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The degree of fineness of cement is the measure of the mean size of the grains in it. There are
three methods for testing fineness: the sieve method—using 90 micron (9 No.) sieve, the air
permeability method— Nurse and Blains method and the sedimentation method— Wagner
turbidimeter method. The last two methods measure the surface area, whereas the first measures
grain size. Since cement grains are finer than 90 micron, the sieve analysis method does not
represent true mean size of cement grains. Also, the tiny cement grains tend to conglomerate
into lumps resulting in distortion in the final grain size distribution curves. Considering these
demerits, fineness is generally expressed in terms of specific area, which is the total surface
area of the particles in unit weight of material.
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X The chemical composition and the degree of calcination influence
the hardness of the clinker and consequently the fineness to which the cement is ground.
Clinker, high in iron or silica, is apt to be hard and difficult to grind. The same is true with a
hard-burned clinker. Fineness is also influenced by the time of grinding and the character of the
pulverizing machinery. It has been found that cement becomes finer with age provided it does
not absorb too much moisture. This is probably due to the decrepitation of the coarser grains
resulting from the hydration of the embedded lime particles.
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X Finer the cement, more is the strength since surface area for hydration will be
large. With increase in fineness, the early development of strength is enhanced but the ultimate
strength is not affected. An increase in the fineness of the cement increases the cohesiveness of
the concrete mix and thus reduces the amount of water which separates to the top of a lift
(bleeding), particularly while compacting with vibrators. However, if the cement is ground
beyond a certain limit, its cementative properties are affected due to the prehydration by
atmospheric moisture. Finer cement reacts more strongly in alkali reactive aggregate. Also, the
water requirement and workability will be more leading to higher drying shrinkage and
cracking.
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: 100 g of cement sample is taken and air-set lumps, if any, in the sample are
broken with fingers. The sample is placed on a 90 micron sieve and continuously sieved for 15
minutes. The residue should not exceed the limits specified below:
Type of cement Percentage of residue Specific surface (m^2 /kg)
by weight not less than- Ordinary Portland Cement (OPC) 10 225
- Rapid Hardening Cement (RHC) 5 325
- Portland Puzzolana Cement (PPC) 5 300
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X The fineness of cement is represented by specific surface, i.e. total
surface area in cm^2 per gram or m^2 per kilogram of cement and is measured by Lea and Nurse
apparatus or by wagner turbidimeter..
The Lea and Nurse apparatus shown in Fig. 5.8 essentially consists of a permeability test
cell—where cement is placed and air pressure is applied, flowmeter—to determine the quantity
of air passing per second through its capillary tube per unit difference of pressure, and
manometer—to measure the air pressure.