Sand

SAND.

Sand is mixed with lime or cement to reduce the cost of the mortar; and is added to lime also to prevent the cracking which would occur if lime were used alone. Any material may be used to dilute the mortar, provided it has no effect upon the durability of the cementing material and is not itself liable to decay. Pulverized stone, powdered brick, slag, or coal cinders may be used; but natural sand is by far the most common, although fine crushed stone, or "stone screenings," is sometimes employed and is in some respects better than natural sand.

In testing cement a standard natural sand or crushed quartz is employed; but in the execution of actual work local natural sand must be employed for economic reasons. Before commencing any considerable work, all available natural sands and possible sub stitutes should be examined to determine their values for use in mortar.

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FOR GOOD SAND. To be suitable for use in mortar, the sand should be sharp, clean, and coarse; and the grains should be composed of durable minerals, and the gradation of the sizes of the grains should be such as to give a minimum of voids, i.e., inter stices between the grains.

The usual specifications are simply: "The sand shall be sharp clean, and coarse." Durability. As a rule ocean and lake sands are more dur able than glacial sands. The latter are rock-meal ground in the geological mill, and usually consist of silica with a considerable admixture of mica, hornblende, feldspar, carbonate of lime, etc.

The silica is hard and durable; but the mica, hornblende, feldspar, and carbonate of lime are soft and friable, and are easily decomposed by the gases of the atmosphere and the acids of rain-water. The lake and ocean sands are older geologically; and therefore are as a rule nearly pure quartz, since the action of the elements has eliminated the softer and more easily decomposed constituents. Some ocean sands are nearly pure carbonate of lime, which is soft and friable, and are therefore entirely unfit for use in mortar. These are known as calcareous sands.

The glacial sands frequently contain so large a proportion of soft and easily decomposed constituents as to render them unfit for use in exposed work, as for example in cement sidewalks. Instead of constructing exposed work with poor drift sand, it is better either to ship natural silica sand a considerable distance or to secure crushed quartz. Crushed granite is. frequently used instead of sand in cement sidewalk construction; but granite frequently contains mica, hornblende, and feldspar which render it unsuitable for this kind of work.

However, as a• rule the physical condition of the sand is of more importance than its chemical composition.

Sharpness.

Sharp sand, i.e., sand with angular grains, is preferable to that with rounded grains because (1) the angular grains are rougher and therefore the cement will adhere better; and (2) the angular grains offer greater resistance to moving one on the other when under compression. On the other hand, the sharper the sand the greater the proportion of the interstices between the grains (compare line 4 of Table 19, page 93, with the preceding lines of the table), and consequently the greater the amount of cement required to produce a given strength or density. For crushing strength a high degree of sharpness is more important than a small per cent of voids; but for tensile strength, economy, and water-tightness a small per cent of voids is more important.

The sharpness of sand can be determined approximately by rubbing a few grains in the hand and noting whether there is any cutting action, or by crushing it near the ear and noting if a grating sound is produced; but an examination through a small lens is a better means. Strictly speaking, the grains of all natural sand are rounded rather than angular. Sharp sand is often difficult to obtain, and the requirement that "the sand shall be sharp" is practically a dead letter in most specifications. For the reasons given in the preceding paragraph, sharpness should not be specified.