Современные проблемы промышленного и гражданского строительства на занятиях английского языка. Кузьмина Е.В. - 21 стр.

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upper end of a rotary kiln heated to 2600 to 3000F by burning oil, gas, or

powdered coal. Because cement production is an energy-intensive process,

reheaters and the use of alternative fuel sources, such as old tires, are used to

reduce the fuel cost. (Burning tires provide heat to produce the clinker and the

steel belts provide the iron constituent.) Exposure to the elevated temperature

chemically fuses the raw materials together into hard nodules called cement

clinker. After cooling, the clinker is passed through a ball mill and ground to a

fineness where essentially all of it will pass a No. 200 sieve (75 m). During the

grinding, gypsum is added in small amounts to control the temperature and

regulate the cement setting time. The material that exits the ball mill is portland

cement. It is normally sold in bags containing 94 lb of cement. Concrete, the

most common use for Portland cement, is a complex material consisting of

Portland cement, aggregates, water, and possibly chemical and mineral

admixtures. Only rarely is Portland cement used alone, such as for a cement

slurry for filling well holes or for a fine grout. Therefore, it is important to

examine the relationship between the various Portland cement properties and

their potential effect upon the finished concrete. Portland cement concrete is

generally selected for structural use because of its strength and durability.

Strength is easily measured and can be used as a general directly proportional

indicator of overall durability. Specific durability cannot be easily measured but

can be specified by controlling the cement chemistry and aggregate properties.

ALUMINOUS CEMENTS

These are prepared by fusing a mixture of aluminous and calcareous

materials (usually bauxite and limestone) and grinding the resultant product to a

fine powder. These cements are characterized by their rapid-hardening

properties and the high strength developed at early ages. Table 4.3 shows the

relative strengths of 4-in cubes of 1:2:4 concrete made with normal Portland,

high-early-strength Portland, and aluminous cements. Since a large amount of

heat is liberated with rapidly by aluminous cement during hydration, care must

be taken not to use the cement in places where this heat cannot be dissipated. It

is usually not desirable to place aluminous-cement concretes in lifts of over 12

in; otherwise the temperature rise may cause serious weakening of the concrete.