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Properties of Clay and Siiale Brick

lime, bricks, sand, mixture, temperature, employed and water

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PROPERTIES OF CLAY AND SIIALE BRICK Good building brick should show a uniform compact structure without laminations. They should have plane, parallel faces and sharp edges, and should not show kiln marks on their edges.

The dry-pressed and re-pressed bricks are usually smoother and more accurate in shape than those made by the soft-mud or stiff mud processes, their density and strength being largely dependent upon the degree of burning and the shrinkage in the kiln. The under burned, salmon bricks are porous and weak, and are usually employed only where strength is not important and in unexposed positions. The well-burned cherry or hard bricks are the best building brick. The overburnecl clinker bricks are more dense and absorb less water, but may be brittle, and are frequently distorted in shape. The overturned and distorted bricks ara sometimes used by architects for special exterior designs with very good effect.

Vitrified bricks, as manufactured for use in paving, are superior in strength and density to common bricks. They frequently show kiln marks on one side, due to softening in the kiln. A clay for making vitrified brick must burn at high temperatures and have considerable range of temperature between the point of incipient fusion and the point of vitrification. It is difficult to maintain the temperature uniformly, so as to burn a large portion of the bricks to the right degree, unless the range of temperature is considerable.

57. made of sand cemented with lilne have been used in a small way for many years. These bricks, as formerly made, were molded and allowed to harden by standing in the air, or in an atmosphere rich in carbon dioxide (CO2). Bricks of this kind are virtually composed of ordinary lime mortar, but with less lime, and are called mortar bricks. They depend, like lime mortar, upon the formation of carbonate of lime for their harden ing, and are weak and of little value as brick, although some struc tures of such materials have proven substantial and durable.

In 1SS1 Dr. of I3erlin patented a process of hardening mixtures of lime and sand by the use of steam at high pressure. He discovered that, in the presence of steam at high temperature, the lone combines with a portion of the silica of the sand, forming a silicate of Tune, which acts as a cementing medium. This silicate

is formed upon the surfaces of the grains of sand and binds the sand into a single hard block.

About fifteen years after Michaelis took out his patent, the manufacture of sand-lime bricks was begun in Germany on a com mercial scale, and soon developed into a considerable industry. In 1901 the first plant was opened in the United States, and the growth of the industry in this country was also very rapid.

the manufacture of sand-lime bricks, four operations are essential: (1) The lime must be completely slaked.

(2) A very uniform mixture of the lime and sand must be obtained.

(3) The material must be formed into bricks under high pressure.

(4) The bricks must be subjected to the action of steam at high pressure for several hours.

The methods employed in different plants for performing these operations vary considerably, depending upon the character and condition of the materials employed.

this process the lime is first slaked to a powder, or a putty, and then mixed with the sand and pressed. The lime may be slaked by any of the methods ordinarily employed in the manufacture of hydrated lime, or it may be reduced to a paste by the use of an excess of water. It is easier to obtain a uniform mixture of the lime and sand when dry hydrated lime arid dry sand are used and the necessary water added afterward. It may, however, be advantageous sometimes to use wet materials, and good results may be obtained by either method if the mixing he thorough and the lime uniformly incorporated in the sand.

Caustic Lime lime is sometimes pulverized and mixed with the sand before slaking. Enough water is then added to slake the lime and reduce the mixture to proper consistency for pressing. High-calcium lime, which slakes quickly, is necessary when this method is used, as sufficient time must be given for the complete slaking to take place before the mixture goes to the press. In some plants the mixture is placed in a silo and allowed to stand for a few hours before pressing, in order to insure that no unslaked lime is left in the mixture when the brick is formed.

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