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Soundness
It is essential that the cement paste, once it has set, does not
undergo a large change in volume.
One restriction is that there must be no appreciable expansion,
which under conditions of
restraint could result in disruption of the hardened cement
paste. Such expansion may occur due
to reactions of free lime, magnesia and calcium sulfate, and
cements exhibiting this type of
expansion are classified as unsound. Free lime is present in the
clinker and is intercrystallized
with other compounds; consequently, it hydrates very slowly
occupying a larger volume than the
original free calcium oxide. Free lime cannot be determined by
chemical analysis of cement
because it is not possible to distinguish between unreacted CaO
and Ca(OH)2 produced by a
partial hydration of the silicates when the cement is exposed to
the atmosphere. Magnesia reacts
with water in a manner similar to CaO, but only the crystalline
form is deleteriously reactive so
that unsoundness occurs. Calcium sulfate is the third compound
liable to cause expansion
through the formation of calcium sulfoaluminate (ettringite)
from excess gypsum (not used up by
C3A during setting). Le Chatelier's accelerated test is
prescribed by BS EN 196-3: 1995 for
detecting unsoundness due to free lime only. Essentially, the
test is as follows. Cement paste of
standard consistence is stored in water for 24 hours. The
expansion is determined after increasing
the temperature and boiling for 1 hour, followed by cooling to
the original temperature. If the
expansion exceeds a specified value, a further test is made
after the cement has been spread and
aerated for 7 days. At the end of this period, lime may have
hydrated or carbonated, so that asecond expansion test should fall
within 50 per cent of the original specified value. Cement
which fails to satisfy at least one of these tests should not be
used. In practice, unsoundness due
to free lime is very rare. Magnesia is rarely present in large
quantities in the raw materials used
for making cement in the UK, but in the US this is not the case.
For this reason, ASTM C 151-05
specifies the autoclave test which is sensitive to both free
magnesia and free lime. Here, a neat
cement paste specimen of known length is cured in humid air for
24 hours and then heated by
highpressure steam (2 MPa (295 psi)) for about 1 hour so that a
temperature of 216 degrees
celcius ( 420 F) is attained. After maintaining that temperature
and pressure for a further 3
hours, the autoclave is cooled so that the pressure falls within
1.5 hours and the specimen is
cooled in water to 23degrees celcius (73 F) in 15 min. After a
further 15 min, the length of the
specimen is measured: the expansion due to autoclaving must not
exceed 0.8 per cent of the
original length. This accelerated test gives no more than a
broad indication of the risk of long-
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term expansion in practice. No test is available for the
detection of unsoundness due to an excess
of calcium sulfate, but its content can be easily determined by
chemical analysis.
Reference: Concrete technology 2nd
edition, A.M. Neville
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Unsoundness:
Unsoundness in cement paste results from excessive volume change
after setting. If there is any
appreciable expansion, however slow, cracking and failure of the
concrete will result.
Unsoundness in cement is caused by the slow hydration of MgO or
free lime, and by the
reactions of gypsum with C3A (see Section 3.1). These expansive
reactions take place very
slowly, and so unsoundness will only appear after many months,
or even years. Therefore, it is
necessary to use some form of accelerated test, so that
tendencies toward unsoundness can be
detected as a quality control measure. Although a number of
soundness tests have been
developed over the years, only one is in common use. The
autoclave expansion test (ASTM C
151) will detect unsoundness due to both excess CaO and excess
MgO, and is the test designated
in ASTM C 150. A cement of normal consistency is molded and
cured normally for 24 h. The
specimens are then removed from the molds, measured, and placed
in an autoclave. The
temperature is raised so that the steam pressure in the
autoclave reaches 2 MPa (295lb/in.2) in 45
to 75 min; this pressure is maintained for 3 h, and the
autoclave cooled so that the pressure is
relieved in 1% h. The specimen is then cooled in water, to 23C
in 15 min. Its length is again
measured after a further 15 min; the expansion must be less than
0.80% if the cement is to be
acceptable. These accelerated tests clearly do not simulate
service conditions, and the amount of
expansion may be affected by a number of factors, such as the
fineness of the cement and the
C3A content. Thus, these tests can only serve as a guide and
cannot provide an absolute
indication of a tendency toward unsoundness for given cement.
However, they provide a
valuable quality control measure, since changes in the expansion
tests would indicate some
anomaly in the cement manufacturing process.
Reference: Mindess
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