The caustic lime process is sometimes modified by grinding the lime with a portion of the sand to a fine powder, which is mixed with the remainder of the sand, and water added to slake the lime and wet the mixture. This is then placed in a silo for a sufficient period to allow the lime to become completely slaked before pressing. It is claimed that grinding the sand and lime together produces an intimate mixture and insures the complete combination during the steaming into silicate which forms the cementing medium of the brick. Grinding the lime and sand together reduces the lime to very fine condition and minimizes the danger from any unslakeci particles of lime left in the mixture, and also fills the voids in the sand more completely, making a more dense brick.
Molding.—The bricks are formed in molds similar to those used for dry clay bricks, and are subjected to high pressure in molding.
Hardening.—After molding, the bricks are loaded upon cars and run into the steaming cylinders, where they are subjected to steam pressure of from 100 to 110 pounds per square inch for a period of six to ten hours, resulting in the combination of the lime with the silica into the cementing substance and binding the sand into a solid block. The brick continue to harden and gain in strength for a time after their removal from the steaming cylinder, as they gradually dry out.
:lIu(eriuls.—High-calcium lime seems preferable for this use, on account of this rapid action and the fine subdivision of its particles. Any good lime may, however, be used for the purpose if care be taken to insure that it be completely slaked.
The requirements for sand to be used in making sand-lime bricks are not essentially different from those for sand to be used in cement mortar. The graduation of sizes to give a dense material is desirable. The presence of more fine material seems to be needed, however, in order to secure a smooth and compact mixture, and to lessen the wear upon the molds, which may become an important item of cost. Coarse sands seem to give stronger brick, but fine sand produces brick with smoother surfaces.
Properties of Sand-lime 13rick.—In strength and durability, sand lime bricks do not differ materially from good average clay bricks. When of good quality they possess sufficient strengths for all the purposes for which building brick are ordinarily employed, and are usually more dense, and absorb less water than common clay bricks.
Sand-lime bricks are usually very uniform in size and shape, and are commonly gray in color, the shade depending upon the sand used in manufacturing them, unless artificially colored.
58. Cement Bricks.—Bricks made of cement mortar or concrete are used in a number of localities. They are commonly of mortar, about one part Portland cement to four parts of sand, or sometimes of a richer mortar, 1 to 2 or 1 to 3, mixed with about an equal quantity of coarser material, varying from ; to .', inch in diameter.
These bricks are made by pressing in hand or power presses, a mixture as wet as is feasible to shape well in the press. About two weeks are required for hardening before the bricks can be used The materials need to be carefully selected, and require the same prop erties as for mortar for use in masonry or concrete. The strength may vary considerably with the grading of the aggregate, the com pression given to the blocks, and the moisture conditions under which the bricks are kept during the period of hardening, the greatest strength will result when they are kept warm and thoroughly dampened. The compressive strength at twenty-eight (lays should not be less than 1000 lb.; and the absorption not more than 15 per cent.
Cement bricks are usually employed as face bricks. Time appear ance will depend upon the texture of the aggregates used and the method of finishing, which may be smooth or roughened by the use of brushes or acids. Color may be given to the bricks by the use of various mortar colors.
59. Test for Building determining the suitability of a brick for structural work, examination is commonly made of the material as to form and texture with reference to the particular needs of the work in hand. Tests for strength and absorption arc sometimes included in specifications for important work, but there is no recognized standard to which such tests conform, and com paratively little data upon which to base a reasonable require ment.
Form.—For neat work, the bricks should he uniform in size with plane faces and sharp edges. (.'are in sorting is usually necessary with clay brick to secure uniformity of color and dimension in par ticular work.
bricks should be uniform and compact in struc ture, should be sound and free from cracks, and the broken surfaces should be free from flaws or lumps. Clay brick should be thoroughly burned, and when struck with a trowel or another brick should give a clear ringing sound. Bricks which meet these requirements are usually suitable for all ordinary work.