Reground samples of natural cement gain strength more rapidly than resifted samples, but eventually the strength attained is about the same. In Portland cements regrinding seems to be of greater value than resifting. A sample of natural cement made from coarse particles reground gains strength rapidly, and for mortars with small proportions of sand, gives good results. The fact that such samples do not give a high strength with large proportions of sand is doubtless due to the fact that the grinding is not thorough, and the indications are that the material of which such coarse particles are composed would form a valuable part of the cement if ground fine enough.
The coarse particles of either natural or Portland cement may be replaced by grains of sand of the same size without materially affecting the strength attained by neat and one-to-one mortars, but for mortars containing larger proportions of sand, such a substitution results in a decreased strength.
95. Finally, it may be said that the process of manufacture and the character of the materials from which cement is made have such an influence on the relative proportions of fine and coarse particles that the percentage of finest particles cannot be determined by testing with a coarse sieve. While it is not known at what point of fineness grains of cement begin to have cementitious value, or what proportion of the cement should be the finest flocculent matter, it is certain that a cement should leave as small a percentage as possible on a sieve having holes .004 inch square, in order to have the greatest sand carrying capacity.
There is, however, a reason for using a comparatively coarse sieve in connection with the fine one. Overburned lime, which is likely to occur in Portland cements, is more dangerous in the form of coarse particles than an equal quantity in a fine condition, because coarse particles slake more slowly and it is better that expansion should occur early in the process of hardening if it is to occur at all. For the same reason a cement that would be unsound normally may be rendered less dangerous by regrinding.
As fine grinding is expensive, it is only a question as to when the increased strength obtained is offset by the extra expense incurred in grinding. There is now little trouble in obtaining either natural or Portland cement of which from 70 to 85 per cent, will pass holes .004 inch square. (See § 77).