CHAPTER XII 

 PROPERTIES AND USES OF CEMENT 



105. Properties. The properties of cement with which 

 every builder is most concerned are those of strength and per- 

 manence. The requirements ordinarily mentioned are proper 

 fineness, proper setting qualities, purity, strength in tension, 

 and soundness. A cement that is fresh, free from lumps, 

 properly packed and stored, is nearly always first-class. 



106. Mortar. Mortar is a mixture of (1) cement or hy- 

 drated lime, or both, (2) sand, and (3) water. It is a plastic 

 mass, the water content being varied with its use. Lime 

 mortars are little used at present because they set slowly, will 

 not set under water, are not very strong, and will deteriorate, 

 due to weathering action. A small amount of lime, 10 to 20 

 per cent, is usually added to cement mortar to make it work 

 well with a trowel and to make it more adhesive. 



107. Definition of Concrete. Concrete is often defined 

 as an artificial stone. It is made by mixing cement with sand 

 and gravel, or broken stone, and water ; or, in other words, it 

 is a mixture of (1) cement, (2) a fine aggregate, (3) a coarse ag- 

 gregate, and (4) water. The addition of water causes the ce- 

 ment to undergo chemical changes forming new compounds 

 that develop the property of crystallizing into a solid mass. 

 The strength and durability of plain concrete (that is, con- 

 crete without reinforcing) varies with: 



a) The quality and amount of cement used. 



b) The kind, size and strength of aggregate. 



c) Correctness of proportioning. 



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