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111scellaneous

slag, cement and cements

111SCELLANEOUS CErlENTS.

Slag Cements are those formed by an admixture of slaked lime with ground blast-furnace slag. The slag used has approximately the composition of an hydraulic cement, being composed mainly of silica and alumina, and lacking a proper proportion of lime to render it active as a cement. In preparing the cement the slag upon coming from the furnace is plunged into water and reduced to a spongy form from which it may be readily ground. This is dried and ground to a fine powder. The powdered slag and slaked lime are then mixed in proper proportions and ground together, so as to very thoroughly distribute them through the mixture. It is of the first importance in a slag cement that the slag be very finely ground, and that the ingredients be very uniformly and intimately incorporated.

Both the composition and methods of manufacture of slag cements vary considerably in different places. They usually con tain a higher percentage of alumina than Portland cements, and the materials are in a different state of combination, as, being mixed after the burning, the silicates and aluminates of lime formed during the burning of Portland cement cannot exist in slag cement.

The tests for slag cement are that briquettes made of one part of cement and three parts of sand by weight shall stand a tensile strain of 140 pounds per square inch (one day in air and in water), and must show continually increasing strength after seven days, one month, etc. At least 90 per cent must pass a sieve containing 40,000 meshes to the square inch, and must stand the boiling test.

Pozzuolanas are cements made by a mixture of volcanic ashes with lime, although the name is sometimes applied to mixed cements in general. The use of pozzuolana in Europe dates back to the time of the Romans.

Roman Cement is a natural cement manufactured from the septaria nodules of the London Clay formation; it is quick-setting, but deteriorates with age and exposure to the air.