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Frequently Asked Questions
Cement & Concrete
Technology Home > FAQs > Ettringite,
DEF, and Concrete Durability
Q: What is ettringite and does it or the sulfate
in cement contribute to expansion and disintegration of portland
cement concrete?
A: Ettringite,
calcium sulfoaluminate, is found in all portland cement concretes
and is commonly referenced in petrographic reports. Calcium sulfate
sources, such as gypsum, are added to portland cement to prevent
rapid setting and improve strength development. Sulfate is also
present in supplementary cementitious materials and admixtures.
Gypsum and other sulfate compounds react with calcium aluminate
in the cement to form ettringite within the first few hours after
mixing with water. Essentially all of the sulfur in the cement is
normally consumed to form ettringite within 24 hours.
The formation of ettringite results in a volume increase in the
fresh, plastic concrete. Due to the concrete’s plastic condition,
this expansion is harmless and unnoticed. If concrete is exposed
to water for long periods of time (many years), the ettringite can
slowly dissolve and reform in less confined locations. Upon microscopic
examination, harmless white needle-like crystals of ettringite can
be observed lining air voids. 
Any form of attack or disintegration of concrete by freeze-thaw
action, alkali-silica reactivity (ASR), or other means, accelerates
the rate at which ettringite leaves its original location in the
paste to go into solution and recrystallizes in larger spaces such
as voids or cracks. Both water and space must be present for the
crystals to form. The space is often provided by cracks that form
due to damage caused by frost action, ASR, drying shrinkage, or
other mechanisms. Ettringite crystals in air voids and cracks are
typically two to four micrometers in cross section and 20 to 30
micrometers long. Under conditions of extreme deterioration, the
white ettringite crystals appear to completely fill voids or cracks.
However, ettringite, found in its preferred state as large needle-like
crystals, should not be interpreted as causing the expansion of
deteriorating concrete.
Another term used in petrographic reports is Delayed Ettringite
Formation (DEF). This refers to a condition usually associated
with heat-treated concrete. Certain concretes of particular chemical
makeup which have been exposed to temperatures over about 70°C
(158°F) during curing can undergo expansion and cracking caused
by later ettringite formation. This can occur because the high temperature
decomposes any initial ettringite formed and holds the sulfate and
alumina tightly in the calcium silicate hydrate (C-S-H) gel of the
cement paste. The normal formation of ettringite is thus impeded.
 In
the presence of moisture, sulfate and alumina desorb from the confines
of the C-S-H to form ettringite in cooled and hardened concrete.
After months or years of desorption, ettringite forms in confined
locations within the paste. Since the concrete is rigid and if there
are insufficient voids to accommodate the ettringite volume increase,
expansion and cracks can occur. In addition, some of the initial
ettringite formed before heating may be converted to monosulfoaluminate
at high temperatures and upon cooling, revert back to ettringite.
Because ettringite takes up more space than monosulfoaluminate from
which it forms, the transformation is an expansive reaction.
Only extreme cases of DEF result in cracking, and often DEF is
associated with other deterioration mechanisms. Air voids can help
relieve the stress by providing a location for the delayed ettringite
to form. Finally, some petrographers or concrete technologists use
the term “secondary ettringite” to refer to both DEF
and harmless ettringite found lining voids (often listed under secondary
deposits in petrographic reports).
References
1. PCA, Ettringite
Formation and the Performance of Concrete, IS417, Portland
Cement Association, 2001.
2. Lerch, William, Effect
of SO3 Content of Cement on Durability of Concrete,
R&D Serial No. 0285, Portland Cement Association, 1945.
3. Day, Robert L., The
Effect of Secondary Ettringite Formation on the Durability of Concrete:
A Literature Analysis,
RD108, Portland Cement Association, 1992.
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