Sintering Behavior of Porous Wall Tile Bodies During Fast Single-Firing Process

In ceramic wall tile processing, fast single-firing cycles have been widely used. In this investigation a fast single-firing porous wall tile mixture was prepared using raw materials from the North Fluminense region. Specimens were obtained by uniaxial pressing and sintered in air at various temperatures (1080 – 1200 °C) using a fast-firing cycle (60 minutes). Evolution of the microstructure was followed by XRD and SEM. The results revealed that the main phases formed during the sintering step are anorthite, gehlenite and hematite. It appears that the sintering process is characterized by the presence of a small amount of a liquid phase below 1140 °C. As a result, the microstructure of the ceramic bodies showed a network of small dense zones interconnected with a porous phase. In addition, the strength of the material below 1140 °C appeared to be related to the type and quantity of crystalline phases in the sintered bodies.


Introduction
Single-firing in the ceramic industry is the procedure that involves subjecting the unfired glaze ware to only one thermal cycle and directly obtaining the finished product 1 .Single-fired wall tiles are conformed by dry pressing with moisture content 4-8%, and the open porosity of the fired ware is usually above 10%.In addition, are required to have flexural strength values over 15 MPa for thickness below 7.5 mm, and for thickness above 7.5 mm a value of flexural strength of 12 MPa is required.They are therefore to be included into Group BIII 2 .In fabrication of this kind of product , the main raw materials are kaolinitic and ilitic clays, and variable carbonate and quartz contents [3][4] .
It is well-known that traditional raw materials such as clays have a complex mineralogical composition, which makes the study of sintering behavior rather difficult.During sintering of phyllosilicates and accessories minerals like quartz, feldspar, calcite, dolomite and hematite, a series of phase transformations occur which determine the final properties of the ceramic products 5 .In the area of mineral raw materials, we must consider firstly the clay minerals, mainly kaolinite.This mineral is often used due to its adequate properties and because it enters into the mineralogical composition of many suitable clays for ceramics 6 .
In South-Eastern Brazil (Campos-RJ), there are important sedimentary environment originated clayey materials deposits, which hold kaolinitic clays 7 .These clays have been extensively characterized for application in red ceramic 8 .However, these raw materials have been little studied for application in ceramic tiles.The local industry ceramic currently produce mainly red ceramic products such as dense bricks, ceramic blocks and roofing tiles.The development of products with higher aggregate value, such as porous wall tile using clayey materials from Campos-RJ is very important for the local ceramic industry and economical development of the region.
The main aim of this work is to study the sintering behaviour of porous wall tile bodies during fast-single firing using raw materials from Campos-RJ region.The following techniques were employed: X-ray diffraction (XRD), thermogravimetry (TG/DTG), differential thermal analysis (DTA) e Scanning Electronic Microscopy (SEM).Moreover, a correlation was done with the microstructure of the sintered bodies and their physical-mechanical properties.

Materials and methods
The ceramic paste studied was formed from a mixture of red clay from Campos-RJ, a calcareous from Italva-RJ and a commercial quartz.The composition of mix used was as follows: red clay 70%, calcareous 15% and quartz 15% 9 .
The raw materials were dry ground, using a laboratory mill, with screening residue 4%, passing in a 250 mesh (63 µm) screen.The dry-ground powders were granulated in a laboratory granulator with moisture content 14% (moisture mass/dry mass).After adjusting the moisture content to 7% the granules were kept enclosed in a dissecator for 24 h to homogenize their moisture content.The granule-size distribution of granulated powders was determined by sieving procedures according to NBR-7181-84.X-ray diffraction analysis was performed with a Seifert URD-65 diffractometer, using monochromatic Cu-K α radiation.The phases were identified from peak position and intensity using reference data from the JCPDS handbook 10 .
Simultaneous DTA/TGA/DTG measurements were carried out in a TA Instruments SDT-2960 simultaneous TG-TGA under air atmosphere from room temperature up to 1150 ºC at a heating rate of 10 ºC.min -1 .
Rectangular ceramic bodies (115 x 25 x 6.75 mm 3 ) were obtained by uniaxial pressing at 35 MPa in order to obtain an apparent density upon drying of 1.9 g.cm -3 .After pressing, the green ceramic bodies were dried for 24h at 110 ºC and sintered in a fast-firing laboratory kiln within the 1080-1200 ºC maximum temperature range with a fast-firing cycle (60 min).The maximum temperatures correspond to temperatures used during firing of industrial ceramic tiles.
The determinations carried out on the sintered ceramic bodies included: shrinkage linear, water absorption and apparent density.In addition, the flexural strength was determined by three-point bending test (model 1125 Instron) at a loading rate of 0.5 mm.min -1 .SEM pictures were used to observe the microstructure of sintered samples using a Zeiss DSM 962 scanning electron microscope.The identification of the phases present in the sintered bodies was performed by X-ray diffraction.

Results and Discussion
Fig. 1 shows the granule size distribution for the granulated powder.One can note that the paste fall into the 150-425 µm granule-size range.The main aim of the granulation was improve a better compactness of the ceramic paste.
The X-ray diffraction for the ceramic bodies sintered in some temperatures (1080-1160 ºC) are showed in Fig. 3.The increase in temperature for singly firing ceramic pastes exceeds the energy threshold of the reactivity of materials and produces a series of reactions and transformations that lead to the formation of new phases and the disappearance of others 11   The fourth endothermic valley (mass loss 5.962%) is due a thermal decomposition of calcite to form calcium oxide (CaO) and CO 2 .A small exothermic peak at 950 ºC was due to further disruption of the lattice, and probably related to the formation of new crystalline phases such as Si-containing γ-Al 2 O 3 with spinel structure and /or 2:1 mullite [11][12] .The changes of phase with sintering temperature in Fig. 3 suggest the following reactions.Firstly gehlenite appears from metakaolinite and calcium oxide reaction.Late, anorthite is formed from gehlenite and metakaolinite reaction [6][7][8][9][10][11][12][13] .The DTA curve reveal only one exothermic phenomenon at 950 ºC characterized by a smoother peak.It seems that the series of reactions, associate with the metakaolinite structural reorganization, could exist simultaneously and progressively.
The fractured surfaces of the specimens sintered at 1110 and 1200 ºC are show in Fig. 5.
At 1110 ºC (Fig. 5a) the microstructure is formed mainly by regions that contain larger pores, resulting from carbonate decomposition.These regions are connected with dense zones.At 1200 ºC the open porosity decreases.It also can be observed that a amount of glassy phase begins to emerge.ºC, presenting a small variation (1.76-1.78g.cm -3 ).Above 1140 ºC, a higher range is presented (1.79-1.84g.cm -3 ).This behaviour is related to the increase of glassy phase, with predominance probably of viscous flow sintering on the material densification.Moreover, the crystallization of ceramic phases such as gehlenite and anorthite from metakaolinite contributes for a higher densification in this range of the temperatures.

Conclusions
The ceramic paste presented dimensional stability (low firing shrinkage), into of range of 1080-1140 ºC.In addition, the variation of water absorption is not relevant in this range, and apparent density is almost constant.Above 1140 ºC relevant variations in all properties studied occur due the formation liquid phase.In general, higher strength data scattering occurs with the sintering temperature.
Two ranges of the temperatures could be analyzed.In the first range (1080-1140 ºC) the sintering is characterized by a small amount of liquid phase.Above 1140 ºC changes significatives occurred probably due the predominance of viscous fluxe sintering mechanism on the densification of the ceramic bodies.
The results revealed that the main phases formed during the sintering step are gehlenite, anorthite and hematite.In general, the ceramic bodies microstructure show a network of small dense zones interconnected with a porous phase.
Figure 1.Granulated powder size distribution by sieving.

Figure 2 .
Figure 2. X-ray diffraction pattern of the ceramic paste.

Figure 3 .
Figure 3. X-ray diffraction patterns of the ceramic paste after sintering at various temperatures.

Figure 7 .
Figure 7.The apparent density as a function of sintering temperature.

Figure 8 .
Figure 8.The flexural strength as a function of sintering temperature.