Ordinary Mortar is composed of lime and sand mixed into a paste with water. When cement is substituted for the lime, the mixture is called cement mortar.
Uses. The use of mortar in masonry is to bind together the bricks or stones, to afford a bed which prevents their inequalities from bearing upon one another and thus to cause an equal distribu tion of pressure over the bed. It also fills up the spaces between the bricks or stones and renders the wall weather tight. It is also used in concrete as a matrix for broken stones or other bodies to be amalgamated into one solid mass; and for plastering and other purposes.
The quality of mortar depends upon the character of the materials used in its manufacture, their treatment, proportions, and method of mixing.
Proportions. The proportion of cement to sand depends upon the nature of the work and the necessity for the development of strength or imperviousness. The relative quantities of sand and cement should also depend upon the nature of the sand; fine sand requires more cement than coarse. Usual proportions are: Lime mortar, 1 part of lime to 4 parts of sand.
Natural cement mortar, 1 part cement to 2 or 3 parts of sand. Portland cement mortar, 1 part cement to 2, 3, or 4 parts of sand, according to the character of the work.
Sand for rlortar. The sand used must be clean, that is, free from clay, loam, mud, or organic matter; sharp, that is, the grains must be angular and not rounded as those from the beds of rivers and the seashore; coarse, that is, it must be large-grained, but not too uniform in size.
The best sand is that in which the grains are of different sizes; the more uneven the sizes the smaller will be the amount of voids, and hence the less cement required.
The cleanness of sand may be tested by rubbing a little of the dry sand in the palm of the hand, and after throwing it out noticing the amount of dust left on the hand. The cleanness may also be judged by pressing the sand between the fingers while it is damp; if the sand is clean it will not stick together, but will immediately fall apart when the pressure is removed.
The sharpness of sand can be determined approximately by rubbing a few grains in the hand or by crushing it near the ear and noting if a grating sound is produced; but an examination through a small lens is better.
To determine the presence of Salt and Clay. Shake up a small portion of the sand with pure distilled water in a perfectly clean stoppered bottle, and allow the sand to settle; add a few drops of pure nitric acid and then add a few drops of solution of nitrate of silver. A white precipitate indicates a tolerable amount of salt; a faint cloudiness may be disregarded.
The presence of clay may be ascertained by agitating a small quantity of the sand in a glass of clear water and allowing it to stand for a few hours to settle; the sand and clay will separate into two well-defined layers.
Screening. Sand is prepared for use by screening to remove the pebbles and coarser grains. The fineness of the meshes of the screen depends upon the kind of work in which the 'sand is to be used.
Washing. Sand containing loam or earthy matters is cleansed by washing with water, either in a machine specially designed for the purpose and called a sand-washer, or by agitating with water in tubs or boxes provided with holes to permit the dirty water to flow away.
Water for flortar. The water employed for mortar should be fresh and clean, free from mud and vegetable matter. Salt water may be used, but with some natural cements it may retard the setting, the chloride and sulphate of magnesia being the principal retarding elements. Less sea-water than fresh will be required to produce a given consistency.
Quantity. The quantity of water to be used in mixing mortar can be determined only by experiment in each case. It depends upon the nature of the cement, upon that of the sand and of the water, and upon the proportions of sand to cement, and upon the purpose for which the mortar is to be used.