Metakaolin

Key takeaways

Metakaolin belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Metakaolin to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Metakaolin from memory before moving on to harder problems.

Reference excerpt

Metakaolin is the anhydrous calcined 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.

Kaolinite sources The quality and reactivity of metakaolin is strongly dependent of the characteristics of the raw material used. Metakaolin can be produced from a variety of primary and secondary sources containing kaolinite: metakaolin is a dehydrated form of kaolinite, a type of clay mineral. Kaolinite-rich minerals are also referred to as china clay or kaolin, which are traditionally utilized in the production of porcelain.

High purity kaolin deposits Kaolinite deposits or tropical soils of lower purity Paper sludge waste (if containing kaolinite) Oil sand tailings (if containing kaolinite)

Forming metakaolin The T-O clay mineral kaolinite does not contain interlayer cations or interlayer water. The temperature of dehydroxylation depends on the structural layer stacking order. Disordered kaolinite dehydroxylates between 530 and 570 °C, ordered kaolinite between 570 and 630 °C. Dehydroxylated disordered kaolinite shows higher pozzolanic activity than ordered. The dehydroxylation of kaolin to metakaolin is an endothermic process due to the large amount of energy required to remove the chemically bonded hydroxyl ions. Above the temperature range of dehydroxylation, kaolinite transforms into metakaolin, a complex amorphous structure which retains some long-range order due to layer stacking. Much of the aluminum of the octahedral layer becomes tetrahedrally and pentahedrally coordinated. In order to produce a pozzolan (supplementary cementitious material) nearly complete dehydroxylation 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 a dead burnt, nonreactive refractory, containing mullite and a defect Al-Si spinel. Reported optimum activation temperatures vary between 550 and 850 °C for varying durations, however the range 650-750 °C is most commonly quoted. In comparison with other clay minerals kaolinite shows a broad temperature interval between dehydroxylation and recrystallization, much favoring the formation of metakaolin and the use of thermally activated kaolin clays as pozzolans. Also, because the octahedral layer is directly exposed to the interlayer (in comparison to for instance T-O-T clay minerals such as smectites), structural disorder is attained more easily upon heating.

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 and 700 °C in an externally fired rotary kiln. It is also reported that HRM is responsible for acceleration in the hydration of ordinary portland cement (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. The amount of slaked lime that can be bound by metakaolin is measured by the modified Chapelle test.

Adsorption properties The adsorption surface properties of the metakaolins can be characterized by inverse gas chromatography analysis.

Concrete admixture 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 wt.% (% by weight) metakaolin produces a concrete mix that exhibits favorable engineering properties, including: the filler effect, the acceleration of OPC hydration, and the pozzolanic reaction. The filler effect is immediate, while the effect of pozzolanic reaction occurs between 3 and 14 days. In the mid-2010s, Limestone Calcined Clay Cement mixture incorporating even more than 20% metakaolin was developed as a lower-carbon cement substitute. The technology is on the commercialization stage in the 2020s.

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. Higher thermal resistance due to increased temperature levels

Uses High performance, high strength, and lightweight concrete Precast and poured-mold concrete Fibercement and ferrocement products Glass fiber reinforced concrete Countertops, art sculptures (see for example the free-standing sculptures of Albert Vrana) Mortar and stucco

See also Concrete Engineered cementitious composite Fly ash Kaolinite Portland cement Pozzolan Rice husk ash (also very rich in SiO2) Silica fume

References

Worked examples

Example 1 — a first encounter with Metakaolin

Start with the simplest possible case. Write down what Metakaolin claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Metakaolin before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Metakaolin ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Metakaolin

In research: Metakaolin appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Metakaolin in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Metakaolin is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cement, Concrete, Silicate minerals, so understanding it makes those chapters shorter.
In everyday life: Look for Metakaolin outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Metakaolin in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Metakaolin means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Metakaolin out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Metakaolin in simple terms?

Metakaolin is the anhydrous calcined 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.

Why does Metakaolin matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Metakaolin?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Metakaolin.