69. Importance Of Fineness

The fineness of cement is always conceded to be one of its most important qualities, and the determination of fineness is omitted in none but the very crudest tests. Unfortunately, however, sieves that are so coarse as to give delusive results are usually employed. It is very easy to show that grains of cement as large as one-fiftieth of an inch in diameter are practically valueless, but much more difficult to determine the point of fineness at which the particles begin to have cementitious value.

70. A moderately coarse sieve is easier to operate than a very fine one, less time being consumed in sifting. The impression seems to be quite general also that there is a fixed relation between the proportions of the different sized grains in different samples. Many specifications require that a certain percentage "shall pass a sieve having 2,500 holes per square inch." Now, there is little doubt that grains of cement larger than .005 inch in one dimension have very little cementitious value, and hence a cement, all of which would pass holes .015 inch square, while but 50 per cent, of it would pass holes .005 inch square, is little better than one which leaves a larger residue on the coarser sieve but the same residue on the finer.

In America and Germany it is the usual practice in the process of manufacture to pass the cement through a screen which will reject particles larger than about .015 inch in diameter; the futility in attempting to determine, with a sieve no finer than this, the proportion of the particles which are fine enough to be of value, is therefore apparent. Since the English cement makers have not been so progressive in the practice of screening, they have obtained the reputation of producing a coarse product. In many cases this reputation is probably a just one, but when tested with a very fine meshed sieve, some of the English cements do not compare so unfavorably with those of German manufacture. It is a curious fact in this connection that the English are the most conservative in holding to the use of the coarse sieve in testing, which makes their cement appear so very much coarser than the American or German product.

71. Sieves

Sieves for cement testing may be made either of wire or silk gauze, set in metal or wood frames. Sieves of perforated metal plate are sometimes employed for sifting sand, but seldom for cement. It is with considerable difficulty that accurate gauze sieves are obtained. They are usually designated by numbers corresponding to the number of meshes per linear inch; this is in some respects an unsatisfactory method, for the size of the wire, which is quite as important as the number of meshes, is frequently not given at all, or stated in terms of some wire gage which is capable of various interpretations.

As usually supplied by different manufacturers, sieves purporting to have the same number of meshes per linear inch may vary in this regard as much as 10 or 15 per cent. Likewise the size of wire used by different makers, in sieves having the same number of meshes per inch, may vary quite as much. Again, on account of irregularities in the gauze, the holes in a given sieve vary one from another; in some cases an opening may be but 60 or 70 per cent, as large in one dimension as an adjacent one.

An ideal sieve should conform to the following requirements: (1) holes to be of uniform size and shape throughout, (2) sides of the holes to be very smooth, and (3) the spaces between the holes to be of such size and shape that particles will not easily rest there.

It is evident that the largest holes determine the character of the sieve. For example, a sieve having half its holes 0.01 inch square and the other half 0.02 inch square, would, if used long enough, separate the cement exactly as it would if all the holes had been 0.02 inch square. Hence, if a very small percentage of the holes are larger than the normal, it seriously impairs the accuracy of the sieve by introducing an indeter-mination; but holes smaller than the normal have no greater objection than that, as the sifting proceeds, they become spaces between the real or larger holes, and as such do not fulfill the third requirement mentioned above. The shape of the holes, whether round, square or hexagonal, seems of minor importance so long as uniformity is maintained. The second requirement is necessary, because, should particles adhere to the sides of the hole, the size of the latter would be decreased to that extent. The third requirement is for convenience, but would require consideration if the style of the sieve were changed to a punched metal plate.

72. The Committee Of The American Society Of CIVIL Engineers

The Committee Of The American Society Of CIVIL Engineers, in their report on " A Uniform System for Tests of Cement" in 1885, recommended three sizes of sieves for cement: No. 50 (2,500 meshes to the square inch) wire to be of No. 35 Stubbs' wire gage; No. 74 (5,476 meshes to the square inch) wire to be of No. 37 Stubbs' wire gage; No. 100 (10,000 meshes to the square inch) wire to be of No. 40 Stubbs' wire gage. For sand, two sieves were recommended, No. 20 and No. 30 (400 and 900 meshes per square inch) wire to be of No. 28 and No. 31 Stubbs' wire gage, respectively. It seems to be impracticable to comply with these sizes of wires, because neither manufacturers nor engineers appear to agree as to what diameters of wire correspond to No. 37 and No. 40 Stubbs' wire gage.

Table 5. Sieves. Number Of Meshes Per Linear Inch And Sizes Of Openings, As Found By Measurement

73. The conferences of Dresden and Munich decided that fineness should be determined by sieves of 900 and 4,900 meshes per sq. cm., respectively, for Portland cement, and 900 and 2,500, respectively, for other hydraulic products, the size of the wires being as follows: for 4,900, .05 mm.; for 2,500, .07 mm.; and for 900, .10 mm. These sieves would have respectively 31,600 (178 X 178), 16,000 (127 X 127), and 5,800 (76 x 76) meshes per square inch, and the sizes of the holes would be approximately .0037 inch square, .005 inch square and .009 inch square, respectively. It was also decided that for sifting sand, punched metal plates were preferable to wire cloth sieves.