Sand-Lime Brick

SAND-LIME BRICK.

Sand-lime brick consist of a mass of sand cemented together with lime. There are two classes of sand-lime brick: one in which the binding material is carbonate of lime, and the other in which the binding material is silicate of lime.

The first is virtually a brick made of ordinary lime mortar, moulded as are soft-mud clay brick, and hardened in the open air or in an atmosphere rich in carbon dioxide (CO,), either with or without pressure. This form may properly he called a lime-mortar brick. It is the older form of sand-lime brick, and was formerly made in a small way where sand and lime were cheap and clay and fuel were expensive; but the brick is so weak and friable that it has not given satisfaction, and needs no further consideration here.

The second kind of sand-lime brick is made from a mixture of sand and lime which is moulded in a press and hardened by being .subjected to steam under pressure. In this case the binding material consists chiefly of hydrosilicate of lime. Probably part of the lime is converted into carbonate by absorbing carbon dioxide; but the most of the lime combines with the silica of the sand and forms hydrosilicate of lime, a stable and comparatively strong cementing material. This form is the only one to which the term sand-lime brick is now applied; but in consulting the past literature on the subject, a careful distinction should be made between the two forms of so-called sand-lime brick. This form of sand-lime brick was first manufactured in Germany about 1880, and has been used there to a considerable extent. It was introduced into this country about 1901. Although the number of sand-lime brick manufactured here is quite small in comparison with the number of clay brick made, the number is so large and is increasing so rapidly as to require a discussion here of this form of brick.

The Sand.

Any sand can be used, but the best results are obtained with a pure silica sand, not containing too many coarse grains, properly graded so as to leave the smallest possible interstitial spaces. If the grains are coarse, the of the brick will not be as smooth as though the grains were finer; and unless there are a good many fine grains, there will not be sufficient surface to allow the most complete combination of the lime and the silica. If the size of the sand grains is not properly graded, too much

lime will be required to fill the voids or interstitial spaces, which will needlessly add to the cost without any compensating advantage.

The Lime.

The quantity of lime used varies from 4 to 10 per cent according to the per cent of voids in the sand, and according to the preference of the manufacturer. Either a high-calcium lime or a magnesian lime (see ˘ 105) may be used, although the former appears to make the stronger brick.

The lime must be reduced to a paste or powder before being mixed with the sand, and there are four ways of doing this: (1) wet slaking, (2) dry slaking, (3) acid slaking, and (4) grinding. In the first process the lime is slaked to a paste in the usual way by drenching with water and by agitation (§ 222); in the second the lime is slaked to a dry powder by adding only enough water that the heat of the chemical reaction will just dry the hydrate; in the third method, after the slaking has begun, a small amount of hydrochloric acid is added to accelerate the slaking process; in the fourth method the quicklime is ground to a powder, and in mixing with the sand only enough water is added to make it possible to work the mixture in the press.

Moulding.

The mixture of sand and lime is thoroughly mixed, and is then forced into moulds under very high pressure, very much as dry-clay brick are moulded. The higher the pressure the stronger and more dense the brick.

Hardening.

The bricks are hardened by being exposed in a closed cylinder to a steam pressure of 100 to 150 lb. per sq. in. for from 12 to 4 hours, respectively. As a rule the bricks are moulded during the day, and are left in the hardening cylinder over night. As soon as the bricks come from the hardening cylinder they can be used; but it is better to allow them to stand a clay or two, as when they dry out they become harder.