are too high, agglomeration begins due to an excess of electrolyte, as verified by Deliormanli and Yayla , in a study analyzing the effect of calcium hydroxide in a ceramic slurry using sodium silicate as a dispersing agent.
Analysis of the samples (Fig. 1a–e) shown a gradual increase in viscosity, compared to the reference sample (Fig. 1a), probably due to the addition of the waste, having a smaller particle size than feldspar. The minimum viscosity occurs when the dispersant reaches 0.57% by weight in compositions M10 (Fig. 1b) and M15 (Fig. 1c), and at 0.65% and 0.68% for M20 (Fig. 1d) and M25 (Fig. 1e), respectively. This increase is attributed to these compositions having a higher amount of finer particles. According to the literature , viscosity increases as particle size decreases, and at a given concentration, the decrease in particle size results in the reduction of the interparticle distance in the dispersed phase. Smaller particle size has a greater number of leachable ions due to the greater surface area of the particles, increasing the area that the suspended ions can react with . Therefore, a higher deflocculant content becomes necessary to disperse the particles. The use of up to 0.8% electrolytes (by weight) on an industrial scale can provide optimum properties for casting of a suspension containing 70% solids by weight [29–31].
Refering to the pH of the suspensions, each material has an isoelectric point at a pH determined by the mass composition. The isoelectric point is the pH at which the zeta potential (æ) in the particle generates zero void mobility  and it is associated with either zero charge on the surface or a balance of positive and negative charges in the particle. At this pH, the repulsion between the particles is at a minimum, resulting in a very viscous suspension . Viscous suspensions form parts with irregular walls , but if the pH of a suspension is sufficiently far from the isoelectric point, it may remain dispersed.
Fig. 1(a–e) shows that the masses containing waste (Fig. 1b–e) had lower pH values than the reference mass (Fig. 1a). It was also possible to verify a direct correlation between the increasing in pH for all suspensions with the increase in deflocculant needded. According to Andreola et al. , increasing the concentration of sodium silicate in a suspension gradually reduces the yield stress and increases pH.
It was noted that a minimum viscosity value was reached in the pH range from 7.5 to 8.5, above it, the viscosity increased slowly. Possibly in this pH range there is a greater stability of the system. According to the literature, the pH of kaolin varies normally between 5 and 6, and between pH 7 and 9 a kaolin-containing suspension is deflocculated when sodium silicate or sodium carbonate is used .
The minimum viscosity value was reached when the pH fell within the range of 7.5 to 8.5. Above this, viscosity slowly increased. In this pH range the system likely has greater stability. According to the literature, the pH of kaolin normally varies between 5 and 6, and between pH 7 and 9, a kaolin-containing suspension is deflocculated when sodium silicate or sodium carbonate is utilized .
Fig. 2 shows the dilatometric curves for the MP, M10, M15, M20, and M25 ceramic mixtures.
Dilatometric curves of the ceramic mixtures.
Compositions having up to 20% waste exhibited similar dimensional variations. All compositions initially showed a slight shrinkage, probably due to the release of adsorbed water. Shrinkage associated with dehydroxylation of the clay minerals was observed, starting at approximately 510°C. A more pronounced expansion occurred in the 573–575°C range due to the polymorphic transformation of alpha quartz into beta. The shrinkage that begins between 890 and 920°C is probably related to the appearance of the glassy phase and the formation of mullite, while above 1030°C the sintering phenomena accelerate, leading to densification of the samples and the occurrence of viscous flow due to the formation of the liquid phases during firing.
It was determined that samples containing waste had a higher shrinkage/densification rate than the standard sample, which may be explained by the smaller particle size of the waste-containing formulations. The addition of waste caused a slight displacement of the maximum shrinkage rate temperature from 1183°C to 1192°C (with 25% waste), a behavior that can be attributed to the higher refractoriness of the waste.
Fig. 3 shows the XRD patterns of the sintered samples at 1200°C.
XRD patterns of samples MP, M10, M15, M20, and M25 sintered at 1200°C: (Q, quartz; M, mullite).
Quartz (JCPDS: 46-1045) and mullite peaks (JCPDS: 15-0776) were observed for all compositions studied. Even with the increased content of waste in the mixtures, no new crystalline phases were formed. These results confirm that the addition of waste does not interfere with the mineralogical characteristics of the ceramic mass, emphasizing the phases that influence the mechanical properties.
Fig. 4(a–e) shows the micrographs of the samples studied.
Micrographs of the polished surfaces attacked by HF of the sintered samples: (a) MP; (b) M10; (c) M15; (d) M20; and (e) M25.
Closed and isolated pores of two types can be observed: non-spherical pores, characteristic of an incomplete densification process due to differentiated crystal behavior; and pores that are surrounded by dense and essentially spherically shaped areas, which indicate the presence of a vitreous phase causing sintering around the pores. Spherical pores can also be caused by problems during processing, such as the entrapment of air bubbles, as suggested by the literature .
It was possible to verify that the ceramic pieces formed from the M10 (Fig. 4b) and M15 (Fig. 4c) mixtures presented a morphology similar to that exhibited by the control MP (Fig. 4a), with a few rounded and isolated pores, indicating the consistent development of the liquid phase during sintering, which promotes the filling of pores . Fig. 4(d) and (e), which refer to compositions M20 and M25, exhibit microstructures that differ from those presented by MP (Fig. 4a), showing a more porous microstructure. The higher number of pores, which act as stress concentrators, can contribute to lower densification of the ceramic bodies produced, with a consequent decrease in mechanical properties.
Table 5 shows the values of the physico-mechanical properties of the ceramic specimens produced.
Physico-mechanical characterization of the specimens.
It can be verified that the apparent density of the samples containing waste did not differ significantly (t-test, p-value>0.05) from the density of the standard mass, remaining in the range of 2.3–2.4g/cm3. With regard to shrinkage from firing, an increase was observed for additions of waste up to 20% by weight, when compared to the standard mass and, beyond that, shrinkage decreased (t-test, p-value<0.05). This behavior corroborates that observed from dilatometry, with higher shrinkage rates for the samples containing waste, and with sample M25 presenting a lower shrinkage rate, and with M10 and M15 not having statistically significant differences. A previous study  performed with ornamental rock waste replacing feldspar in an aluminous porcelain body, observed a decrease in linear shrinkage from firing as the amount of waste was increased, which could be attributed to two combined effects: (i) incorporation of a greater quantity of quartz particles in the mixture, considering the high silica content in the chemical composition of the waste; and (ii) formation of closed porosity characteristic of overheating, resulting in expansion of the structure.
However, the shrinkage from firing results all fell between the range of 9.8–11.8%, which is in compliance with the range recommended for ceramic sanitary ware, according to the Brazilian standard for Sanitary Apparatus, ABNT/NBR 15097-1  and also with the literature , which recommends a final shrinkage of less than 12%, as higher values would imply an unacceptable deformation. The high shrinkage of the product during the sintering process is a consequence of the large amount of liquid phase that forms, caused by the presence of flux in the mixture .
The M10 and M15 samples showed no statistically significant difference when compared to the standard sample (t-test, p-value>0.05) for the water absorption and apparent porosity parameters. However, those properties increased when 20% and 25% of waste was used. Apparent porosity varied between 1.1% and 2.2% and water absorption varied between 0.5% and 0.9%. Only ceramic masses with up to 15% waste can be recommended because they have values of approximately 0.5%, corroborating with other authors  who investigated the absorption of water in sanitary ware. The pieces obtained from compositions M20 and M25 presented inadequate water absorption values for sanitary ware, according to ABNT/NBR 15097-1 , which recommends a maximum absorption of up to 0.5%.
In this study, the quartzite waste particles were smaller than those of feldspar, which may have contributed to the behavior of the masses containing up to 15% waste during firing. On the other hand, the presence of higher waste content in the M20 and M25 compositions probably difficult packaging of the particles during sintering, leading to a lower densification rate, as verified in the dilatometric analysis.
As shown in Table 4, it was found that additions of up to 15% waste improved the hardness of the ceramic bodies, with higher values (t-test, p-value<0.05) than those obtained by the reference composition MP, probably due to the lower amount of vitreous phase. The modulus of rupture values also increased, but there was no statistically significant difference in these values (t test, value p>0.05) for mixtures of up to 20% waste. Above this, there was a decrease in the modulus, which is related to the higher temperature of maximum shrinkage and lower shrinkage rate of sample M25, verified previously. According to Ustudang et al. , in studies using waste from rock-cutting, the substitution of feldspar with larger amounts of waste may result in a lower densification rate with the concomitant formation of closed pores and expansion of the structure. Other authors  who analyzed the properties of ceramic pieces obtained from slips, attributed the reduction of mechanical resistance to the type of microstructure formed. As previously pointed out in the microstructural analyzes, there are anisometric pores and others of spherical shape, which may have acted to concentrate tension forces.
In the present study, the influence of quartzite waste in partial substitution of feldspar on the mixture used to produce sanitary ware ceramics was studied. The quartzite waste had a chemical composition similar to feldspar. The mixtures containing residue shown granulometry similar to that of the standard mixture. The phases formed after firing, for all compositions, were mullite and quartz. The specimens produced from compositions containing quartzite waste of up to 15%, presented physical and mechanical characteristics within the range recommended for sanitary ware, with mechanical resistance values superior to 35MPa and water absorption near 0.5%. Similar microstructural aspects were observed for the standard mass and the masses containing up to 15% waste. Therefore, the results showed that it is feasible to add up to 15% quartzite waste as replacement for feldspar in sanitary ware formulations.
The authors would like to acknowledge the financial support received from the PDSE program of CAPES and CNPq. The authors would also like to thank the Institute of Ceramics and Glass (ICV) in Madrid, Spain, where this work was partially done.
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