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BUILDING MATERIALS MORTAR

The mortars used in bricklayers' work consist of an admixture of lime, or Portland cement, and sand. A knowledge of the properties of these materials is very necessary to the craftsman, if he is to obtain the best results from' his labours.

Lime is manufactured by the calcination, or burning, of a carbonate of calcium, of which chalk is the commonest example. During calcina­tion, decomposition occurs, and carbonic acid and water are driven off, an oxide of calcium (quicklime) remaining.

If water be added to lumps of quicklime, rapid combination ensues, great heat and volumes of steam being generated. The lumps disintegrate with a series of small explosions, and are eventually reduced to a very fine powder. This process is termed slaking; and when making mortar it is highly necessary that it should be thoroughly carried out, as any unslaked particles subsequently expand and seriously damage the work.

Limes may be divided into three distinct classes — 1. Rich limes. 2.Poor limes. 3. Hydraulic limes.

Rich limes contain not more than, 6 per cent of impurities, slake very rapidly, and are entirely dependent on external agents for setting power. They are chiefly used for interior plasterers' work.

Poor limes contain from 15 per cent to 30 per cent of useless impurities, and possess the general properties of rich limes, only to a lesser degree. They are only fit for unimportant work.

Hydraulic limes contain certain proportions of impurites, which, during calcination, combine with the lime, and endow it with the valu­able property of setting under water, or without external agents. The proportions of these impurities determine whether a lime is emi­nently, moderately, or only feebly hydraulic. The principal limes used in making mortar for constructional work are of the Greystone variety. These have hydraulic properties, and will take a large proportion of sand, without weakening their setting powers. The usual proportions are from two to four parts of sand to one of lime.

The setting of lime depends largely upon its absorption of carbonic acid from the atmosphere. The particles return to their original form of a carbonate, and crystallize. These crystals have a tendency to adhere to anything rough, such as sand or the surfaces of a brick.

Pure lime mortars built into thick walls never harden in the inte­rior. The crystallization of the exterior of the joint when set prevents access of carbon dioxide to the inside of the wall. For this reason,
pure lime mortars should not be used for constructional work, only those which are not entirely dependent on external agents. For more important work, where great strength is required, Portland cement is
used instead of lime.

Portland Cementis an artificial cement, manufactured by calcining chalk and clay, or river mud containing certain chemical constituents in definite proportions. The chalk and clay are ground and mixed into a slurry, which after being strained through very fine sieves, is pumped into an orifice in the top of an inclined revolving cylinder. A blast of intense flame is directed, through this cylinder, which is lined with firebrick. As the slurry drops through the flame, it is burned into small clinkers, which are afterwards ground exceedingly fine in specially constructed mills, and then passed through sieves, having as many as 35,000 meshes to the square inch. The powder is aerated by being spread on wooden floors, with an occasional turning, to en­sure the thorough slaking and cooling of all particles. It is then put up in sacks ready for use.

This process of aeration has now been superseded in many cement works by the addition of a small quantity of gypsum (plaster of Pa­ris), which retards the otherwise rapid-setting tendency of a freshly ground cement.

Sand. — When used for mortar, sand should be angular in grain, free from

clay or dirt, and moderately coarse. If too fine, the proportion of lime or cement will have to be considerably increased.

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