Cement: Soundness Test
It is essential that the cement
concrete does not undergo large change in volume after setting. This is ensured
by limiting the quantities of free lime and magnesia which slake slowly causing
change in volume of cement (known as unsound). Soundness of cement may be
tested by Le-Chatelier method or by autoclave method. For OPC, RHC, LHC and PPC
it is limited to 10 mm, whereas for HAC and SSC it should not exceed 5 mm.
Importance: It is a very
important test to assure the quality of cement since an unsound cement produces
cracks, distortion and disintegration, ultimately leading to failure.
Conditions Affecting Soundness:
The main cause for unsoundness in Portland cement is the hydration of the
uncombined lime encased within the cement particles. Exposed, finely ground,
free lime in small percentages, hydrates before the cement sets and produces no
injurious effect. The uncombined lime in cement is a result of either
underburning the clinker or of excess lime in the raw materials. Freshly ground
cement is often unsound due to the presence of uncombined lime. Cement is thus
allowed to aerate for two to three weeks, allowing the lime to hydrate, to
overcome unsoundness.
Fine grinding of the raw material
and clinker help to produce a sound cement. By grinding fine the raw materials,
it is possible to produce a homogeneous mixture before burning where the lime
is uniformly distributed. The coarse grains of cement may imprison minute
particles of uncombined lime which do not hydrate. These lime particles on
hydralion produce disintegration.
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