Metakaolin is an amorphous, alumino-silicate mineral,
manufactured by calcining kaolin at temperatures between
650°C and 850°C. It is classed as a pozzolan, defined
by international standards agency ASTM as a siliceous or
aluminous and siliceous material, which reacts chemically with
calcium hydroxide (lime) at ordinary temperatures, to form
compounds possessing cementitious properties.
Sold in bags as a fine dry powder, the colour of metakaolin
varies from white to orange or pink, depending on the amount of
coloring oxides present, such as iron oxide
(Fe2O3) and titanium dioxide
According to Transparency Market Research, the global
metakaolin market was valued at $91.8m in 2012 and is expected
to reach $124.2m by 2019, expanding at a compound annual growth
rate (CAGR) of 4.4%.
In terms of volume, demand was 250,000 tonnes in 2012. The
amount of metakaolin sold to the concrete market that year
was valued at $48m, accounting for more than 50% of total
Calcination of metakaolin is traditionally performed using
gas-fired rotary kilns, in which the temperature and rate of
throughput are carefully controlled in order to maximise the
amount of amorphous metakaolin produced. Over-calcining, at
temperatures above 900° C, will alter the
material’s mineralogical structure to spinel and
mullite, which are non-pozzolanic.
Flash calcination can also be used, a process which often
forms spherical agglomerates. The spherical structure of
flash-calcined metakaolin can impart additional benefits
compared to traditionally calcined material, including lower
water demand, better workability and higher cement
substitution. According to California, US-based materials
technology firm Demeter Technologies Inc., energy consumption
of flash-calcined metakaolin material is up to two and a half
times less than that of traditional calcining.
High-reactivity metakaolin (HRM) is metakaolin produced from
consistent, refined kaolin, and is distinct from that produced
from low-grade, unrefined kaolin. The latter often includes
unwanted non-pozzolanic minerals such as quartz and feldspar.
Another aspect of HRM is that the sum of aluminium oxide
(Al2O3) and silicon dioxide
(SiO2) in the chemical composition is greater than
The presence of significant potassium oxide (K2O)
or sodium oxide (Na2O) in metakaolin suggests that
mica or feldspar were present in the pre-calcination feed. As a
result, the metakaolin product will contain unreactive
images courtesy of of Dr Denis Bezard of
Newchem AG, Switzerland
|432 Park Avenue, New York City. At 425
metres tall, this is the third
highest building in the US and the tallest residential
building in the
western hemisphere. HRM supplied by German chemicals
BASF was used in the formulation of the 760mm-thick
to achieve a compressive strength of 16,100 psi.
Anthony Quintano, via Flickr
Pozzolans in concrete
The raw materials used to produce cement include limestone
or chalk and shale or clay. After crushing and milling, the
mineral powder is heated in huge rotary calciners to around
1,450°C to form cement clinker, which is then ground back
to powder. It can be mixed with water and poured to set as a
solid mass or used as an ingredient in making mortar or
concrete, which are combinations of cement, water and
Pozzolanic materials containing high proportions of silica
and/or alumina are known as supplementary cementing materials
(SCMs) and include fly ash, granulated ground blast furnace
slag (GGBFS), silica fume, pumice and metakaolin.
SCMs may be added to concrete to reduce the volume of cement
required to make it and to improve its physical properties,
such as strength and durability. This enhanced concrete is
known as high performance concrete (HPC), defined by the
American Concrete Institute as "a concrete which meets
combinations of performance and uniformity requirements that
cannot be achieved routinely using conventional constituents
and normal mixing, placing and curing practices".
Despite being more expensive than conventional concrete, HPC
is economical because it extends service life, guaranteeing
reduced damage through corrosion and lowering overall cost.
When ordinary Portland cement reacts with water it produces
a hard cementitious substance but also releases calcium
hydroxide (Ca(0H)2), an unwanted by-product which
can lead to concrete cancer ** and other problems. High
concentrations of Ca(0H)2 are found at the
interfacial zone – the layer of cement paste directly
adjacent to particles of aggregate. Metakaolin has the effect
of densifying this zone by reacting with Ca(0H)2,
increasing its strength and reducing porosity.
Unlike competitor SCMs, such as fly ash, GGBFS and silica
fume, metakaolin is manufactured for its intended use, with its
chemical composition, brightness, particle size distribution,
chapelle reactivity and other properties tailored to produce a
consistent and effective pozzolan.
The pozzolanic performance of metakaolin and other SCMs can
be measured using procedures outlined by ASTM, in which 7 and
28-day compressive strengths of mortar cubes with a 20% mass
replacement of cement by pozzolan are compared to those of a
control without pozzolan.
Other test procedures include the Chappelle Test, which
measures the ability to absorb calcium hydroxide and the
Fratinni Test, which measures the OH-content (contained in
calcium hydroxide) of concrete mixes after eight days of
When used to replace cement at levels of 5-15% by weight,
the concrete produced is more cohesive and less likely to bleed
(experience water separation), thereby reducing the intensity
of pumping and finishing required. The compressive, tensile and
flexural strength of the hardened concrete is also improved,
increasing stability and reducing deflections in tall
buildings. The cement matrix has lower porosity and
permeability, increasing resistance to corrosion by sulphate
and chloride ions and mineral and organic acids. Freeze-thaw
resistance is also enhanced.
The amount of heat needed to hydrate cement can be reduced
by 50% when metakaolin is used, which is important for very
large structures such as dams where an increase in temperature
during the early setting stages can lead to thermal stress and
cracks. Fly ash and GGBS provide similar benefits, but silica
fume, which is highly reactive, accelerates temperature
Using HPC allows for tall but slender tower blocks to be
built where space is at a premium in city centres. A good
example is 432 Park Avenue in Manhattan, US – a
building distinguished by a height-to-width ratio of 15:1
(anything greater than 7:1 is considered to be slender) and a
low footprint of only 28.5 metres2.
At 425 metres tall, it is the third highest building in the
US (see image) and the tallest residential building in the
western hemisphere. HRM supplied by German chemicals
manufacturer BASF was used in the formulation of the
760mm-thick concrete walls to achieve a compressive strength
of 16,100 psi (ordinary Portland cement concrete is
approximately 4,500 psi).
In some cases, like 432 Park Avenue, the aesthetics of a
structure are important, often requiring a white finish.
Metakaolin manufactured in the US, India, China and elsewhere
from low-iron and titania-feed kaolin can achieve brightness
levels of 80-85 and are the preferred materials for such
There is still a large market for low-brightness metakaolin,
however, where the emphasis is on its technical rather than
aesthetic qualities, such as in concrete used for dams,
aqueducts and bridges.
Another important application is the exploration of oil and
gas deposits, where concrete is used to fill the space between
the borehole walls and the outer steel casing of the drill.
Metakaolin improves the compressive and flexural strength of
the hardened concrete and reduces its permeability to liquids
and gases. Finished oil wells are plugged with concrete to
|Table 1: Chemical Properties of metakaolin
and other SCMs
|*Flash Calcined ** K2O + Na2O = 1.0% ***
CaO + MgO = 0.6%
Source: Published technical data
|Table 2: Physical properties of metakaolin
and other SCMs
Source: Published technical data
According to the US Environmental Protection Agency,
production of Portland cement releases approximately one tonne
of carbon dioxide (CO2) per tonne of cement,
accounting for around 6% of total man made global CO2 emissions
(1.2 tonnes in every 20).
The Swiss Federal Institute of Technology investigated
CO2 emissions from the production of metakaolin and
calculated a typical figure of 270kg per tonne, which included
clay extraction, drying and gas-fired calcination. The use of
biogas, as used by Argeco Development, a metakaolin producer
based in southwest France, is calculated to produce around 92kg
of CO2 tonne.
The LC3 Project, funded by the Swiss Agency for Development
and Cooperation through its global programme for climate
change, is an initiative that aims to develop a new blend of
concrete based on 50% clinker; 30% metakaolin; 15% limestone;
and 5% gypsum.
It is claimed that LC3 can reduce CO2 emissions by up to 30%
and can be made in existing cement plants without the need for
|Table 3: Typical prices of
|Source: First Test
Global metakaolin producers
The US, China and India are the major global producers of
metakaolin. Tables 1 and 2 list some of the major players in
the industry and detail chemical and physical properties for
leading commercial grades of metakaolin compared to other
SCMs (fly ash, silica fume and GGBFS).
Most US production is based in Georgia and Carolina, where
metakaolin is manufactured from bright sedimentary feedstock.
Companies including BASF, French industrial minerals
conglomerate Imerys and US producers Thiele Kaolin Co. and
Burgess Pigment Co. sell high-brightness metakaolin for use in
concrete and other industries such as paints and PVC.
Canadian firm I-Minerals Inc. is developing a new metakaolin
plant in Idaho to produce material exclusively for the
concrete industry. The facility is expected to be on stream
India is home to several metakaolin producers, notably 20
Microns Ltd, English Indian Clays Ltd (EICL) and Ashapura
These companies and a number of others are located close to
India’s west coast, in Kerala or Gujarat and are
logistically well placed to supply the Middle East, where
significant infrastructure and construction investment is
planned in the Gulf Cooperation Council (GCC) countries in
the coming years.
Leading Chinese metakaolin companies include Jinyu Kaolin
Chemical Co., Shanxi Jinyang Calcined Kaolin Co., Beihai Rede
Kaolin Co. and Shijiazhuang Jinli Mineral Co. China is also the
world’s largest producer of fly ash and GGBFS.
Outside these major metakaolin-producing countries, notable
suppliers include Imerys in France and Ukraine; Poraver in
Germany and Canada; Whitemud Resources Inc. in Canada:
Metacaulim do Brasil in Brazil; Plast Rifey LLC in Russia;
and Calix Ltd in Australia.
Outlook for metakaolin demand
According to Ruben Snellings of the Sustainable Materials
Management section of Vlaams Institute of Technology in
Belgium, global cement production in 2015 was 4.2bn tonnes,
corresponding to the use of approximately 800m tonnes of
SCM’s (20% replacement of cement by
Concrete manufacture is expected to grow significantly
between now and 2019 due to an increasing number of civil
engineering projects in both developed and developing
In April 2015, the UAE made it mandatory to use SCMs in
concrete, the first and so far the only country to do so, but
the move is likely to encourage other GCC countries to pass
In the US, more than 50% of all concrete production includes
Fly ash dominates the SCM market, accounting for around 70%
of global volume. GGBFS is the next biggest seller, followed by
much smaller volumes of silica fume and metakaolin.
To meet growing demand for cement and concrete while cutting
CO2 emissions, cement producers will need to
investigate the use of more SCMs without compromising
performance and durability. GGBFS supply is already limited and
an increase in metals recycling is likely to lead to further
reductions, while fly ash supply is not likely to expand at the
pace of concrete demand.
This opens up opportunities for metakaolin and other
materials such as pumice in concrete markets.
But for consumption of these materials to grow, suppliers
need to convince architects, ready-mix concrete manufacturers
and other decision makers in the construction sector that
metakaolin is a beneficial option.
Research into further improvements for concrete includes the
development of ultra-high performance concrete (UHPC) and
geopolymers, both of which benefit from the inclusion of
Main benefits of adding
metakaolin to concrete
- Increased compressive, tensile and flexural
- Reduced permeability
- Reduced sulphate and chloride corrosion
- Reduced freeze-thaw damage
- Prevention of alkali silica reaction
- Prevention of efflorescence
- Reduced CO2 footprint
Structural uses of metakaolin
Jupia Dam, Brazil (1962)
This was the first use of metakaolin in concrete. The primary
purpose was to suppress alkali silica reaction (sometimes
called concrete cancer).
Brayton Point Cooling Towers, Massachusetts, US
Metakaolin was used in the concrete of two cooling towers in
a coal-burning power plant adjacent to the sea, providing
reduced permeability, improved chloride resistance and
increased durability and strength.
Palais Royale, Mumbai, India (2013)
One of the first projects in India where metakaolin was used
in HPC, the Palais Royale is approximately 300 metres
Bacalan Bridge, Bordeaux, France (2013)
"Argical" metakaolin supplied by Imerys was used in the
construction of concrete pillars which form the vertical lift
structure for the $195m bridge, providing improved strength
and aesthetic colouring.
Dr Denis Bezard, Newchem AG
Scientific World Journal: "Effects of Different Mineral
Admixtures on the Properties of Fresh Concrete", by Sadaqat
Ullah Khan, Muhammad Fadhil Nuruddin, Tehmina Ayub and Nasir
Dezeen architecture and design magazine
The International Journal of Current Engineering &
The Concrete Centre "Tall Buildings"
The 2012 NRMCA Supplementary Cementitious Materials Use
Ruben Snellings, Vlaams Institute of Technology,
*Frank Hart is the owner and director of First Test
**Concrete cancer is a term given to the reaction which
occurs over time in concrete between the highly alkaline cement
paste and the reactive non-crystalline (amorphous) silica found
in many common aggregates