Industrial Minerals · Buyer's Guide
Metakaolin: Buyer's Guide to Calcined Kaolin Pozzolan
Metakaolin is kaolin clay calcined until it loses its crystal water and becomes a highly reactive pozzolan. Concrete producers add it to raise strength and durability; it is traded under HS 2507, which covers kaolin whether or not calcined. This guide covers production, performance, specification and shipping.
Metakaolin at a glance
| Item | Typical value |
|---|---|
| Raw material | Kaolin clay (kaolinite) |
| Most-quoted calcination range | 650–750 °C |
| Kiln type | Rotary kiln or flash calciner |
| Particle size | Average 1–3 µm; finer than cement, coarser than silica fume |
| Cement replacement in concrete | 8–20% by weight; about 8–10% in most high-performance mixes |
| Reactivity | About twice that of most other pozzolans |
| Standard | ASTM C618, Class N (natural pozzolan) |
| HS heading | 2507 — kaolin and other kaolinic clays, whether or not calcined |
How metakaolin is produced
Kaolin is dried, calcined and ground. Heating drives off the kaolinite's structural water (dehydroxylation, from about 530–630 °C depending on how ordered the clay is) and leaves an amorphous, reactive aluminosilicate. High-reactivity grades are usually calcined at 650–700 °C in an externally fired rotary kiln.
For anyone costing a production line, the variables that decide product quality are the kaolinite content of the feed clay, the kiln type and its temperature control (too cold leaves unreacted clay, too hot lowers reactivity), and the final grinding fineness.
Where metakaolin is used
High-performance concrete
Replacing part of the cement raises compressive and flexural strength and cuts permeability, including chloride penetration, which protects reinforcement.
Durability problems
It reduces the effects of alkali-silica reaction, lowers efflorescence, improves chemical resistance and reduces shrinkage through better particle packing.
Precast and GFRC
Used in precast products, glass-fibre reinforced concrete and fibre-cement composites, where a lighter colour and early strength matter.
Mortar, stucco and countertops
Improves workability and finish in mortars, renders and decorative concrete.
What to specify in your request for quotation
- Reactivity evidence: strength activity index or pozzolanic activity test results
- Chemistry: SiO2 + Al2O3 + Fe2O3 total and loss on ignition, against ASTM C618 Class N
- Fineness: particle-size distribution or residue on a 45 µm sieve
- Colour or brightness if appearance matters, as in precast and decorative work
- Moisture, packaging (FIBC bulk bags or paper sacks), quantity, Incoterm and destination port
- Certificate of analysis per lot, and third-party inspection if your contract requires it
Customs classification and shipping
- HS 2507 covers kaolin and kaolinic clays whether or not calcined, so metakaolin sits under this heading. Check the national tariff line with the HS Code Explorer.
- Keep it dry. Metakaolin is a fine, reactive powder; ship in sealed bags and dry containers.
- Bags, not loose bulk, is the norm for smaller lots: FIBC bulk bags or multi-wall paper sacks on pallets.
- Plan loads with the CBM calculator and the container specifications library, and compare FOB and CFR offers with the Incoterms® 2020 guide and the landed cost calculator.
About this guide
This is independent reference information for buyers and specifiers. IMPUTIX is published by Mada Trade Co. in Alexandria; the guide is not an offer to sell this material, and product availability should always be confirmed with your own suppliers.
Shipping a bulk mineral? See export packaging for FIBC bulk bags and export packaging options.
Market context: industrial minerals market research. Other mineral guides: Anhydrite · Calcined shale · Perlite.
Frequently asked questions
What is metakaolin?
Kaolin clay calcined at roughly 650–750 °C until it loses its crystal water, leaving a highly reactive pozzolan used in concrete and mortar.
How much cement can metakaolin replace?
Typically 8–20% by weight, with about 8–10% common in high-performance concrete.
What is the HS code for metakaolin?
HS 2507, which covers kaolin and other kaolinic clays whether or not calcined. National tariff lines add further digits.
Is metakaolin covered by ASTM C618?
Yes. It is specified as a Class N natural pozzolan under ASTM C618.
Sources
- Metakaolin — Wikipedia (calcination temperatures, replacement range, reactivity)
- Use of Supplementary Cementitious Materials for Concrete — National Academies
- SCMs in concrete: natural pozzolans — NPCA
- ASTM C618 — Coal Ash and Raw or Calcined Natural Pozzolan for Use in Concrete
- HS 2507 — Kaolin and other kaolinic clays, whether or not calcined
Last reviewed: September 2026.