Metakaolin
Encyclopedia
Metakaolin is a dehydroxylated
Hydroxylation
Hydroxylation is a chemical process that introduces a hydroxyl group into an organic compound. In biochemistry, hydroxylation reactions are often facilitated by enzymes called hydroxylases. Hydroxylation is the first step in the oxidative degradation of organic compounds in air...

 form of the clay mineral kaolinite
Kaolinite
Kaolinite is a clay mineral, part of the group of industrial minerals, with the chemical composition Al2Si2O54. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra...

.

Rocks that are rich in kaolinite are known as china clay or kaolin, traditionally used in the manufacture of porcelain
Porcelain
Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and...

. The particle size of metakaolin is smaller than cement
Cement
In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...

 particles, but not as fine as silica fume
Silica fume
Silica fume, also known as microsilica, is a fine-grain, thin, and very high surface area silica.It is sometimes confused with fumed silica and colloidal silica...

.

Forming metakaolin

Between 100–200 °C, clay minerals lose most of their adsorbed
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...

 water. Between 500–800 °C kaolinite becomes calcined
Calcination
Calcination is a thermal treatment process applied to ores and other solid materials to bring about a thermal decomposition, phase transition, or removal of a volatile fraction. The calcination process normally takes place at temperatures below the melting point of the product materials...

 by losing water through dehydroxilization. The dehydroxilization of kaolin to metakaolin is an endothermic
Endothermic
In thermodynamics, the word endothermic describes a process or reaction in which the system absorbs energy from the surroundings in the form of heat. Its etymology stems from the prefix endo- and the Greek word thermasi,...

 process due to the large amount of energy required to remove the chemically bonded hydroxyl ions. Above this temperature range, kaolinite becomes metakaolin, with a two-dimensional order in crystal structure. In order to produce a pozzolan
Pozzolan
A pozzolan is a material which, when combined with calcium hydroxide, exhibits cementitious properties. Pozzolans are commonly used as an addition to Portland cement concrete mixtures to increase the long-term strength and other material properties of Portland cement concrete, and in some cases...

 (supplementary cementing material) nearly complete dehydroxilization must be reached without overheating, i.e., thoroughly roasted but not burnt. This produces an amorphous
Amorphous solid
In condensed matter physics, an amorphous or non-crystalline solid is a solid that lacks the long-range order characteristic of a crystal....

, highly pozzolanic state, whereas overheating can cause sintering
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...

, to form the dead burnt, nonreactive refractory
Refractory
A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...

, called mullite
Mullite
Mullite or porcelainite is a rare silicate mineral of post-clay genesis. It can form two stoichiometric forms 3Al2O32SiO2 or 2Al2O3 SiO2. Unusually, mullite has no charge balancing cations present...

.

High-reactivity metakaolin

High-reactivity metakaolin (HRM) is a highly processed reactive aluminosilicate
Aluminosilicate
Aluminosilicate minerals are minerals composed of aluminium, silicon, and oxygen, plus countercations. They are a major component of kaolin and other clay minerals....

 pozzolan, a finely-divided material that reacts with slaked lime at ordinary temperature and in the presence of moisture to form a strong slow-hardening cement. It is formed by calcining purified kaolinite, generally between 650–700 °C in an externally fired rotary kiln
Rotary kiln
A Rotary kiln is a pyroprocessing device used to raise materials to a high temperature in a continuous process. Materials produced using rotary kilns include:* Cement* Lime* Refractories* Metakaolin* Titanium dioxide* Alumina* Vermiculite...

. It is also reported that HRM is responsible for acceleration in the hydration of ordinary portland cement
Portland cement
Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...

 (OPC), and its major impact is seen within 24 hours. It also reduces the deterioration of concrete by Alkali Silica Reaction
Alkali Silica Reaction
The alkali–silica reaction is a reaction which occurs over time in concrete between the highly alkaline cement paste and reactive non-crystalline silica, which is found in many common aggregates....

 (ASR), particularly useful when using recycled crushed glass or glass fines as aggregate. The amount of slaked lime that can be bound by metakaolin is measured by the modified Chapelle test.

Concrete application

Considered to have twice the reactivity of most other pozzolan
Pozzolan
A pozzolan is a material which, when combined with calcium hydroxide, exhibits cementitious properties. Pozzolans are commonly used as an addition to Portland cement concrete mixtures to increase the long-term strength and other material properties of Portland cement concrete, and in some cases...

s, metakaolin is a valuable admixture
Admixture
Admixture can refer to:* Genetic admixture, the result of interbreeding between two or more previously isolated populations within a species.* Racial admixture, admixture between humans, also referred to as Miscegenation* Hybrid...

 for concrete/cement applications. Replacing portland cement
Portland cement
Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...

 with 8–20% (by weight) metakaolin produces a concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

 mix, which exhibits favorable engineering properties, including: the filler effect, the acceleration of OPC hydration
Mineral hydration
Mineral hydration is an inorganic chemical reaction where water is added to the crystal structure of a mineral, usually creating a new mineral, usually called a hydrate....

, and the pozzolanic reaction
Pozzolanic reaction
The Pozzolanic reaction is the chemical reaction that occurs in hydraulic cement, a mixture of slaked lime with amorphous siliceous materials , forming non-water-soluble calcium silicate hydrates...

. The filler effect is immediate, while the effect of pozzolanic reaction occurs between 7 to 14 days.

Advantages

  • Increased compressive and flexural strengths
  • Reduced permeability (including chloride permeability)
  • Reduced potential for efflorescence
    Efflorescence
    In chemistry, efflorescence is the loss of water of crystallization from a hydrated or solvated salt to the atmosphere on exposure to air.-Examples:...

    , which occurs when calcium is transported by water to the surface where it combines with carbon dioxide from the atmosphere to make calcium carbonate, which precipitates on the surface as a white residue.
  • Increased resistance to chemical attack
  • Increased durability
  • Reduced effects of alkali-silica reactivity
    Alkali Silica Reaction
    The alkali–silica reaction is a reaction which occurs over time in concrete between the highly alkaline cement paste and reactive non-crystalline silica, which is found in many common aggregates....

     (ASR)
  • Enhanced workability and finishing of concrete
  • Reduced shrinkage, due to "particle packing" making concrete denser
  • Improved color by lightening the color of concrete making it possible to tint lighter integral color.

Uses

  • High performance, high strength, and lightweight concrete
  • Precast and poured-mold concrete
  • Fibercement and ferrocement products
  • Glass fiber reinforced concrete
  • Countertops, art sculptures
  • Mortar and stucco

See also

  • Concrete
    Concrete
    Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

  • Engineered Cementitious Composite
    Engineered Cementitious Composite
    Engineered Cementitious Composite , also called bendable concrete, is an easily molded mortar-based composite reinforced with specially selected short random fibers, usually polymer fibers. Unlike regular concrete, ECC has a strain capacity in the range of 3–7%, compared to 0.1 % for ordinary...

  • Fly ash
    Fly ash
    Fly ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal...

  • Kaolinite
    Kaolinite
    Kaolinite is a clay mineral, part of the group of industrial minerals, with the chemical composition Al2Si2O54. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra...

  • Portland cement
    Portland cement
    Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...

  • Pozzolan
    Pozzolan
    A pozzolan is a material which, when combined with calcium hydroxide, exhibits cementitious properties. Pozzolans are commonly used as an addition to Portland cement concrete mixtures to increase the long-term strength and other material properties of Portland cement concrete, and in some cases...

  • Rice husk ash (also very rich in )
  • Silica fume
    Silica fume
    Silica fume, also known as microsilica, is a fine-grain, thin, and very high surface area silica.It is sometimes confused with fumed silica and colloidal silica...

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