Answer / sjtbehera

It is very important that the cement after setting shall
not undergo any appreciable change of volume. Certain
cements have been found to undergo a large expansion after
setting causing disruption of the set and hardened mass.
This will cause serious difficulties for the durability of
structures when such cement is used. The testing of
soundness of cement, to ensure that the cement does not
show any appreciable subsequent expansion is of prime
importance.


The unsoundness in cement is due to the presence of excess
of lime than that could be combined with acidic oxide at
the kiln. This is also due to inadequate burning or
insufficiency in fineness of grinding or thorough mixing of
raw materials. It is also likely that too high a proportion
of magnesium content or calcium sulphate content may cause
unsoundness in cement. For this reason the magnesia content
allowed in cement is limited to 6 per cent. It can be
recalled that, to prevent flash set, calcium sulphate is
added to the clinker while grinding. The quantity of gypsum
added will vary from 3 to 5 per cent depending upon C3A
content. If the addition of gypsum is more than that could
be combined with C3A, excess of gypsum will remain in the
cement in free state. This excess of gypsum leads to an
expansion and consequent disruption of the set cement
paste. Unsoundness in cement is due to excess of lime,
excess of magnesia or excessive proportion of sulphates.
Unsoundness in cement does not come to surface for a
considarable period of time. Therefore, accelerated tests
are required to detect it. There are number of such tests
in common use. The appartus is shown in figure. It consists
of a small split cylinder of spring brass or other suitable
metal. It is 30 mm in diameter and 30 mm high. On either
side of the split are attached two indicator arms 165 mm
long with pointed ends. Cement is gauged with 0.78 times
the water required for standard consistency (0.78 P), in a
standard manner and filled into the mould kept on a glass
plate. The mould is covered on the top with another glass
plate. The whole assembly is immersed in water at a
temperature of 27°C – 32°C and kept there for 24 hours.


Measure the distance between the indicator points. Submerge
the mould again in water. Heat the water and bring to
boiling point in about 25-30 minutes and keep it boiling
for 3 hours. Remove the mould from the water, allow it to
cool and measure the distance between the indicator points.
The difference between these two measurements represents
the expansion of cement. This must not exceed 10 mm for
ordinary, rapid hardening and low heat Portland cements. If
in case the expansion is more than 10 mm as tested above,
the cement is said to be unsound.

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