Metakaolin

Metakaolin is a dehydroxylated form of the clay mineral kaolinite.

Rocks that are rich in kaolinite are known as china clay or kaolin, traditionally used in the manufacture of porcelain. The particle size of metakaolin is smaller than cement particles, but not as fine as silica fume.

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Forming metakaolin

Between 100-200°C, clay minerals lose most of their adsorbed water. Between 500-800°C kaolinite becomes calcined by losing water through dehydroxilization. The dehydroxilization of kaolin to metakaolin is an endothermic 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 (supplementary cementing material) nearly complete dehydroxilization must be reached without overheating, i.e., thoroughly roasted but not burnt. This produces an amorphous, highly pozzolanic state, whereas overheating can cause sintering, to form the dead burnt, nonreactive refractory, called mullite.^[1]

High reactivity metakaolin

High reactivity metakaolin (HRM) is a highly processed reactive aluminosilicate 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. It is also reported that HRM is responsible for acceleration in the hydration of OPC and its major impact is seen within 24 hours. It also reduces the deterioration of concrete by alkali silica reaction (ASR), particularly useful when using recycled crushed glass or glass fines as aggregate.{fact}

Concrete application

Considered to have twice the reactivity of most other pozzolans, metakaolin is a valuable admixture for concrete/cement applications. Replacing portland cement with 8% - 20% (by weight) metakaolin produces a concrete mix which exhibits favorable engineering properties, including: the filler effect, the acceleration of ordinary portland cement (OPC) hydration, and the pozzolanic reaction. 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, 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 (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
• Engineered Cementitious Composite
• Fly ash
• Kaolinite
• Portland cement
• Pozzolan
• Silica fume

References