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Cementing Materials - Lime

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CEMENTING MATERIALS - LIME Classification.—The cementing materials employed in the construction of masonry and concrete structures include common lime, hydraulic lime, Portland cement, natural cement, and puzzolan. These materials are formed by the calcination of limestones, or of mixtures of limestones with siliceous or argillaceous materials, and their properties vary with the nature and porportions of the sub stances combined in them.

Common Lime.—When limestone composed of nearly pure car bonate of lime is burned, the resulting clinker, known as quicklime, possesses the property of breaking up, or slaking, upon being treated with a sufficient quantity of \water. The slaking of lime is due to its rapid hydration when in contact with water, and the process is accompanied by a considerable increase in the volume of the mass of lime and by a rise in temperature. If the quantity of water be only sufficient to cause the hydration of the lime, the quicklime is reduced to a dry powder; while if the water be in excess it becomes a paste.

The slaked lime thus formed possesses the further property, when mixed to a paste with water and allowed to stand in the air, of hardening and adhering to any surface with which it may be in contact. This hardening of common limes will take place only when exposed to the air and allowed to become dry.

When Tune is nearly pure and its activity very great it is known as fat lime.

If the lime have mixed or in combination with it considerable impurities of inert character, ~which act as an adulteration to lessen the activity of the lime, causing a partial loss of the property of slaking and diminishing its power to harden, it is known as meager or poor lime. .

Hydraulic Lime.—When the limestone contains about 10 to 20 per cent of silica or clay mixed with the carbonate of lime, the material resulting from the burning is known as hydraulic lime. This clinker will slake when treated with water like common lime, but with reduced activity. The slaked lime thus obtained pos sesses the further property, when mixed with water to a paste, of hardening under water and without contact with the air.

In hydraulic lime the silica and alumina are combined with a portion of the lime, forming compounds which harden under water, while part of the lime is left uncombined. This free lime expands when hydrated by addition of water, causing the material to slake.

Hydraulic Cement.—When the proportion of siliceous or argil

laceous materials in limestone, or mixed with it, is sufficient to combine with all the lime, leaving no lime in a free state, the prod uct of burning is known as hydraulic cement. This clinker will not slake, but must be reduced to powder by grinding. The cement powder, when mixed with water, has the property of setting and hardening under water, and of adhering firmly to any surface with which it may be in contact.

Portland Cement is the name given to hydraulic cement which is formed by burning and grinding an intimate mixture of powdered limestone and argillaceous matter in accurately determined pro portions. In making Portland cement, the ingredients are care fully proportioned to secure the complete combination of the lime with the silica and alumina into active material, and it is necessary to reduce the materials to a very fine state and secure uniform incorporation of the ingredients before burning.

Natural Cements are made by burning limestones which con tain proper proportions of argillaceous materials, and grinding the resulting clinker to powder. Natural cements are less rich in lime than Portland cements, complete combination of the argillaceous materials not being effected. They are burned, like lime, without the pulverization of the raw materials, and require a much lower temperature in burning than Portland cement.

The term Puzzolan is commonly applied to a class of materials which, when made into a mortar with fat lime or feebly hydraulic lime, impart to the lime hydraulic properties and cause the mortar to harden under water. It derives its name from Pozzuoli, a city of Italy near the foot of Mount Vesuvius, where its properties were first discovered. It was extensively used by the Romans in their hydraulic constructions, being mixed with slaked lime for the for mation of hydraulic mortar. Puzzolan is essentially a silicate of alumina in which the silica exists in a condition to be attacked readily by caustic alkalies, and hence easily combines with the lime in the Puzzolan Cement is formed by mixing slaked lime with puzzolan and grinding the mixture to a fine powder. Certain materials of volcanic origin are frequently used for this purpose in Europe, while considerable quantities of cement of this class have been made by the use of blast furnace slag, both in Europe and the United States.

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