The
fineness of cement has an important bearing on the rate of hydration and hence
on the rate of gain of strength and also on the rate of evolution of heat.
Finer cement offers a greater surface area for hydration and hence faster the development
of strength, (Figure). The fineness of grinding has increased over the years.
But now it has got nearly stabilized. Different cements are ground to different
fineness. The particle size fraction below 3 microns has been found to have the
predominant effect on the strength at one day while 3-25 micron fraction has a
major influence on the 28 days strength. Increase in fineness of cement is also
found to increase the drying shrinkage of concrete.
AIM:
To determine the fineness
of cement.
Apparatus:
· Sieve IS-90 microns,
· Balance,
· Brush, etc.
Procedure:
1.
Fit the tray under the sieve, weigh
approximately 10 g of cement to the nearest 0.01 g and place it on the sieve,
being careful to avoid loss. Fit the lid over the sieve. Agitate the sieve by
swirling, planetary and linear movement until no more fine material passes
through it.
2.
Remove and weigh the residue. Express its
mass as a percentage, R1, of the quantity first placed in the sieve to the
nearest 0.1 percent. Gently brush all the fine material off the base of the sieve
into the tray.
3.
Repeat the whole procedure using a fresh 10 g
sample to obtain R2. Then calculate the residue of the cement R as the mean of
R1, and R2, as a percentage, expressed to the nearest 0.1 percent.
4.
When the results differ by more than 1
percent absolute, carry out a third sieving and calculate the mean of the three
values.
Percentage
of fineness (%)= W1-W2/W1 X 100
Where,
W1= Total weight of cement sample
W2= Weight of residue
Conclusion: The
fineness of a given sample of cement is _______ %.
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