The fineness of cement is a measure of the size of particles of cement and is expressed in terms of specific surface area of cement. Fineness of cement is measured as the % weight retained on 90µm IS sieve over the total weight of sample.
In laboratory, fineness of cement is tested by sieving and by determination of specific surface. For ordinary cement, the weight of residue left on the sieve shall not exceed 10%.
Fineness of cement is the significant factor affecting rate of hydration, rate of gain of strength, setting time and rate of evolution of heat.
Rate of strength development increases with fineness. Since specific surface area is inversely proportional to the size of particle, the surface area is more for finer cement than for a coarser cement. Therefore, finer the cement, higher the surface area for hydration and hence faster the development of strength.
Smaller cement particles react much quicker than the larger particles. A cement particle with diameter 1 µm will react entirely in 1 day, whereas the particle with diameter 10µm takes about 1 month. It is found that the particle size fraction below 3 microns affect one day strength and 3 to 25 micron fraction influence the 28 days strength.
Setting time decreases with increase in fineness of cement. Too much of smaller particles in cement results in quick setting, thereby leaving no time for mixing, handling and placing. Also, such cements are susceptible to airset and early deterioration. To increase the setting time of cement, it must be manufactured in a different range of particle sizes.