Phase and Micro-Structural Characterization of Sanitary- Ware Fired at Different Temperature

The three main ingredients of sanitary-ware are clay, feldspar and quartz. This ware is being widely used and has therefore, attracted the attention of researchers from time to time. Consequently, it has been extensively investigated. The present study describes the phase and micro-structural analysis of sanitaryware samples collected from local (Durr Ceramics Peshawar) industry. XRD (X-Ray Diffraction) of samples fired at 1100C reveals the presence of α-quartz and primary mullite only. In addition to a-quartz and primary mullite, elongated needles of secondary mullite were also present in samples fired at 1200 and 1300C. Unlike typical vitreous ceramics bodies, regions containing elongated secondary mullite originating from the clay relict and growing into the feldspar relict were few in number which is consistent with the high clay content in the starting body ingredients of the investigated samples. Another sample investigated for comparison purposes, contained all the phases mentioned above along with some corundum grains which indicated that the composition of sanitary ware varied from manufacturer to manufacturer. EDS (Energy Dispersive Spectroscopy) detected high concentration of Fe in some regions in the bulk but the glaze did not contain any Fe.

The properties of materials are strongly dependent upon their processing conditions, constituent phases and microstructure, and the same is the case with sanitaryware. Therefore, the processing, phase and micro-structural evolution of vitreous ceramics including sanitary ware has been investigated previously (Klein,[2], Weymouth [3], Jackson [4], Cole [5] and Lee and Iqbal [6]).

MICROSTRUCTURE PROPERTY RELATIONSHIP OF SANITARY-WARE
Sanitary-ware products are required to have minimum water absorption index, high scratch resistance, resistance to chemical detergents, crazing resistance, thermal shock resistance and maximum mechanical strength (Manfredini et. al. [7]). The manufacturers aim at producing good quality products and therefore, the al. [8] reported that the Kaolin transformed to Meta-kaolin at 550 o C. However, Qiu et. al. [9] reported the formation of meta-kaolin by de- The Al 2 O 3 :SiO 2 ratio in primary and secondary mullite has been reported to be 2:1 and 3:2 respectively (Iqbal and Lee [10]).
Various proportions of two types of feldspar are commonly used in the production of commercial white-ware bodies namely potash feldspar and soda feldspar (Schramm and Hall [11]). The viscosity of the glassy phase in sodafeldspar containing porcelain bodies is less than that containing potash feldspar. The consequent increase in the fluidity of the melt accelerates the transformation of primary mullite into secondary mullite at high temperatures ( ≥ 1200ºC). Therefore, the type and proportion of initial body ingredients can be altered to improve the properties of the product (Schuller [12]).
At 1200 o C, the dissolution of α-quartz speeds up and increases the quantity of liquid phase in the body. Pure clay relicts contain primary mullite (~7nm) and occasionally secondary mullite (~30nm) in clay relicts adjacent to large amount of feldspar relict at 1000ºC.
Secondary mullite forms as a consequence of the diffusion process between feldspar and clay. It depends upon the diffusion of K 2 O and Al 2 O 3 . Feldspar melting point is lower than the other body ingredients so that the clay with high contents of feldspar possesses low viscosity. The clay relict agglomerates containing feldspar are more fluid than those without feldspar, so secondary mullites crystals grow larger in less viscous clay-feldspar mixture as compared to primary mullite in non-viscous feldspar-freeclay agglomerates (Iqbal and Lee [13]). According to Vaughan and Dinsdale [19].

W=k√t
Where, W is volume expansion in time "t", and k = 10 -8 g.cm -2 day ½ (sorption rate). For example, the recommended value of moisture expansion during one year for a sanitaryware product of internal area 0.5m 2 (5x10 3 cm 2 ) is 0.1%0.  [20]).
Important factor that influence the mechanical strength of the sanitary-ware, is the grain size of the filler such as α-quartz. Cracks commonly observed in and around large quartz grains occur because of the large thermal expansion mismatch between the crystalline quartz (α ≈ 23x10 -6 K -1 ) and the glassy phase (α ≈ 3x10 -6 K -1 ) in the temperature range 20-750 o C (Iqbal and Lee [13] cited in Lundin [21]).
According to Knudsen relation (Knudsen [22] cited in Dinsdale and Wilkinson, [23]) quantitatively strength of the body is: Here S and D are strength and particle diameter and "K" and "a" are constants.This shows that the strength increases as the size decreases. However, as the grain size is reduced, a maximum strength is reached, beyond which further reduction in size is accompanied by a reduction in strength.
Coarse filler forms an unconnected dispersion with intervening smaller clay and flux particles. When flux particles melt, the area of contact between the glassy matrix and alumina particles increases, and the strength increases. When the filler size becomes very small, this situation no longer holds, as the filler may now be more in number to be connected (Dinsdale and Wilkinson, [23]).
The bending strength of porcelain can be increased by decreasing the size of α-quartz particles to 10-30 μm (Bradi [24]).

Samples Fired at 1100 o C and Commercially Fired
Micro-structural and elemental analyses were carried out using EDS and SEM. The compositions of sample fired at Soda feldspar is used to lower the firing temperature (Iqbal and Lee [13]). Fig. 3 is a SEIwith marked EDS result from a general area of the sample fired at 1100°C. α-Quartz grains are not clearly visible in this sample because its melting begins at high temperatures i.e. >1200°C (Kobyashiet. al. [27]). In samples fired at 1100°C, the solution rims around α-quartz grains coming from their partial dissolution are absent and therefore, the grains cannot be resolved from the matrix and will require compositional analysis to establish their identity.  Regions containing Primary mullite of scaly appearance are shown in Fig. 5. This is consistent with previous studies performed at temperatures close to or at 1100°C ([Lee and Iqbal [6] Iqbal and Lee [10,13] Weight (%)) of iron has also been detected by EDS (Fig.   5).

Comparison of α α α α α-Quartz and Mullite Grains
Partial dissolution of α-quartz begins at temperatures above 1200°C, therefore, α-quartz grain cannot be resolved from the matrix at low temperatures (1100 o C). Glaze composition and morphology is shown in Fig. 9.

Sample Fired at 1300 o C
The microstructure of sample reheated at 1300 o C contains elongated needles of secondary mullite (Fig. 10). The high quantity of iron oxide detected (Fig. 10) in these needles makes the precise analysis ambiguous. Crystals of similar morphology observed at temperatures above 1200 o C in others studies of vitreous ceramics using pure raw materials are identified as secondary mullite ([Lee and Iqbal [6,13]). The presence of iron oxide may be due to the iron-rich glass covering the crystals. The presence of other elements tabulated in (Fig. 10) also indicates that the SEM probe used for chemical is not precise enough to isolate the crystal from the glassy matrix and peaks due to other elements in the vicinity overlap the peaks from the marked region (Fig. 10). Fig. 11 shows the denteric growth of mullite in a region, where 50% SiO 2 exists (Table 3). Sodium feldspar conferring the fluidity upon the matrix is generally believed to allow faster mulllite growth than other alkali silicates.
In our study, the amount of Na detected in the vicinity of crystal is low enough to enhance mullite growth to the observed size ([Schuller [12] and Kobayashi et. al. [28]).
In vitreous ceramics, the mixing of raw materials is always incomplete and the composition changes from region to region which may be the possible reason for enhanced

CONCLUSIONS
(i) XRD revealed the presence of α-quartz and mullite as major crystalline phases in all the sanitary ware samples fired at 1100 and 1300 o C.
A small number of low intensity XRD peaks were also present which could be due to albite and sanidine, however, their peak intensities decreased with increase in temperature. (iv) One of the samples also contained corundum grains but the absence of XRD peaks due to corundum from the relevant spectra reveal that the percentage of corundum added was too low to be detected with XRD.
(v) The level of impurities such as titanium and iron in the raw materials used was much higher than those used in the previous studies.

ACKNOWLEDGEMENT
It is difficult to acknowledge fully those who directly or indirectly have influenced me either by personal contacts, in lectures, in the laboratory or by their written work. It will be apparent from the cited works within the text that we are indebted to many such people.