Limestone Clays for Ceramic Industry

2.1 Kaolinite

The kaolinite with structural formula Al₂O₃⋅2SiO₂⋅2H₂O has a dioctahedral structure, which consists of a tetrahedral layer linked by an octahedral layer. Pure kaolinites usually have low plasticity, see Figure 1.

2.2 Montmorillonite

Montmorillonites are a set of family of clay minerals, composed of dioctahedral and trioctahedral silicate sheets, see Figure 2(a) and (b). The most outstanding feature of these minerals is their ability to absorb water molecules [8, 9]. It has 80% of exchangeable cations in the galleries and 20% on the lateral surfaces. The modification of montmorillonite clays has aroused scientific and technological interest for providing significant improvements when incorporated into pure polymeric materials and conventional composites. The clay modification process occurs preferably through the ionic exchange of the exchangeable cations of its crystalline structure.

2.3 Illite

The basic structural unit of the illites is the same as that of the montmorillonites except that in illites, the silicon atoms in the silica layers are partially replaced by aluminum. Therefore, there are free valences in the boundary layers of the structural units, which are neutralized by K cations, arranged between the overlapping units. The structural scheme of the illites is shown in Figure 3. The K cation is the one that best adapts to the hexagonal meshes of the oxygen planes of the layers of silica tetrahedron and is not displaced by other cations. The water adsorption and cation exchange capacity is due only to the broken connections at the ends of the layers. The average diameter of the illites varies between 0.1 and 0.3 μm. When the replacement of silicon in the tetrahedron layers by aluminum in the illites is small, the connections between the structural units provided by the K cations may be deficient and will allow water to enter. When this occurs, the properties of the illites are close to the properties of montmorillonites [3].

2.4 Chlorite

Chlorites are minerals made up of four hydrated aluminum and magnesium silicate layers, containing Fe (II) and Fe (III) as shown in Figure 4.

The most common clay minerals are interstratified, characteristic of mixtures of clay minerals, classified by subgroup and mineralogical species, see most common classification in Table 1. Clay minerals are divided into several classes. A large majority of clays do not have in just one crystalline phase. Two or more chemical species may be present.

3.1 Heat action on clays

According to Mackenzie [10], when a ceramic raw material is subjected to the action of heat, it experiences volumetric variations, usually permanent and irreversible, which can be classified as:

• Oxidation of organic matter

• Decomposition of compounds containing oxygen, such as sulfates, carbonates, etc.

• Dehydroxylation of the clayey mineral

• Crystallization by increasing the temperature

• Vitreous phase formation

• Solid solutions: adjacent crystals of two different materials but of similar structure can react with each other, forming a solid solution.

Kaolinitic clay: the scheme according to Figure 5 shows an endothermic peak between 560 and 590°C referring to the elimination of hydroxyls from the constitution water present in the clays, and an exothermic peak between 980 and 1000°C, due to the formation of mullite, which can be represented by the reactions 1 and 2 [8].

E1

E2

Montmorillonite: montmorillonites have water that lodges in the mineral structure, that is, hydration water of adsorbed ions. The elimination of hydroxyl groups occurs at 700°C. At 850°C, a small endothermic peak may occur due to the loss of montmorillonite crystallinity. Illites can present loss of adsorbed water between 100 and 200°C and water loss in the constitution between 550 and 600°C, see Figure 6.

3.2 Most common impurities present in clays

Quartz: it appears in clays in colored or colorless round grains, whose percentage ranges from 0 to 60%. For high levels of quartz, the clay is called sandy and has low plasticity [11].

Hematite: iron can be present in the forms of hematite (α-Fe₂O₃), goethite (α-FeO⋅OH), and lemonade (a mixture of iron oxides and hydroxides of a weakly crystalline nature), or simply as Fe³⁺ ions in the clay structure. In the illite group, Fe³⁺ ions can replace Al³⁺ ions in the octahedral structure [11]. Fe₂O₃ is formed during sintering under oxidation conditions and from minerals in the clays, giving a reddish color to ceramic materials.

Feldspar: feldspars refer to a group of aluminum silicate minerals. The feldspar contained in the clays is a source of sodium and potassium oxides and plays an important role in ceramic materials with quality of flow agents, temperatures such as sintering temperatures, porosity after firing and facilitating phase formation [6]. The most representative are the orthoclase (KAlSi₃O₈) and albite (NaAlSi₃O₈).

Carbonates: calcium or magnesium carbonates can appear as coarse or small grains. If they are presented as large grains (>125 μm), they may not react completely and the resulting oxides may rehydrate causing expansion according to reactions [12, 13].

E3

3.3 Use of calcite in the ceramic and chemical industry

Ceramic enamels and frits: can be used in matte enamels as a source of CaO to form crystals such as wollastonite, anorthite, gehlenite or in transparent enamels giving shine.

Masses for ceramic coating: as a source of CaO up to the limit of 3%, CaCO₃ assists in the formation of the vitreous phase. CaO levels that vary from 8 to 14% favor the formation of crystalline phases such as gehlenite, wollastonite, pseudo wollastonite, and anortite.

Putties for limestone porcelain: calcium carbonates provide the CaO that are used as a flux in limestone porcelain masses.

Ceramic pigments: the calcium carbonate provides calcium oxide, which together with SnO₂ produces pink pigments.

Glasses: glasses based on NaOH and CaO use CaCO₃ in their composition.

Obtaining settlement mortars: as a plasticizing agent for water retention and aggregate incorporation.

Steel: CaCO₃ acts as a flux and pH regulator in water treatment and as lubricant for drawing steel rebars.

3.4 Specifications of raw materials for ceramic tiles

Sánchez et al. [14] defined some specification parameters for choosing raw materials for formulations of coating masses, as shown in Table 2 below.

Calcium or magnesium carbonates can appear as coarse or small grains. If they are presented as large grains (>125 μm), they may not react completely, and the resulting oxides may rehydrate causing expansion.

In compositions of ceramic floor covering with low water absorption, CaCO₃ acts as a flux until the limit of 3%; above this value, CaCO₃ increases porosity and can be accepted up to 40% in porous coatings.

Enrique [15] recommends that the CaCO₃ particle size should be less than 125 μm, because particles of larger sizes, the CaO resulting from the dissociation of carbonates when calcined at 900°C, do not react with the SiO₂ present in the clays and feldspars that should form the pseudo-wollastonite and wollastonite phases, which can give rise to Ca(OH)₂ formed by the hydration of CaO, when the part comes into contact with the humidity of the air, generating problems of expansion by humidity, with consequent cracking.

The ceramic tile and brick industry have grown enormously in recent years in Brazil. The clays must have sufficient plasticity to provide mechanical resistance when forming by pressing, in order to guarantee the integrity of the piece in the path between the press and the oven. The feldspar contained in the clays are sources of sodium and potassium oxides, acting as fluxes at temperatures above 800°C for bricks and above 1100°C for ceramic tiles, which facilitates the formation of a vitreous phase and reduces porosity [16, 17].

Quartz is mixed with clay during geological formation. If it is present in a smaller proportion, it helps in the formation of the vitreous phase, in the degassing of organic matter and water. However, large proportions of quartz lead to a drastic reduction in mechanical strength after firing [18]. Iron oxide is present in ceramic raw materials in the form of hematite or goethite, giving the finished product a red color.

Calcite, which appears in most clays used in the production process of ceramic tiles of type BIIb, is a mineral that needs special care in its use due to its high loss to fire. When present in a proportion equal to or less than 3%, this mineral acts as a flux. However, in higher proportions, calcite can cause an increase in the final porosity of the product. In addition, the size of the calcite particle for processing ceramics must be less than 125 μm. For larger sizes, it is observed that the CaO resulting from the dissociation of carbonates can hydrate after burning, promoting variations in the dimension of the piece. Therefore, the use of limestone clays is a challenge, requiring care in processing and control in the formulation and burning of coatings. To ensure the correct sintering of the product, proper grinding and pressing of the raw material are necessary, in addition to efficient, fast burning with the lowest possible energy consumption.

4.1 Chemical analysis

Table 3 shows the chemical compositions of a typical Brazilian limestone clay used in ceramics [19]. The chemical compositions of the raw materials were determined by X-ray fluorescence spectroscopy by wavelength dispersion (WDFRX), in a Bruker S8 Tiger equipment, in which the percentages of constituent oxides were estimated by the method semi-quantitatively. For these measurements, samples with a mass of 10.0 g were pressed as discs with 40.0 mm diameter and 4.0 mm thickness. During measurements, the samples were kept in a vacuum of 10⁻⁶ bar. A mixture of P-10 (90% argon and 10% methane) was used in the proportional counter.

The results show that all clays are composed mainly of SiO₂ and Al₂O₃. These elements are associated with clay minerals, quartz, and feldspar structures [17]. The highest amount of SiO₂ was determined for sample C1. This component is important for the manufacture of ceramic tiles, as it improves workability and favors compaction. However, SiO₂ can also cause low mechanical strength of sintered ceramic bodies, in addition to reducing shrinkage during firing.

The amount of Fe₂O₃ detected in the samples was between 4.7 and 7.1%. These values are acceptable for use in ceramic tiles, such as bricks and tiles, this element being responsible for the reddish color of the sintered pieces as well as being a powerful flux [20]. The high content of calcium oxide in C4 (12%) and C3 (7%) stands out, characterizing these clays as limestone [21]. C4 clay was previously studied in Alcântara [16], which reports the formation of stains on the ceramic bodies produced with this material, after sintering at 1120°C. This behavior was associated with a high content of CaO, estimated at 10%, which during the burning phase, the dissociation of CaCO₃, promotes a high mass loss. C4 (13%) generates many pores, reducing water absorption and resistance of the final product. Thus, the higher the CaO content, the higher the CaCO₃ content and in addition, the higher the mass loss.

Analyzing the levels of alkaline oxides, it is observed that the sample C2 has the highest concentration of K₂O, while the concentration of Na₂O is approximately the same in the four samples studied. Alkaline and alkaline earth compounds have a melting effect, which facilitates the formation of liquid phase and linear shrinkage during burning [13].

Table 4 was arranged according to the increasing amount of CaO present in the clays. Note that C1 and C2 have CaO content below 3%. According to Enrique [15], CaO acts as a flux until the limit of 3% in masses of ceramic coating. The percentage of alkali oxides (Na₂O and K₂O), also presented in Table 3, is another major factor for the densification process, due to the great tendency of liquid phase formation during burning. Considering the sum of the percentages of CaO and alkali oxides in samples C3 and C2, it can be concluded that C2 has a higher proportion of fluxing oxides, suggesting that this sample is the most promising. On the other hand, clays with a high limestone content, such as C3 and C4, tend to have greater porosity and less mechanical resistance after firing. Additionally, these two raw materials have lower alkaline oxide ratios than those observed for C3 and C2.

4.2 X-ray diffractometry

The X-ray diffraction patterns of the clays are shown in Figure 7 and correlate positively with the results observed by X-ray fluorescence. The X-ray diffractometry (XRD) technique was used to determine the crystalline phases. The samples were dried in an oven at 110 °C for 24 h, ground, and passed through a 150-μm mesh sieve. The diffraction patterns were obtained in a Rigaku D-MAX 100 equipment, using Cu Kα1 radiation (λ = 1.5418 Å). All measurements were carried out in the continuous scanning mode with speed of 1°/min, in the range of 5 to 65° and in the range of 2 to 15° in samples saturated with ethylene glycol for 1 h to identify montmorillonite by displacing the diffraction peaks at smaller angles compared to dry sample testing. The crystalline phases were identified through Match! (Phase Identification by Powder Diffraction) in the demo version, according to the ICSD (Inorganic Crystal Structure Database).

The main phases identified were quartz, kaolinite, muscovite, montmorillonite, calcite, feldspar, and hematite. Minerals from kaolinite and montmorillonite clay were identified in all analyzed clays. According to Celik [20], these clay minerals provide the necessary plasticity to guarantee conformation through the pressing process. The percentage of each crystalline phase present in the samples was estimated from the relative intensity of the main peaks in each phase. The values are shown in Table 5. The percentage of carbonates increases from 0.9% in C1 to 12.4% in C4.

4.3 Dilatometric tests

To verify the dimensional changes of expansion and thermal retraction of the samples, dilatometry tests were performed on a Netzsch dilatometer, model DIL 402PC, under synthetic air flow at 130 ml/min. For these analyses, the samples were compacted in a cylindrical shape, 12.0 mm in length and 6.0 mm in diameter. Under a constant heating rate of 10°C/min, the length of the compacted body is measured as a function of time and temperature, which varied from room temperature to 1150°C.

In Figure 8 we can observe a slight expansion in all curves up to approximately 850°C, and at 573°C, the expansion was more pronounced due to the transformation of α quartz to β [22, 23], except for C2, which presents a lower percentage of free quartz. From 573°C, there was a gradual reduction in the expansion rate, occurring or starting with sintering, followed by an exponential retraction [22].

The results shown in Table 5 with the percentages of CaO, Na₂O, and K₂O recommended by XRF measurements point out that sample C2 has a greater amount of funds (calcium carbonate up to a limit of 3% and alkaline oxides), or what is known as a greater linear shrinkage. Despite its advantages over the other samples, the C2 clay underwent deformation during firing up to 1150°C. This effect, known as pyroplastic deformation, may be due to the large proportion of funds in the sample, a high content of Fe₂O₃, and, even, the amount of organic matter [24]. One of the ways to control deformation during firing is to adjust the thermal cycle through the dilatometric curves, so that the plate remains within the required standards [25].

4.4 Firing clays with limestone

Clays containing limestone when subjected to burning, CaCO₃ after heating, in the temperature range between 850 and 920°C, form CaO and release CO₂. An intense endothermic peak of approximately 35–44% of the mass loss can be observed in differential thermal analysis. In ternary diagrams, it is observed that there is a eutectic point (above 1170°C), which reduces the dimensional stability in ceramic products, which can melt quickly (Figure 9).

Clays when mixed with limestone can behave differently, as shown by Sánchez [25]. Figure 10 shows a standard clay with 5 and 10% of incorporated limestone. It was observed that as the limestone and temperature increase, respectively, the dimensional instability increases. In other words, the retraction increases constantly, when it undergoes an exponential increase, reaching the melting point.

This phenomenon can be explained as follows: when exhibiting CaO up to the limit of 3%, this, associated with SiO₂ and Al₂O₃ present in clays and feldspars, helps in the formation of eutectic systems at 1170°C, with consequent formation of liquid phase and contributing to obtain the desired mechanical strength and porosity. When introduced in percentages above 4%, CaCO₃ levels are increased, and the composition moves from the eutectic line, forming crystalline phases such as CaSiO₃ (pseudo-wollastonite) and 2CaO⋅Al₂O₃⋅SiO₂ (gehlenite). So, a larger number of pores is left by the eliminated CO₂. In this way, the porosity of the final product is increased, as shown in Figure 11. In Figure 12 is shown a photo of a clay mass with 10% calibration in which the porosity exerted can be observed.