Before using Portland cement for important work, the undermentioned points should be inquired into: -
The cement should be ground to a fine powder.
This can be roughly tested by rubbing it between the fingers, or, accurately, by passing it through a sieve with meshes of known size.
With regard to the exact degree of fineness that is advantageous, there is some difference of opinion.
There seems, however, to be no doubt that properly burnt cement, when ground extremely fine, is, as compared with one coarsely ground, much stronger when used with sand, and also safer, for there are none of the coarse particles which exist in well burnt and coarsely ground cements, especially when they have any tendency to excess of lime.
A heavy well burnt cement is difficult to grind properly, and it will often contain a considerable proportion of coarse particles which ought to be separated and reground.
The experiments by Messrs Grant, Colson, Mann, and others show that when used neat (i.e. without admixture of sand) a coarse-grained cement is stronger than one finely ground.
When mixed with sand, however, as it generally is in actual use, the finely ground cement makes stronger mortar than the other, the difference in its favour being greater as the proportion of sand in the mortar is greater.
A lightly burnt cement is easily ground fine, and "at 7 or even 28 days may appear to be superior to heavy, which is with difficulty ground as fine as the lightly burnt, but in the long run the heavy, if not too coarsely ground, will surpass the lightly burnt, and if the heavily burnt were as finely ground as the light it would be a great deal stronger from the beginning, the time of setting being of course the same. Fine cement, as it takes more sand, goes farther than coarse, it is also much safer when it verges on the blowing point from excess of lime."l
1 Dent's Cantor Lectures.
Grinding is better than sifting, "Heavy clinker ground fine will when tested give higher results than lighter cement of equal fineness obtained by sifting."1
Mr. Mann, in his experiments on the adhesive strength of cement, found that with cement sifted through a sieve having 176 meshes to the lineal inch, - i.e. 30,976 to the square inch, the so-called "coarse" grains stopped even by this fine mesh influenced the cement as follows : -
Adhesive strength after 7 days; in lbs. per sq. inch.
The fine particles only..
Ditto with 25 per cent of the coarse grains
Ditto with 75 per cent of coarse grains ..
Coarse grains only ....
Mr. Mann, says that this fine sieve was found to "afford more definite and reliable results than those having larger meshes."
The experiments of the late Mr. Grant, Mr. Mann, and others, have shown that the larger grains in a coarsely ground cement, besides being in many cases a source of danger, are almost useless, sometimes quite useless, as cement, being more or less inert, so that even if safe they play no other part than that of additional sand.
Mr. Grant found that "coarser cement than would pass through the sieve of 2580 to the square inch, was at least no better than sand, and that when it contained free lime it was a source of weakness if not of danger." 2
There is no general consensus of opinions or practice among engineers with regard to the degree of fineness which it is best actually to require in specifications for Portland cement.
At first the cement used for the Metropolitan Main Drainage Works was specified to be ground "extremely fine," but the exact size of mesh it should pass is not fully determined.
In 1876 Mr. Mann said, "l-50th inch square (2500 holes per square inch) is as fine a mesh as can be conveniently used in practice, smaller ones clogging very easily; on the other hand, cement reduced to this fineness has a very appreciable superiority with sand, as compared with even slightly coarser samples." 3
In the second series of very elaborate and useful experiments made by the late Mr. Grant, the resident engineer of the Metropolitan Main Drainage Works, the cement tested had all been passed through a sieve of only 400 holes to the square inch, the weight of the sifted cement being 110.56 lbs. per bushel.
1 Grant, M.P.I.C.E., vol. "lxii. p. 102.
2 M.P.I.C.E., vol. lxii. p. 243.
3 Captain Innes, R.E., R.E. Corps Papers, vol. xxi. p. 4.
Of late years, however, manufacturers in Germany and Austria have introduced cements ground to a much greater degree of fineness.
Cements are easily procurable which will entirely pass sieves of 2580 meshes to the inch (400 to the square centimetre), leaving only 10 per cent on sieves of 5806 meshes to the inch (900 to the square centimetre), and it has been stated that cement can be procured of which only 3 to 10 per cent is rejected by a sieve of 32,000 meshes to the inch.1
These German cements are not much, if at all, used in engineering works in England, but they are used by the manufacturers of patent cement paving and similar materials. •
With regard to ordinary English cements, Mr. Mann found, that of the cement received from nine makers, from 38 to 50 (average 45.6) per cent was stopped by a sieve with 30,976 meshes per square inch, and with eight varieties Mr. Grant found the residue to average 49.5 per cent.
Experiments made by a friend of the writer's showed that a cement which left 10 per cent core on a 2500 mesh sieve, left 20 per cent on the 30,976 mesh sieve. A cement which left only 10 per cent core on the 30,976 mesh sieve cost about 4s. a ton more for grinding than the other. It appears therefore that to obtain 10 per cent more cement cost 4s., and that the extra grinding is not economical, except for cements, which cost more than 40s. a ton delivered on the works, as they do sometimes abroad.
Where fineness of grit is alluded to in specifications, as it always should be, 2500 meshes to the square inch is frequently specified, though the Metropolitan Board of Works and a few engineers specify that not more than 10 per cent by weight shall be rejected by a sieve of 5800 meshes to the square inch, and there seems no doubt that this requirement, which is estimated to add only 1/10 to the cost of the cement, is a very desirable one to enforce until still finer grinding can be obtained.
Great care must be taken, however, that finely ground cement is not lightly burnt, to prevent which the weight, or better still (see p. 167) the specific gravity, of the cement should be specified too.
Where cement is to be sent abroad, and thus rendered expensive by the addition of freight, or it seems especially desirable to have a material which gives the maximum strength with the minimum bulk.
The table (page 169) shows the degree of fineness and other particulars specified by various public departments and on various works.