In composition and classification of ceramic tiles, some important methods like light-firing, kaolins and pyrophyllitic are used.
Clays are used to make tiles, so their commercial classification depends on the technological and aesthetic requirements of each type of ceramic body:
first, the color after firing; second, the behavior during the tile-making process, which involves qualities like slip viscosity, plasticity, drying sensitivity, fusibility, pore-forming ability, and so on, all of which are inextricably linked to clay mineralogy and particle size distribution.
First, the color of the ceramic body after firing is used to distinguish between light-firing (from white to light brown) and dark-firing (from pink to dark brown).
Even if other substances like TiO2 and CaO that may change the hue to yellowish or pinkish colors also play a part, iron oxide is the main factor determining this color.
In general, clays used in light-firing and dark-firing bodies may be distinguished from one another pretty effectively by having a Fe2O3 composition of about 3% wt.
In any case, there is a connection between this threshold and technological properties. For example, the iron content of kaolin used to whiten a given body is acceptable for values well below the limit (e.g., 1% Fe2O3), whereas the iron oxide content of bentonite used to increase green bending strength is acceptable even at slightly higher levels.
In the paragraphs that follow, other distinctions pertaining to the technological characteristics of clays (rheological properties and behavior during grinding, compaction, drying, and burning) will be discussed.
Light-firing Ceramic Clays
Light-firing ceramic clays include a number of industrial varieties that go by the general names of kaolin and ball clay (as well as pyrophyllite and, to a lesser extent, bentonite), each of which refers to a variety of mineralogical and granulometric traits.
From a mineralogical perspective, they mostly consist of phyllosilicates (mainly kaolinite) and silica phases (quartz and especially cristobalite, tridymite, and opal), while other elements (feldspars, iron and aluminum.
oxyhydroxides, organic materials) may also be present. In decreasing order of frequency, clay minerals other than kaolinite include illite, interstratified I/S, halloysite, smectite, pyrophyllite, sericite, dickite, interstratified K/S, vermiculite, and chlorite.
In spite of genetic, compositional, or technological characteristics, these names are employed relatively liberally for commercial reasons outside of the general distinction between kaolin and ball clay.
Depending on the ultimate purpose, it is common practice to refer to similar raw materials as “kaolin” or “ball clay”; other clays with quite varied compositions and particle sizes may also be categorized under the same name. Because of this, we suggest a classification based on the following standards:
Portion of quartz and other phyllosilicates, mainly pyrophyllite and expandable clay minerals: smectite + interstratified I/S; iron oxide content;
amount of kaolinite group minerals (kaolinite, halloysite, dickite); particle size distribution; and plasticity (Atterberg consistency limits and Methylene Blue Index).
Accordingly, low iron oxide clays (thus light-colored after burning) can be classified into the following groups by the quantity of kaolinite group minerals:
N 75% Illite-mica, Al-oxyhydroxides, and non-plastic components (quartz, feldspar, rock fragments, titanium dioxide) may be present in low concentrations (below 15%) in high-grade kaolins (HK), which are distinguished by high to very high levels of kaolinite and/or halloysite.
In addition to being mostly kaolinitic, 50–75 percent of low-grade kaolins (LK) also contain low to large levels of illite and/or interstratified I/S, as well as an invariably significant non-plastic component made up of quartz, feldspars, etc.
Although it can be less than 50% in the case of illite-rich materials, 25-75% Ball Clays (BC) have an amount of kaolinite group minerals comparable to low-grade kaolins.
In actuality, the general mineralogical makeup of BC greatly resembles that of kaolins. Here, the difference between ball clays and low-grade kaolins is proposed in terms of plasticity.
When kaolinite content is at least 50%, ball clays can be recognized from raw kaolins by their substantial particle fraction below 2 m, which must be at least 40% by weight.
25 to 50 percent Raw Kaolins (RK) are essentially heavily non-plasticized parent rocks (in the case of main deposits) (particularly rock fragments, quartz, feldspars and mica).
In any case, the quantity of phyllosilicates must be large enough to provide the raw kaolin enough flexibility, albeit at a low level, to function as a clay material.
Kaolinite-containing sedimentary deposits known as kaolinitic loams (KL) usually contain various clay minerals such as smectite, interstratified I/S, illite, and chlorite in addition to a substantial non-plastic component (mostly quartz) and kaolinite.
The particle size distribution of raw kaolins and kaolinitic loams is coarse-grained with a frequent considerable sandy proportion (N25% over 63 m).
b25% Pyrophyllitic Clays (PC) or White Bentonites (WB) are particular examples of light-firing clays that are deficient in kaolinite and contain more than 20% of pyrophyllite or smectite, respectively.
Premium Kaolins (HK) High-grade kaolins are used to make ceramic tiles because they are easily dissolved in water but difficult to press and very refractory, which promotes the creation of mullite and whiter hues during firing.
To demonstrate a plastic behavior, more refractory concepts frequently need to be milled, hence flint clay examples are shown.
A few percent in glazes and engobes (to thicken and stabilize suspensions) and no more than 10-15% in porous and vitrified bodies are the typical HK usage rates (unglazed porcelain stoneware and white birapida).
Kaolins from Cornwall, UK, Bayern, Germany (Keck, 1991; Kitagawa and Köster, 1991), and various grades from Kütahya, Turkey are examples of HK used in tilemaking.
Kaolins Ceramic Clays
Components related to kaolinite in the plastic and nonplastic fractions have an impact on how low-grade kaolins behave in the ceramic clays cycle.
For instance, although their use is limited to vitrified and porous bodies (typically in percentages larger than HK) and is typically not.
used in engobes and glazes, the technological performance of materials higher in kaolinite and poorer in feldspars and expandable clay minerals does not significantly differ from that of high-grade kaolins.
Due to the increased non-plastic component, which implies lesser plasticity and confirms challenges in pressing and sintering, the technical behavior is obviously different from that of HK when.
the number of kaolinite group minerals exceeds 50%. However, the presence of feldspars (increasing fusibility and encouraging sintering) and expandable clay minerals can help to some part overcome these limitations (improving plasticity).
In any case, clays with a relatively low plasticity are defined by the LK class (MBIa b7.5). Kaolins from Santa Severa, Italy, Sinitsa and Dedovka, Belarus, and several grades from Muang Ranong, Thailand are examples of LK used in tilemaking.
Since using raw kaolins to make tiles is not simple, it is commonly overlooked. In actuality, due to their weak plasticity, rawkaolins cannot satisfy standard standards for ceramic clays.
Due to equivalent proportions of kaolinite, quartz, and flux (feldspars, rock pieces), RK, however, may function as a form of mixture whose technical behavior must be balanced by the other clays and fluxes making up the body.
When the distance between the mine and the tile production is close, the inclusion of raw kaolin proves to be beneficial and allows for a low cost.
Raw materials from Romana, Piloni di Torniella, Italy, or Michoacán, Mexico are examples of RK used in the production of tiles.
As the raw kaolin fraction, some sedimentary deposits used in the ceramics industry are largely composed of quartz, with a little amount of feldspar and occasionally carbonates.
Due to their coarse grain size distribution, which has a significant impact on their technical behavior, these clay minerals are referred to here as kaolinitic loams.
Due of the common coexistence of expandable clay minerals and kaolinite, this might be characterized as a compromise between coarse grain size and relatively high plasticity.
With such an odd composition, KL becomes a rather adaptable raw material that can be used for vitrified bodies up to 20–25%.
Italian clays from Florinas, Sardinia, and Lozzolo, Piedmont, are two examples of KL used in tile manufacturing.
Ball clay is a fine-grained, highly pliable, primarily kaolinitic sedimentary clay, the better grades of which fire to a white or nearly white color, according to the traditional description.
In practice, regardless of compositional characteristics, such a description is stretched in the production of ceramic tiles to encompass proper (or even just acceptable) behavior during wet milling, pressing, and sintering.
As a result, the phrase “ball clay” in commerce refers to a remarkable variety of compositions and technological attributes (ICerS, 1995-2010).
A large range of kaolinite (20-80%), illite (0-60%), quartz (0-60%), and feldspars (0-30%) concentrations are implied by the fact that BC used in tilemaking plot throughout nearly the full Kaolinite, Illite, Quartz, + Feldspar ternary diagram.
Although they are not always present, expandable clay minerals, which are often interstratified illite-smectite and occasionally smectite, are common components that can exceed 25% and may have a considerable impact on plasticity and behavior during pressing and drying.
Other components include organic or carbonaceous materials (often in the 0.1-0.7% range) and iron oxyhydroxides (typically below 2%, though slightly greater quantities may be tolerated).
There are occasionally traces of other minerals, such as pyrophyllite, interstratified K/S, chlorite, gibbsite, and carbonates.
The majority of the time, the particle size distribution is fine-grained, with a clay component of at least 50% and typically over 75%.
However, some raw materials marketed as “ball clay” have grain sizes that are relatively coarse and have a silty proportion of above 50%.
These extensive collections of granulometric and mineralogical data, which cover a wide spectrum of BC’s technological characteristics, including plasticity and behavior during compaction, drying, and burning, are combined.
Ball clays have a wide range of MBIa (ranging from 8 to 40 meq/100 g), with generally greater values in comparison to LK.
This image enables the classification of LFC based on technological performance: Low plasticity clays (MBI b 7.5; commonly found in kaolins: HK, LK, RK);
medium plasticity clays (7.5 b MBI b 12; BC1); high plasticity clays (12 b MBI b 16; BC2); and very high plasticity clays (16 b MBI b 30; BC3 and some WB); exceptionally high plasticity only occurs in bentonites among ceramic raw materials.
These restrictions deviate from established reference schemes and can be linked to the technological behavior in the production of tiles.
There are numerous instances of BC being utilized in the production of tiles; examples include clays from the Donbass, Ukraine; Santa Cruz, Argentina; and Westerwald, Germany.
Pyrophyllitic Ceramic Clays
Pyrophyllite can occur in ceramic clays materials used to make ceramic tiles in concentrations ranging from a few percent to 80% weight.
Pyrophyllite differs technologically from kaolinite, so it is convenient to classify raw materials containing a significant amount of pyrophyllite (N20%) as Pyrophyllitic Clays (PC) because they typically exhibit lower loss on ignition, easier compaction, and lower refractoriness than kaolins.
PC are often clay stones of various origin that include pyrophyllite, quartz, feldspars, and frequently kaolinite and/or illite or sericite in addition to the aforementioned minerals.
Pyrophyllitic clays are used to make tiles, particularly in India; Argentina; they can replace BC in various applications and are roseki in Japan and Korea.
In order to improve the plasticity of overly lean bodies, clays with a high smectite concentration and a low iron content (Fe2O3 at most 6%, but preferably below 2%) are employed in the production of tiles.
Despite the fact that the firing color of the terms richer in iron is relatively dark, this type of clay is frequently referred to as White Bentonites. Smectite and other phyllosilicates (interstratified I/S and less frequently kaolinite or illite), silica phases.
(quartz, cristobalite, tridymite, opal), feldspars, volcanic glass, and rock fragments are abundant in somewhat complicated mineralogical assemblages.
Although their maximum tolerable amount is limited by rheological properties during wet milling and spray-drying (smectite induces a sharp increase in slip viscosity and thixotropy) as well as by drying sensitivity, WB is largely used in tilemaking to increase the mechanical strength of unfired bodies.
Due to these technological limitations, it is recommended to have WB with a moderate activity and a little amount of smectite (i.e., Ca-exchanged terms). Italy, Greece, and Argentina are three countries that use WB in the production of tiles.
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