247. Various kinds of rock are capable of producing sand of good quality. The natural sands are usually siliceous in character, but calcareous sands are also met with and may give excellent results in mortar. Good artificial sand may be made from almost any kind of rock that is not liable to chemical decay, even though it be only moderately hard. One of the most essential features of a good sand is that the grains should be perfectly sound. Evidences that chemical decay is going on in the grains would indicate that the sand is of very inferior quality.

248. Shape And Hardness Of The Grains

It is generally believed that the grains of sand should be angular in order to give the best results; this is probably true, although in testing three varieties of calcareous sand, M. Paul Alexandre 1 obtained results which seemed to indicate that if rounded grains are disadvantageous, the other properties of the sand may readily counterbalance this disadvantage.

M. Alexandre used three sands which were reduced to the same fineness by sifting into different sizes and then remixing them in fixed proportions (equal parts of five sizes). The three sands were, 1st, white marble, very hard with sharp corners; 2d, moderately hard limestone; and 3d, chalk, very soft with rounded grains. The proportions used were 400 kg. of cement to one cubic meter of sand, the amount of water varying from twenty-five to thirty per cent, of the sand, according to the amount required to produce plasticity. The tensile strength of the mortars, in pounds per square inch, is given in Table 45.

Table 45. Results Obtained With Three Varieties Of Calcareous Sand

As these sands varied in the structure and hardness as well as in the shape of the grains, it cannot be concluded that rounded grains are as good as sharp and angular ones for mortar-making. There is little question that if two samples of pure quartz sand, differing in sharpness but alike in all other respects, including the percentage of voids, were tested side by side, the rounded grains would be found inferior. (See also § 253).

1 "Recherches Experimentales sur Les Mortiers Hydrauliques".

M. Alexandre also made tests on sands differing both in chemical and physical characteristics, but having the same fineness, namely, twenty per cent, each of five sizes of grain. Some of the results are given in Table 46.

Table 46. Results Obtained With Various Sands

249. Siliceous Vs. Calcareous Sands

The above tests would seem to show that sand to be used in mortar need not be siliceous. In. experimenting on different varieties of sand, both natural and artificial, the author has obtained results that point to a similar conclusion. Some of these tests are given in Tables 47 to 50.

Table 47 gives the results obtained with four varieties of siliceous sand. The first was an artificial sand made by crushing sandstone, the second and third were natural sands containing a large percentage of quartz grains, and the fourth appeared to be almost pure quartz. Only the fine particles of the sands were used in the tests given in this table. 'The differences in strength at the end of two years are not great, but the two natural sands appear to give somewhat lower results.

In Table 48 the two natural sands were again compared, but this time in connection with a calcareous sand formed by crushing limestone. The latter gave the best results. Only the finer grains were used in these tests.

250. Tables 49 and 50 are more valuable in this connection, since the coarser particles of the sand were used with the fine. The sand was separated into four sizes by sifting, and then remixed in equal proportions. Table 49 gives the results obtained with natural cement, and Table 50 refers to Portland. The superiority of the screenings is very clearly shown, the limestone giving especially good results. Indeed, the strength obtained with three parts limestone screenings to one part of either Portland or natural cement is remarkably high. The mortar made from such sand is peculiarly plastic when fresh, and soon gains a high strength which it appears to maintain.

Table 47. Values Of Different Varieties Of Fine Siliceous Sand For Use In Portland Cement Mortar

Two Parts Sand to One Cement by Weight.

Note. — Holes in No. 40 sieve 0.015 inch square, holes in No. 100 sieve about 0.0065 inch square.

Table 48. Different Varieties Of Fine Sand For Portland Cement Mortar

Table 49. Values Of Different Varieties Of Sand For Natural Cement Mortar

1 13.6 per cent, water, trifle dry.

Note. — Fineness Mx. means 25 per cent, each of 20-30, 30-40, 40-50 and 50-80.

Expression 20-30 means passing No. 20 sieve and retained on No. 30 sieve.

Table 50. Values Of Different Varieties Of Sand For Portland Cement Mortar

1 Trifle dry, plastic. 2 13.3 per cent, water. 3 14.3 per cent, water. Note. — Fineness Mx. means 25 per cent, each of 20-30, 30-40, 40-50 and 50-80.

251. Slag Sand

To turn to good account some of the immense quantities of blast furnace slag produced yearly, the use of granulated slag in place of ordinary sand has been advocated. In a paper read before the Engineers' Society of Western Pennsylvania, in March, 1904, Mr. Joseph A. Shinn described some experiments he had made, in which it was shown that "slag sand," with Portland cement, natural cement, or common lime, gave a higher strength than the sample of river sand used in the comparison.

The "slag sand" is produced by projecting two flat jets of water into the stream of molten slag, the resulting sand being heavier, finer and more nearly uniform in size of grain than the ordinary slag granulate.