Современные проблемы промышленного и гражданского строительства на занятиях английского языка. Кузьмина Е.В. - 22 стр.

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Aluminous cements are much more resistant to the action of sulfate waters than

are Portland cements. They also appear to be much more resistant to attack by

water containing aggressive carbon dioxide or weak mineral acids than the

silicate cements. Their principal use is in concretes where advantage may be

taken of their very high early strength or of their sulfate resistance, and where

the extra cost of the cement is not an important factor. Another use of

aluminous cements is in combination with firebrick to make refractory

concrete. As temperatures are increased, dehydration of the hydration products

occurs. Ultimately, these compounds create a ceramic bond with the aggregates.

NATURAL CEMENTS

Natural cements are formed by calcining a naturally occurring mixture of

calcareous and argillaceous substances at a temperature below that at which

sintering takes place. The ‘‘Specification for Natural Cement’’ requires that the

temperature be no higher than necessary to drive off the carbonic acid gas.

Since natural cements are derived from naturally occurring materials and no

particular effort is made to adjust the composition, both the composition and

properties vary rather widely. Some natural cement may be almost the

equivalent of Portland cement in properties; others are much weaker. Natural

cements are principally used in masonry mortars and as an admixture in

Portland-cement concretes.

LIMES

These are made principally of calcium oxide (CaO), occurring naturally in

limestone, marble, chalk, coral, and shell. For building purposes, they are used

chiefly in mortars.

Hydraulic Limes

These are made by calcining a limestone containing silica and alumina to

a temperature short of incipient fusion so as to form sufficient free lime to

permit hydration and at the same time leave unhydrated sufficient calcium

silicates to give the dry powder its hydraulic properties.

Because of the low silicate and high lime contents, hydraulic limes are

relatively weak. They find their principal use in masonry mortars. A hydraulic

lime with more than 10% silica will set under water.