SAND FOR MORTAR Quality of Sand.—As hydraulic cement is commonly mixed with certain proportions of sand, when used in construction, the nature and quality of sand used, and the method of manipulating the materiaLs in forming the mortar have quite as important, an effect upon the final strength of the work as the quality of the cement itself.
In testing cement a standard sand is employed. This sand may be obtained quite uniform in quality. In the execution of work, however, local sand must generally be used; this varies widely in character, and should always be carefully considered upon any Nvork where the development of strength and lasting qualities are of importance.
Size of Sand Grains.—It is usual to class as sand all material less than 1-inch diameter; pieces larger than this being classed as gravel. Coarse sand is superior to fine sand for use in cement mortar. Coarse sand presents less surface to be coated with cement and the interstices are more easily filled with cement paste. Fine sand requires More water in mixing to the same consistency, and gives usually weaker and more porous mortar than coarse sand.
The use of a mixture of grains of different sizes is usually desir able, giving less voids to be filled by the cement; and it is frequently found, when the cement is not in considerable excess, that the strength obtained by such a mixture is much greater than is given by either the large or small grains alone. Sand of mixed sizes, giving a minimum of voids, requires less cement to make a mortar of maximum density and strength than that of more uniform sizes.
Shape of Grains.—Sand with angular grains usually gives better results in mortar than that with rounded grains, and specifications frequently call for sharp sand. This difference is, however, much less important than that of proper gradation of sizes, and should not be given undue weight in the selection of sand for use in mortar.
Stone Screenings.—The screenings from crushed stone are fre quently used in place •of natural sand. Ordinarily screenings from stone of good quality give mortar of rather better strength than natural sand. This, however, depends in most instances upon the gradation of sizes in the two materials. The sharpness of grain
is favorable to the screenings, and the presence of a certain amount of very fine stone dust in the screenings seems to be of value in the mortar. When the screenings are derived from soft rock, the dust may be present in too large amount and need to be screened out before the screenings can be successfully used.
Chemical Composition.—Sands as commonly used for mortar are composed mainly of silica. In most cases, sand which has a proper granulometric composition is satisfactory for use. The failure of concrete work has, however, in a number of instances been found to be due to the use of sand low in silica. Sand containing less than 95 per cent silica needs to be carefully tested before being used, although some sands as low as 75 per cent silica have given good results. The composition of sands have not been sufficiently studied to determine the differences of composition which cause failure in one case and success in another.
The presence of erica in sand or screenings is supposed to injuri ously affect the strength of mortar in which the material is used. The results of experiments upon the effect. of mica are not conclusive, although they semi to indicate that mica may sometimes be injuri ous. Sated containing mica should be carefully tested before being used.
Effect of Impurities.—Sand for use in mortar should be clean, and as free from loam, nntci, or organic matter as possible. In general the presence of any foreign matter is to be avoided, though a small amount of fine clay distributed through sand has sometimes been found to increase the strength of cement mortar, and also helps to make the mortar work more smoothly, sometimes decreasing its permeability. The effect of the clay depends upon the character of the sand and upon the richness of the mortar. Fine clay may help to fill the voids in an otherwise porous mortar with good effect, but may be deleterious in a rich mortar, or when it is not finely divided and uniformly distributed through the sand. In a particular instance, the effect of such an adulteration can be judged only by testing it.