Calcined Kaolin Powder, also known as Metakaolin, is a processed form of kaolin clay that has been heated at high temperatures (typically between 600 and 800 degrees Celsius) to remove water and other volatile compounds. This calcination process alters the physical and chemical properties of kaolin, making it suitable for various applications in the construction industry.
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Here are some of the common applications and uses of calcined kaolin powder in construction:
Cement and Concrete: Calcined kaolin can be used as a supplementary cementitious material (SCM) in the production of cement and concrete. When added to cement or concrete mixtures, it enhances the strength, durability, and workability of the material. It acts as a pozzolan, reacting with calcium hydroxide produced during the hydration process of cement to form additional cementitious compounds, resulting in improved performance and reduced environmental impact.
Mortars and Grouts: Calcined kaolin is often used in the formulation of mortars and grouts to improve their workability, reduce shrinkage, and enhance adhesion. It acts as a filler, providing a smoother consistency to the mixture and reducing the amount of water required for proper hydration.
Geopolymer Concrete: Geopolymer concrete is an alternative to traditional cement-based concrete that uses a binder composed of aluminosilicate materials. Calcined kaolin, with its high aluminum content, can serve as a valuable precursor for geopolymer binders, helping to create strong and environmentally friendly construction materials.
Surface Coatings: Calcined kaolin powder is widely used in the production of paint, coatings, and sealants for construction applications. It acts as an extender pigment, enhancing the opacity, brightness, and durability of the coatings. It also contributes to improved rheology and reduced cracking.
Fiber Cement Products: Fiber cement boards, tiles, and panels are widely used in the construction industry due to their durability and fire resistance. Calcined kaolin is added to the formulation of fiber cement products as a filler, providing improved strength, dimensional stability, and resistance to weathering.
Ceramic and Porcelain Tiles: Calcined kaolin is utilized in the manufacturing of ceramic and porcelain tiles to enhance their strength, whiteness, and firing characteristics. It improves the workability of the clay mixture, reduces shrinkage during firing, and helps achieve a smooth and glossy finish.
Insulating Materials: Calcined kaolin can be incorporated into insulating materials, such as thermal insulation boards, refractories, and firebricks, due to its high melting point, low thermal conductivity, and good resistance to high temperatures.
Road Construction: Calcined kaolin powder can be used in road construction as an additive to bituminous materials, such as asphalt, to enhance their stability, durability, and resistance to rutting and cracking.
USES of CALCINED KAOLIN POWDER OTHER THAN construction
Calcined kaolin powder, apart from its applications in the construction industry, finds utility in several other fields. Here are some additional uses of calcined kaolin powder:
Ceramics and Refractories: Calcined kaolin is extensively used in the ceramics industry as a component in the production of ceramic bodies and glazes. It improves the strength, whiteness, and thermal stability of ceramic materials. Additionally, it is employed in the manufacturing of refractory bricks and castables due to its high melting point and resistance to heat.
Paper and Packaging: Calcined kaolin is utilized as a filler and coating pigment in the paper and packaging industry. It enhances the smoothness, brightness, opacity, and printability of paper products. It also improves the ink absorption properties and reduces ink bleed, resulting in sharper printed images.
Paints and Coatings: Calcined kaolin acts as an extender pigment in the formulation of paints and coatings. It enhances opacity, provides excellent hiding power, and contributes to improved durability and weather resistance. It is widely used in architectural coatings, industrial paints, and automotive coatings.
Plastics and Rubber: Calcined kaolin is used as a reinforcing filler in plastic and rubber applications. It improves mechanical properties, such as tensile strength, modulus, and impact resistance. It also enhances dimensional stability, reduces shrinkage, and improves the surface finish of plastic and rubber products.
Personal Care Products: Calcined kaolin is employed in various personal care products, including cosmetics, skincare formulations, and hair care products. It serves as an absorbent, bulking agent, and opacifying agent. It helps control oiliness, provides a silky texture, and imparts a matte finish in cosmetic products.
Pharmaceuticals: Calcined kaolin is used in pharmaceutical formulations as an excipient and filler. It aids in tablet formation by improving flow properties and compressibility. It can also be used in topical ointments and creams for its absorption properties and rheological benefits.
Polishes and Polishing Compounds: Calcined kaolin is utilized in the production of polishes and polishing compounds. It provides a fine abrasive action, which helps in achieving a smooth and glossy finish on various surfaces, including metals, plastics, and glass.
Catalyst Support: Calcined kaolin can be used as a support material for catalysts in chemical processes. It offers a high surface area, pore volume, and thermal stability, making it suitable for catalytic applications.
These are just some of the applications and uses of calcined kaolin powder in the construction industry. Its versatile properties make it a valuable ingredient in various construction materials, offering improved performance, sustainability, and aesthetic appeal.
These are some of the prominent uses of calcined kaolin powder outside of the construction industry. Its unique properties make it a versatile material with applications in diverse sectors, including ceramics, paper, paints, plastics, personal care, pharmaceuticals, and more.
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Alternate Names: Cal Kaolin, Meta Kaolin
Kaolin is pure clay mineral, having a fired chemistry of 1 molar part Al2O3 and 2 parts SiO2. However the raw clay crystals are hydrated, having 12% crystal-bound water. This is the secret to their plasticity. Calcining the material removes that water and destroys the plasticity. Calcined kaolin is not commonly used in traditional ceramics, except for the specifics mentioned below, but it is a commodity material in many industries. Various grades of calcined kaolin are produced by varying parameters in the calcination process (e.g. retention time in the rotary kiln, firing temperature). Calcination initially converts it to metakaolin (whose reactivity makes it useful in cements). Further heating whitens it more and reduces reactivity while still keeping abrasiveness low. Still, more calcination converts the crystals to mullite, making them much more abrasive.
Calcined kaolin has an important use in ceramic glazes. Al2O3 is essential to the chemistry of the vast majority of glazes and its most readily-meltable natural supplier is feldspars, however they most often oversupply KNaO in providing the needed Al2O3. Raw kaolin is an ideal second or even primary source of Al2O3. Not only does it readily decompose in the melt but it also supplies SiO2 that all glazes need. And it acts to suspend the slurry and harden the dried layer. But there is a problem with raw kaolin: Once recipe percentages pass 20% shrinkage becomes too high (causing crawling). In these cases substituting part of the raw kaolin for calcined kaolin solves the problem, maintaining the chemistry of the glaze but reducing the shrinkage and cracking. Of course, mixing the raw and calcined materials must take into account the LOI of the raw material (12% less calcined is needed).
Calcined kaolins are also useful in tuning the shrinkage and plasticity of slips (engobes) which are applied to wet or leather-hard ware. Engobes contain higher clay percentages than glazes and it is more important to control their drying shrinkage. Thus, like for glazes, they can be substituted for part of the raw kaolin to tune drying while maintaining fired properties.
Calcined kaolin is refractory and softens at about cone 35. It is thus useful in refractory castables and furniture, thermal insulation bodies, low expansion bodies, permeable ceramic compositions, and investment casting (see molochite).
You can make calcined kaolin by simply bisque firing any raw powdered kaolin (in a small enough bisque vessel and slow enough ramp that the heat penetrates well). Roasting the powder at red heat, e.g. F, is sufficient to destroy the plasticity. It is a good idea to measure weight loss on calcined or roasting a compensate by that amount when substituting for raw kaolin. The material is a good example of how we can alter the mineralogy of a material to affect its working properties while maintaining the chemistry to retain fired properties.
If your drying pottery glaze is doing what you see on the left, a spider web crack pattern, do not smooth it with your finger and hope for the best. It is going to crawl during firing. Wash it off, dry the ware and change your glaze or process. The first thing to check is water content. If the glaze has worked fine in the past then it is likely going on too thick because the specific gravity is too high - just repeat cycles of adding a little water and dip testing. But that was not the issue here. Glazes need clay to suspend and harden them, but if there is too much it can mean trouble. This was Ravenscrag Slip, a clay, being used pure as a cone 10R glaze. The glaze appeared to go in perfectly and it dried to the touch in ~20 seconds. But shrinkage continues after that, revealing after a couple of minutes. Fixing the issue was a matter of adding some roasted Ravencrag Slip to the bucket. That reduced the shrinkage and therefore the cracking. Any glaze containing excessive kaolin can be fixed the same way (trade some of the raw kaolin for calcined kaolin). Some glazes that contain plenty of clay also have bentonite - a simple fix for these is to simply remove it.
These are two 10 gram GBMF test balls of Worthington Clear glaze fired at cone 03 on terra cotta tiles (55 Gerstley Borate, 30 kaolin, 20 silica). On the left it contains raw kaolin, on the right calcined kaolin. The clouds of finer bubbles (on the left) are gone from the glaze on the right. That means the kaolin is generating them and the Gerstley Borate the larger bubbles. These are a bane of the terra cotta process. One secret of getting more transparent glazes is to fire to temperature and soak only long enough to even out the temperature, then drop 100F and soak there (I hold it half an hour).
It was spray applied on the dried bowl (no bisque fire) an was too thick (not to mention under fired). But the main problem was a glaze recipe having too high a clay content. If a glaze has more than about 25% clay, consider a mix of the raw clay and calcined. For example, you can buy calcined kaolin to mix with raw kaolin. Or you can calcine the clay in bowls in your kiln by firing it to about F.
These three cups are glazed with GS at cone 03. The glaze is the most crystal clear achieved so far because it contains almost no gas producing materials (not even raw kaolin). It contains Ferro frits and plus 11 calcined kaolin and 3 VeeGum. Left is a low fire stoneware (LT), center is Plainsman L212 and right a vitreous terra cotta (LF). It is almost crystal clear, it has few bubbles compared to the kaolin-suspended version. These all survived a 300F/icewater IWCT test without crazing!
Calcined kaolin has zero plasticity. 25% bentonite had to be added to make it plastic enough to make this piece. Why bother? Because this will flash heavily in reduction firing.
These are two cone 6 matte glazes (shown side by side in an account at Insight-live). GZ is high calcium and a high silica:alumina ratio. It crystallizes during cooling to make the matte effect and the degree of matteness is adjustable by trimming the silica content (but notice how much it runs). The GC has high MgO and it produces the classic silky matte by micro-wrinkling the surface, its matteness is adjustable by trimming the calcined kaolin. CaO is a standard oxide that is in almost all glazes, 0.4 is not high for it. But you would never normally see more than 0.3 of MgO in a cone 6 glaze (if you do it will likely be unstable). The GC also has 5% tin, if that was not there it would be darker than the other one because Ravenscrag Slip has a little iron. This was made by recalculating the Moore's Matte recipe to use as much Ravenscrag Slip as possible yet keep the overall chemistry the same. This glaze actually has texture like a dolomite matte at cone 10R, it is great. And it has wonderful application properties. And it does not craze, on Plainsman M370 (it even survived a 300F-to-ice water IWCT test). This looks like it could be a great liner glaze.
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