A Primer
on Pozzolans
Portland cement consists primarily of calcium silicates.
When mixed with water, a chemical reaction known as
hydration creates calcium silicate hydrate (CSH). Needle
like crystals of CSH interlock to hold concrete together,
almost like a glue. Hydration also produces calcium
hydroxide, also known as lime. In fact, approximately
15 percent to 25 percent of hydrated portland cement
is lime. Lime does not contribute to the strength of
concrete and can even be detrimental to a project.
For example:
• Lime migrates out of concrete to form efflorescence
on the concrete surface.
• When lime leaches out of concrete, it creates
voids that increase the porosity of concrete, allowing
moisture penetration and corrosion.
• Lime is highly alkaline and contributes to
alkali silica reaction (ASR) that can crack concrete.
Pozzolans are supplementary cementitious admixtures
that react with lime to form more CSH. The additional
CSH increases the cementitious bonds in the concrete
to make it stronger. This also makes concrete denser
and less porous so concrete is more resistant to penetration
by water and chemicals. And because pozzolans consume
lime, there is less efflorescence and ASR. Highly reactive
pozzolans like metakaolin outperform other pozzolans
and can produce high-performance mixtures with higher
strength and lower porosity than could otherwise be
achieved.
In this simulated microscopic view of cement paste, hydration begins
when Portland cement particles are surrounded by water.
The Portland cement begins to dissolve in the water, forming a gel.
Crystals begin to form as a result of chemical reactions.
Inside the concrete, the highly alkaline lime can react with certain
types of aggregate to form alkali silica reaction (ASR). ASIR expands
in the presence of water, creating forces that can crack concrete.
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Calcium silicate hydrate (CSH) shown here in white, creates interlocking
crystals that give concrete its strength. However, as much as 25
percent of the products of hydration become calcium hydroxide (lime),
shown in green, and does not contribute to the strength of concrete.
Voids are formed by excess water in a mixture.
The voids create pores that allow water to penetrate concrete. Since
lime is soluble, it dissolves to create even larger pores and allow
more liquid and gases to penetrate into the concrete where they can
accelerate corrosion. The lime can also be deposited on the concrete
surface where it forms efflorescence and disfigures concrete.
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As an alternative to ordinary concrete mixtures, high reactivity metakaolin
can be used to replace 5 percent to 20 percent of the Portland cement
in a mixture. The small size of the metakaolin particles fills in
the gaps between cement particles to create a denser and creamier
cement paste.
As a pozzolan, metakaolin reacts with lime formed during cement hydration
to form additional CSH. Because there is less lime, the potential for
efflorescence and ASR are reduced.
The additional CSH formed by metakaolin results in a denser concrete
that is stronger and has greater durability due to lower porosity.
Plus, the white color of metakaolin brightens the concrete.
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Images © Engeelhard Corporation
A unique animation explaining how pozzolans improve
concrete is available at www.engelhard.com as
part of an online continuing education program. |
