Man's physical universe

xanabras

COMBUSTION 651

is much used in industries whenever a mild alkah is desired. Hydrated

Hme is an important constituent of mortar, plaster, and stucco. Ordinary

mortar consists of a mixture of hydrated lime, sand, and water;

when exposed to the air, the water evaporates, and the mortar slowly

sets as the calcium hydroxide reacts with the carbon dioxide of the air

to form artificial limestone, CaCOs, containing sand.

Interior plasters contain plaster of Paris as their principal constituent.

Plaster of Paris is obtained from naturally occurring gypsum,

CaS04 •

2 H2O, by heating it to drive off a part of its water to form

CaS04


|- H2O. When plaster sets, the reverse reaction slowly takes

place. As plaster of Paris sets, it swells slightly and is therefore very

useful for preparing casts of various kinds.

Stucco, i.e., exterior plaster, usually contains both lime and cement.

2. Portland Cement May Be Made by Heating the Proper Mixtures

of Certain Very Abundant Rocks. The invention of Portland cement

revolutionized the building and construction industries by enabling

man to produce artificial stone in any shape desired, thus saving immense

amounts of labor that would otherwise have been expended in

cutting stones.

Modern Portland cement is produced by intermingling raw materials

rich in lime, CaO, with other materials rich in alumina, AI2O3, and

silica, Si02. The lime is usually obtained from low-grade limestone,

chalk, or shells. The alumina and silica are furnished by clay, shale,

slate rock, volcanic ash, or other materials such as blast-furnace slag.

These raw materials are analyzed and mixed in the proper proportions.

The manufacturing process consists in grinding the material very finely,

mixing it intimately, partially fusing it in a rotary kiln at a very high

temperature, and then grinding it to an extremely fine powder in

rotating tubes partially filled with steel rolls or balls.

When Portland cement is mixed with water or with a mixture of

water, sand, and gravel to form concrete, it takes an initial set within

a few hours and continues to harden slowly for months or years.

This

hardening process consists, in part at least, in the hydration, i.e.,

chemical combination with water, of the anhydrous compounds produced

during the burning of the cement, to produce synthetic silicate

rocks. These synthetic rocks may be made stronger than natural rocks

by the use of reinforcing steel. Concrete should be kept wet when

setting in order to provide plenty of water for the hydrating process.

3. Ceramic Products Have Also Replaced Natural Stone and Provided

Entirely New Building Materials. The manufacture of brick

and other clay products such as pottery was among the first advances

made by man in the realm of material transformations.

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