What Is Metakaolin? A Practical Guide to This High-Reactivity Pozzolan
If you work with high-performance concrete, you have probably seen metakaolin listed on a spec sheet next to silica fume and slag cement. So what is metakaolin, and why are ready-mix producers, precasters, and engineers paying closer attention to it? In short, metakaolin is a highly reactive pozzolan made by heating kaolin clay. Used as a partial cement replacement, it makes concrete stronger, denser, and far more resistant to chlorides and chemical attack.
This guide covers how metakaolin is made, how it works inside a concrete mix, what independent test data shows, and where it fits best.
How Metakaolin Is Made
Metakaolin starts as kaolin, a naturally occurring clay mineral (kaolinite) that has been used for centuries to make porcelain. On its own, raw kaolin does very little in concrete. The change happens during calcination. When kaolin is heated to roughly 600 to 800°C (about 1,100 to 1,470°F), the chemically bound water is driven out of its crystal structure. What remains is an amorphous, highly reactive aluminosilicate.
After calcining, the material is ground to a very fine powder. Metakaolin particles are smaller than portland cement grains but coarser than silica fume, which puts them in a useful middle ground for particle packing.
How Metakaolin Works in Concrete
When portland cement hydrates, it produces two main products: calcium silicate hydrate (C-S-H), the glue that gives concrete its strength, and calcium hydroxide (CH). Calcium hydroxide adds little strength. It is soluble, it can leach to the surface as efflorescence, and it tends to concentrate in the weak zone around aggregate particles.
Metakaolin changes that balance through the pozzolanic reaction. Its reactive silica and alumina combine with calcium hydroxide to form additional C-S-H and calcium aluminate hydrates. The practical result is:
- A denser cement paste with a finer, less connected pore structure
- A stronger bond between the paste and the aggregate
- Less free calcium hydroxide available to leach, react, or weaken the matrix
- Lower permeability to water, chlorides, and sulfates
What the Test Data Shows
Tech Silica distributes R-E-D Dynapoz metakaolin, and its technical data sheet includes third-party test results comparing a control mix to mixes with 10% and 15% metakaolin replacement:
- 28-day compressive strength rose from 5,630 psi (control) to 7,360 psi at 10% and 7,690 psi at 15%.
- 90-day strength reached 8,950 psi at 15% replacement, about 41% above the control.
- Rapid chloride permeability (ASTM C1202) at 28 days dropped from 4,295 coulombs to 1,544 at 10% and 1,065 at 15%.
- Initial and final set times stayed within about 20 minutes of the control.
That combination of higher strength, much lower permeability, and a normal setting profile is why metakaolin shows up in so many demanding specifications.
What Is Metakaolin Used For?
- Bridges, parking structures, and transportation infrastructure exposed to deicing salts
- Marine and underwater structures
- Ultra-high performance concrete (UHPC)
- Precast and architectural concrete where finish and efflorescence control matter
- Projects that call for chemical resistance or a lower carbon footprint
Metakaolin Specifications
Metakaolin is classified as a calcined natural pozzolan. Dynapoz metakaolin meets ASTM C618 and AASHTO M295 Class N requirements, which makes it straightforward to submit on DOT, commercial, and public projects that already reference those standards. If you are comparing options, our guide to metakaolin vs silica fume breaks down the differences.
Frequently Asked Questions
Is metakaolin the same as kaolin?
No. Kaolin is the raw clay. Metakaolin is kaolin that has been calcined at controlled temperatures, which removes bound water and turns it into a reactive pozzolan suitable for concrete.
Is metakaolin a cement replacement?
Yes. Metakaolin is typically used to replace a portion of portland cement by weight. Test data for Dynapoz metakaolin covers 10% and 15% replacement, and final dosage should be confirmed with trial batches.
Is metakaolin a supplementary cementitious material?
Yes. Metakaolin is an SCM, grouped with silica fume, fly ash, slag cement, and ground glass pozzolan. It reacts with calcium hydroxide to form additional strength-building C-S-H.
What ASTM standard covers metakaolin?
Metakaolin falls under ASTM C618 as a Class N calcined natural pozzolan. AASHTO M295 Class N is the equivalent transportation specification.
Where can I buy metakaolin in the United States?
Tech Silica, based in Atlanta, Georgia, supplies Dynapoz metakaolin in 40-lb bags, 2,000-lb bulk bags, and bulk truckloads, with flexible domestic and international shipping.
Get Metakaolin for Your Next Project
Tech Silica has supported concrete producers since 1980. Contact our team for pricing, samples, and the full Dynapoz technical data sheet, or visit our metakaolin product page to learn more.