253. The size and shape of the grains are important elements in the quality of sand. Considering grains of the same shape but differing in size, the larger grain will have a smaller surface area in proportion to the volume than the smaller grain, since the volume varies approximately as the cube of one dimension while the surface varies as the square. Since, in order to obtain the best results in mortar, each grain of sand must be coated with cement, it follows that, other things being equal, the coarser grained sands will give the best results, because they will be more thoroughly coated; this will be especially true when the amount of sand in the mortar is relatively large.

Following the same reasoning given above as to the relative volume and superficial area of sand grains, it would appear that spherical grains would be better than cubical or angular ones (see § 248). This, however, is not thought to be the case, for the better bond obtained with angular grains seems to counterbalance the advantage which the small superficial area would appear to give to the spherical grains. For this reason a lenticular shaped grain, while having a very large area relative to its volume, will give excellent results in mortar if otherwise suited to the purpose.

It is usually desirable to have all of the voids in the sand filled by the cement paste, as this renders the mortar less porous, and makes it more certain that all the grains are coated with cement. On this account a mixture of fine and coarse particles is excellent.

Table 51. Effect On Tensile Strength Of Varying Fineness Of Limestone Screenings Used With Portland Cement

Significance Of Fineness

Notes. — Three parts screenings to one cement by weight.

All briquets made by one molder and immersed in one tank. Variations in consistency were slight, the largest percentage of water being used for the finest particles.

254. Tests On Effect Of Fineness Of Sand

Many of the experiments made to show the effect of the fineness of sand on the strength of the mortar are defective, because the sand used varies in the shape of the grains and in chemical characteristics as well as in fineness. The experiments given in Table 51 were made with screenings obtained in crushing limestone, and thus all causes of variation aside from the fineness of the sand were absent, except the differences in consistency of the mortar, the uniformity in consistency depending on the judgment of the operator. The results show quite clearly the superiority of the coarser sand.

255. The Relative Effect Of Fine Sand On Portland And Natural Cement

The tests in Table 52 were made to determine the relative effects of fine sand on Portland and natural cements. Limestone screenings of two sizes of grain were used in connection with two brands of each kind of cement. At twenty-eight days the natural cement shows the decrease in strength due to the use of fine sand more than Portland cement does.

Table 52. Coarse And Fine Sand, Relative Effects With Portland And Natural Cement

Notes. — Sand, limestone screenings; three parts to one cement by weight.

20-30 means sand passing sieve with 20 meshes per linear inch, and retained on sieve with 30 meshes per linear inch.

Columns 5 and 9 show percentage that strength with finer sand is of the strength with coarser sand.

At six months the fine sand seems to have about the same effect on Portland and natural, but the two-year results indicate that the ultimate effect is less on the natural cement than on the Portland; the mean ratio of the strength obtained with fine sand to that given by coarse sand being ninety-six in the case of natural, and only eighty-six in the case of Portland. The effect of fine sand appears to decrease with age, especially with natural cement.

The fineness of sand will be treated further in the following article relating to voids.