Sand is known as "argillaceous," "siliceous," or "calcareous;' according to its composition.
River Sand is not so sharp or angular in its grit, the grains having been rounded and polished by attrition.
It is commonly fine and white, and therefore suited for plastering.
When sand contains lumps or stones it should be "screened," or, if required of great fineness, passed through a sieve.
Sand found to contain impurities, such as clay, loam, etc., which unfit it for almost every purpose, should be washed by being well stirred in a wooden trough having a current of water flowing through it which carries off the impurities. It is sometimes washed by machinery, such as an Archimedean screw revolving and carrying up the sand, while a stream of water flows down through it.
Clean sand should leave no stain when rubbed between the moist hands. Salts can be detected by the taste, and the size and sharpness of the grains can be judged of by the eye.
Where this is specified, as it is in connection with the cement and sand test for Portland cement (see p. 170), it is generally required that the sand should pass through a sieve of 400 meshes to the square inch and be retained by one of 900 meshes.
Burnt Clay is sometimes used as a substitute for sand in mortar.
It is prepared by piling moistened clay over a bonfire of coals and wood. As the clay becomes burnt and the fire breaks through, fresh layers of clay and coal, "breeze," or ashes, are piled on, and the heap may be kept burning until a sufficient supply has been obtained.
The clay should be stiff. Care must be taken that it is thoroughly burnt Raw or half-burnt pieces would seriously injure mortar.
Sand is sometimes very economically obtained by grinding the refuse "spalls" left after working the stones for walling. It is generally clean if carefully collected, but the sharpness of its grit depends upon the nature of the stone from which it is procured.1
1 Mr. Kinniple's experiments show that mortar of 1 Portland cement and 1 crushed sandstone is 55 per cent stronger than that of 1 Portland cement 1 pit sand. - M.P.I.C.E., vol. lxiv. p. 330. Experiments by a friend of the writer's showed Scoriae from ironworks, old bricks, Clinker from brick kilns, and Cinders from coal, make capital substitutes for sand when they are quite clean and properly used. Wood Cinders are too alkaline. Crushed Slag from furnaces may be dangerous if it contains lime.