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 the faster and greater the development of strength. The fineness of grinding has increased over the years,
But now it has got nearly stabilised. Different cements are ground to different fineness. The disadvantage of fine grinding is that it is susceptible to air-set and carly deterioration. Maximum number of particles in a sample of cement should have a size less than about 100 microns. The smallest particle may have a size of about 1.5 microns. By and large an average size of the cement particles may be taken as about 10 micron. The particle size fraction below 3 microns has been found to have the predominant effect on the strength at one day while the 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. In commercial cement it is suggested that there should be about 25-30 percent of particles should be of less than 7 micron in size.
