Preparation, microstructure studies and mechanical properties of glazes ceramic sanitary ware based on kaolin

In this paper, the use of local kaolin coming from Djebel Debbagh (denoted
 DD1) in the composition of ceramic glazes for sanitary ware was examined.
 Because of its natural abundance, low price and good characteristics, this
 kaolin represents an interesting economic alternative to other mineral
 clays. The chemical composition showed that this kaolin contains 38.49 wt.%
 Al2O3 and 44.85 wt.% SiO2. Two glazes based on kaolin DD1 denoted as GaDD1
 and GbDD1 were prepared with conventional ceramic processing techniques at
 temperature 1250 oC. As a reference, another glaze (Gref) based on kaolin
 Remblend was also prepared in the same conditions. The samples were
 characterized with X-ray diffraction and Scanning Electron Microscope. The
 results revealed that zircon and quartz are the crystalline phases present
 in these glazes. In the sample GaDD1, it was found that the degree of
 whiteness is very high and reaches 93.30 %. However, the water absorption
 coefficient is low which is about 0.19 ? 0.04 %. In addition, the flexural
 strength and the Vickers microhardness are respectively, about 56.07 ? 5.61
 MPa and 7952.80 ? 101.76 MPa. These properties are compatible with those of
 the glaze reference Gref and commercial sanitary ware glazes, indicating the
 potential use of kaolin DD1 in the production of glazed ceramic for sanitary
 ware applications.


Introduction
Many local raw materials, like kaolin, feldspar, quartz, calcite and dolomite, are very abundant and they are used in industrial ceramics because of availability and low cost. Several research studies have already been published about the valorization of raw materials for the production of ceramic membranes [1], dental porcelains [2], bioceramics [3], technical porcelain [4], glass [5] and glass-ceramic [6]. Therefore, the exploitation of new raw material resources satisfying the criteria of ceramic industries is important.
Algeria has large deposits of kaolin raw materials, particularly in the east of the country. Several work projects were carried out in recent years to assess the Algerian kaolin properties such as formation of anorthite containing cordierite materials throught reaction sintering kaolin, MgO and CaO precursors [7], preparation and microfiltration membranes and their supports using kaolin (DD 2 ) and CaCO 3 [8], elaboration of tubular supports for membranes filtration based on kaolin DD 3 and lime extracted from limestone [9]. Chargui et al. [10] showed that kaolin, whose main constituent is kaolinite (Al 2 Si 2 O 5 (OH) 4 ), undergoes successive structural and microstructural transformations during its firing. The last transformation step is the vitrification of cristobalite, which occurs at a temperature generally above 1400 o C. The presence of some impurities, such as CaO, Na 2 O and K 2 O, in the initial kaolin favors cristobalite vitrification at lower temperatures. But to date, there have been no studies on the possibility of using this kaolin in glazes production in the literature.
It is well known that glazes are stable glassy coatings applied to ceramic to attain stunning surfaces and formerly obtained by cooling oxides applied on the surfaces of ceramic objects. Originally, glazes were considered as an innovation for sanitary ware when associated with a porous ceramic substrate because it is making a clean and hygienic surface [11]. Most of the glazes are applied to ceramic bodies in order to ensure their liquid and gas impermeability while remaining mechanically stronger, resistant to scratching, readily cleanable, chemically more inert. The aesthetic qualities of the product are also enhanced [12].
Generally, glaze, as used for sanitary ware, contains many raw materials such as zircon, feldspar, limestone, quartz, talc, ZnO, calcite, sodium carbonate, wollastonite, pegmatite and kaolin. Each of those materials gives special properties to the glazes. Among them, kaolin was used to introduce alumina and, to a lesser extent silica in glazes. The addition of kaolin has a stabilizing effect on the aqueous suspension of glaze; its application to the body allows obtaining a consolidated glaze layer that gives rise to the final glaze coating on firing [13]. However, its use was limited due to the possibilities of high iron content that could tint the glaze to give brown color.
This work aims to study and characterize the Algerian kaolin DD 1 for potential use in industrial glazes. This kaolin was chosen because of its natural abundance, low price and good characteristics. Kaolinite has a hydrothermal geological formation [14], its valorization for the manufacture of low-cost glazes ceramic products in multicomponent oxides system as well as the improvement of whiteness, mechanical properties and chemical durability of glazes are the main objectives of this work.

Materials and Experimental Procedures 2.1 Selection and characterization of the raw materials
A numerical approach based on calculated crystalline indices CI 1 and CI 2 from the intensities of selected vibration modes and structural OH bands [15] of the FTIR spectrum of kaolin DD 1 was applied using the equations (1) and (2): (1) where I(ɤ 1 ) and I(ɤ 3 )are intensities of the OH stretching bands at 3693 and 3620 cm -1 and I(ɤ 2 ) is the intensity of the OH bending band at 910 cm -1 . According to the obtained values of crystallinity indices, kaolinites are classified as poorly ordered structures when (CI 1 < 0.7, CI 2 > 1.2); partially ordered structures if (0.7 < CI 1 < 0.8, 0.9 < CI 2 < 1.2) and ordered structures if (CI 1 > 0.8, CI 2 < 0.9). The mineralogical composition of DD 1 was determined using an X-ray Panalytical diffractometer with Cu Kα X-ray radiation λ=1.5406 Å, in the 2θ range of 0-80°.

Glazes preparation and characterization
Initially, the glazes formulations in suitable proportions are presented in Table II. The experimental glazes GaDD 1 and GbDD 1 were prepared using kaolin DD 1 . As for the sample, Gref (as a reference) was prepared in the same conditions as our glazes using kaolin Remblend.
The glazes slurries were prepared by directly milling the weighed starting raw materials with water in a porcelain jar during 3 h, with added milling balls (10 mm). The raw materials: balls: water ratio was 1: 1: 0.5. During the homogenization process, 0.5 % sodium silicate (99.90 %, grade ACS reagent) was added to the slurry to obtain a better dispersion. The slurries were sieved through a sieve of 63 µm to remove coarse particles [16]. The fluidity behavior of the slurries was determined at 20 °C in a Ford cup time, these values vary between 20 and 30 s. The required density of the glaze suspension onto a ceramic substrate ranges between 1.70 and 1.72 g/cm 3 [17]. The glaze slurries were sprayed on dried sanitary ware bodies with a compressed-air sprayer. The glazed bodies were then dried for 1 h at 100 o C.
Later the glazed bodies were fired at temperature of 1250 o C in a tunnel kiln for 21 h. At firing zone, all the raw materials are fused and glazes were fused evenly. At cooling zone, sudden cooling is done to create a glossy surface.
To identify the crystalline phases formed after the thermal treatment, the glazes were analyzed by X-ray diffraction (XRD). The powdered samples were inspected using a panalytical diffractometer, Cu Kα X-ray radiation, λ = 1.5406 Å, in the 2θ range of 10-70°.
The microstructure of the glazes was examined using a Scanning Electron Microscope WD S, JEOL JSM 6360LV.
The grain size distributions of glaze powders were measured with a laser granulometer (HORIBA, model analyzer LA-960).
The unfired glaze powder was subjected to differential thermal analysis (DTA) and thermo-gravimetric analysis (TGA) using DSC/DTA/TGA analyzer (SDT Q 600 -TA instrument) with α-alumina as the reference material. About 40 mg of raw powder mixture was placed in an alumina crucible and heated at a rate of 10 o C/min from room temperature to 1050 o C.
The whiteness parameters L, a* and b* of the fired samples were measured using a Minolta CR-400 series Chroma Meter. L (whiteness) from absolute white L = 100 to absolute black L = 0, a* and b* whiteness indices; a* (a*> 0 red; a*< 0 green), b* (b* > 0 yellow; b*< 0 blue). Further, a* represents the variation between green and red colors; and b* indicates the variation between blue and yellow colors [18].
The water absorption percentage (W abc ) was determined using the following method [19]: the samples with dimensions 8 cm × 2 cm × 1 cm were dried for 12 h at 110 o C and its weight (M 1 ) measured using an analytical balance; these samples were then immersed in distilled water and boiled for 2 h, cooled in situ for 12 h and re-weighed (M 2 ). After this, W abc was calculated using the formula (3): The flexural strength of the glazes ceramic was measured by a three-point bending test (sample dimensions 9 cm × 2 cm × 2 cm) carried out with a machine NETSZH, using the formula (4): where P is the force at the fracture load (N), L is the distance between supports (mm), b is the width (mm) and h is the height (mm). Each value illustrates the average of measurements made on eight individual specimens. The Vickers microhardness measurements were done for glazed samples (2 cm × 8 cm × 1 cm) by using a Vickers Hardness Tester (AFFRI DM2D Digital) at the load of 1000 gf for 25 s. The Vickers microhardness HV was measured using the formula (5) [20]: where F is the indentation load (N), d is the diagonal of the imprints (µm), and 1.8544 is a geometrical constant of the diamond pyramid which was calculated from the specific geometry of the indenter. Chemical resistance was tested according to the NF D14-506 and NF D14-508 standard method. Then, the studied glaze test pieces (dimensions 2 cm × 2 cm × 1 cm) were immersed in an aqueous solution of HCl (pH 1.8, grade ACS reagent) and NaOH (pH 13.5, grade ACS reagent) for 7 days at 20 o C [21]. The glazes were weighed before and after the tests using an analytical balance. The samples were visually examined to verify possible changes in color and gloss.

Characteristics of the raw material DD 1
The chemical composition of kaolin DD 1 is given in Table I The FTIR spectrum corresponding to kaolin DD 1 (see Fig. 1 2 shows the X-ray diffractogram of kaolin DD 1 . It is evident that the main mineral constituents of the clay are kaolinite (Al 2 Si 2 O 5 (OH) 4 ) and halloysite (Al 2 Si 2 O 5 (OH) 4 ·2H 2 O) identified respectively by using the JCPDS database files 14-0164 and 29-1489. Halloysite is a naturally occurring hydrated polymorph of kaolinite and has a similar structure and chemical composition; however, the unit layers are separated by an additional monolayer of water molecules [27].
The observation of the SEM microstructure of kaolin powders (Fig. 3) revealed randomly shaped, elongated and oriented in all directions. In kaolin, randomly shaped and intermixed agglomerates favor a good porosity.      5 shows the microstructure of glazes GaDD 1 , GbDD 1 , where two distinct crystalline phases are observed. The first represents zircon whose grain size is submicron (indicated by rectangles in Fig. 5), the second crystalline phase is quartz (many pieces of dark areas indicated by ellipses in Fig. 5). The dispersion of the zircon grains in the glassy matrix (indicated by arrows in Fig. 5) seems homogeneous and random. This uniform distribution is very important to improve the surface densification of glazed ceramic. Fig. 6 shows the particle size distributions of the samples GaDD 1 , GbDD 1 and Gref. Glazes prepared with local kaolin GaDD 1 and GbDD 1 exhibited a bimodal particle size distribution with the same particle populations but with different rates. GaDD 1 and GbDD 1 glazes show a mean particle size D 50 of 42.21 and 19.76 µm respectively. In contrast, the initial Gref glaze powder distribution is composed of three particle populations, with a mean particle size D 50 of 58.92 µm, the first two populations of Gref glaze powder particles are similar to those of the kaolin DD 1 glazes. The third family of particles consists of large particles of a few hundred µm, such results are consistent with SEM observations. Gorodylova et al. studied the impact of particle size reduction on glaze-melting, they noted an increase of milling time, resulting in an improved homogeneity and a decrease of the mean size values [28], which agrees with the low values of mean size obtained for the samples GaDD 1 and GbDD 1 .

Thermal analysis of glazes powder
Thermogravimetric (TGA) and differential thermal analysis (DTA) were employed to study weight change and all the transformations taking place during the heating cycle. TGA/DTA curves of the three investigated glazes from ambient temperature to 1050 o C are reported in Fig. 7. An initial weight loss of about 3 % is observed in the TGA curve, which is attributable to the evaporation of adsorbed water [29,30]. The departure of the structural water or the dehydroxylation of kaolin DD 1 occurs between 500 and 550 o C [31], accompanied by a loss of mass of the order of 3 %. The DTA curve does not show the endothermic and exothermic peaks of its transformations given its low content in the mixture relative to feldspar and quartz. At temperature around 570 o C, the α-quartz is transformed into β-quartz without weight loss [32]. A mass decrease of about 5 % was observed during the decomposition of calcium carbonate [33] to CO 2 and CaO between 600 and 750 o C. The decomposition of dolomite (CaMg (CO 3 ) 2 ) [34] into CaO and MgO with a new mass loss of 2 % and the recrystallization of kaolinite into mullite and cristoballite occur at temperatures comprising between 750 and 1050 o C. Overall, the mixture of the starting powders undergoes a total mass loss estimated to be around 9 to 11 % after heating at 1050 o C. With increasing of temperature, the sodium feldspar begins to melt congruently and decompose at about 1100 o C [35], favored by the presence of sodium oxide. At temperatures above 1200 o C, the mixture melts to form the glaze, the zircon remains stable in this temperature range and dispersed in the glassy matrix (see Fig. 5), it dissociates in zirconia and silica only above 1500 o C [36]. According to the thermal analysis results, samples GaDD 1 and GbDD 1 showed some differences compared with sample Gref.  As can be seen from Table III, the L value for GaDD 1 sample is 93.30 % which is higher than that of GbDD 1 (92.04 %) and Gref (89.15 %). In general, opaque glazes exhibit high whiteness values [37]. The values of L in GaDD 1 and GbDD 1 are superior to 92 %, indicating, therefore, a high degree of whiteness of glazes, there is also a tendency to shift slightly in the left quadrant (green and yellow), which is probably due to the presence of zircon crystals in the glaze. It may also be noted that the kaolin DD 1 containing compositions GaDD 1 and GbDD 1 showed higher L values due to the presence of lower amounts of Fe 2 O 3 and TiO 2 compared to the kaolin Remblend containing composition Eref [38]. Benkacem et al. found the optimum value of whiteness for opaque glazes used in ceramic sanitary ware is 87.00 % due to the presence of zircon crystals and thus effectively scatters light to ensure opacity [39]. Also, the Mie scattering calculations identify that maximum light scattering and whiteness with zircon occur with a particle size range of 0.60-0.75 µm and a mass fraction 0.16 [40].

Water absorption percentage of elaborated glazes
The water absorption percentages of the glazes studies are summarized in Table IV, the low values of water absorption percentages suggest a high degree of vitrification. Moreover, low water absorption percentage (< 0.5 %) is essential to ensure hygiene during the product life cycle of glazes ceramic sanitary ware [41]. On the other hand, during the firing process, liquid phase viscosity decreases and contributes to reduce the pore size [42,43].

Flexural strength and microhardness of Vickers
According to the results of Table IV, it is noticed that GaDD 1 exhibits higher flexural strength (56.07 ± 5.61 MPa) than the reference sample Gref. At higher firing temperatures, the mechanical strength of a glaze increases with decreasing water absorption, which is inversely proportional to the flexural strength of samples. It is easy to understand that the highest mechanical strength corresponds to the maximum development of the crystalline phase and the development of the glassy phase insofar as it must not be in excess, but in a sufficient quantity to allow good cementation of the crystalline grains [44].
The comparison of the Vickers microhardness shows that GaDD 1 exhibits higher microhardness than other glazes which is about 7952.80 ± 101.76 MPa. Generally, the microhardness of glaze ceramic is related to both crystalline and residual glassy phases [45]. It is evident that there is an increase in the amount of zircon crystals in both GaDD 1 and GbDD 1 samples, which leads to the increase in microhardness values. According to Levistskii et al. [46], the microhardness values of the opacified glazes produced by high temperature firing for sanitary ware range approximately from 6500 to 7500 MPa, in good agreement with our results.

Chemical resistance
According to the NF D14-506 and NF D14-508 standard methods, glazes GaDD 1 , GbDD 1 and Gref were classified class AA and showed very good chemical resistance to aqueous solutions of HCl and NaOH with no visual changes on the surfaces of the samples after the tests. This good resistance can be attributed to the very well balanced composition of raw materials. The mass loss after 7 days attack using HCl and NaOH solutions was very low, detected only in the last decimal digit, showing that it is possible to synthesize glazes with excellent chemical resistance to strong solution attack starting from kaolin DD 1 combined with commercial raw materials.
The physico-chemical properties of the samples based on kaolin DD 1 are the result of a proper combination of crystalline phases and their distribution in the glassy matrix, forming a uniform glass-crystal structure.

Conclusion
In this paper, an Algerian kaolin (DD 1 ) was tested as raw glaze ceramic for sanitary ware production. From the obtained results, the following conclusions can be drawn: • XRD and SEM analysis revealed the presence of zircon and quartz crystals in glazes.
• Zircon crystals contribute to the opacity and enhancing the properties of the final glazes. • Glazes prepared with local kaolin exhibited a bimodal particle size distribution.
• Glazes ceramic sanitary ware based on kaolin DD 1 developed interesting characteristics in terms of higher whiteness (93.30 %) and smaller coefficient of water absorption (0.19 ± 0.04 %).
• The values of flexural strength and microhardness of the glazes prepared with kaolin DD 1 were higher than those of glaze reference and commercial sanitary ware glazes. • Studied glaze samples show good chemical resistance.