| Concrete Experts International has extensive, world-wide experience with
deteriorated concrete suffering
from sulfate attack caused by water containing sulfate. Diagnosing
external sulfate attack is an integrated part of our
petrographic analysis of concrete.
What is External
Sulfate Attack?
External sulfate attack
is a chemical breakdown mechanism where sulfate ions from an external
source attack components of the cement paste. Such attack can occur when
concrete is in contact with sulfate containing water e.g. seawater,
swamp water, ground water or sewage water. The often massive formation
of gypsum and ettringite formed during the external sulfate attack may
cause concrete to crack and scale. However, both laboratory studies and
examinations of field concrete show that external sulfate attack is
often manifested, not by expansion or cracking, but by loss of cohesion
and strength.
Microscopic appearance
The microscopic
appearance of concrete suffering from external sulfate attack appears to
be quite variable. Some diagnostic features such as
are, however, often found
associated with external sulfate attack.
The above-mentioned
features are usually most pronounced near the attacked surface.
Sometimes external sulfate attack causes the paste to expand and gaps
around aggregates are formed. All the features do not necessarily have
to be observed to diagnose an external sulfate attack. The features
present depend on many factors such as the quality of the concrete
(including w/c and cement type), exposure time, temperature,
concentration, and chemistry of the ambient water.
There is a general
agreement that concrete suffering from external sulfate attack develops
a more and less pronounced mineralogical and chemical zoning which can
be studied in the optical
fluorescence microscope and the scanning electron microscope.
ASTM C856 recommends
chemical analysis to verify that the sulfate content of the concrete has
been increased over that expected from the concrete constituents in
order to diagnose external sulfate attack. Please
to not hesitate to contact CXI if you have some problems regarding
external sulfate attack or any other deterioration mechanisms.
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