354. The Proportion Of Sand

The rate of change in the strength of mortars as the proportion of sand is increased varies greatly for different cements. The fineness and chemical composition of the cement, and the quality of the sand, are the most important factors influencing this rate of change upon which the question of the relative economies of different mortars is so largely dependent.

Table 67 gives the results of tests with two brands of Portland cement mixed with from two to ten parts of river sand, the age of briquets being six months and two years. It is of interest to notice that the strengths of the mixtures are approximately in the inverse ratio of the number of parts of sand used. Thus the strength with six parts sand is approximately two-sixths of the strength with two parts, while with ten parts sand, the strength is nearly two-tenths of that with mortar containing two parts.

Table 67. Rate Of Decrease In Strength With Addition Of Sand

Portland Cement; River Sand, " Point aux Pins ".

355. In Table 68 similar results are given for two samples of Portland cement and two kinds of sand, neat cement specimens being included in the comparison. The one-to-one mortars give a higher strength than neat cement, and even the mortar containing two parts of the limestone screenings is stronger than the neat specimens. From the one-to-one mortars the strengths decrease rapidly as more sand is added, until five parts sand are used, but the strengths then decrease less rapidly as larger additions of sand are made.

Table 68. Rate Of Decrease In Strength With Addition Of Sand

Portland Cement, Brand R; Sand, Crushed Quartz and Limestone Screenings.

356. In Table 69 two samples of natural cement are treated in a similar manner, from one to eight parts river sand being used in the mortars. With Sample II the strength is diminished rapidly until five parts sand have been added, but with further additions of sand, the strength is decreased more slowly. Sample 18 S gives quite a different curve, as the one-to-two mortar is stronger, and the one-to-three mortar is but little weaker than the one-to-one. With four parts sand the mortar shows a marked falling off in strength, but further additions of sand diminish the strength more slowly.

357. Increase In Tensile Strength With Time

In Table 70 are given the results obtained in tests of tensile strength with twelve samples of Portland cement, illustrating the rates of increase in strength from seven days to three years. It is seen that rich mortars gain strength rapidly, neat and one-to-one mortars showing usually eighty to ninety per cent, of their ultimate strength in twenty-eight days. Mortars containing not more than four parts sand to one cement give practically their ultimate strength at six months. It is also of interest to notice that the variations in strength among the several samples are not very great. The lowest strength at the end of two to three years is seventy-five to eighty per cent, of the highest.

Table 69. Rate Of Decrease In Strength With Addition Of Sand. Natural Cement, Brand Gn; River Sand, "Point Aux Pins"

358. In the case of natural cements, results for ten brands of which are given in Table 71, only fifty to seventy per cent, of the ultimate strength is gained in the first twenty-eight days; with mortars containing three parts sand to one cement the average result at twenty-eight days is less than forty per cent, of the strength at two years. Most of the samples gain some strength after six months, but two samples fail at two years which had given a fair result at six months. The variations in strength among the several samples are very much greater than with Portland cements; even omitting the two samples that failed, the strength of the highest is two or three times the strength of the weakest sample at two years.

Table 70. Portland Cement, Twelve Samples. Strength Of Mortar And Fineness

Table 71. Comparative Tests On Ten Brands Of Natural Cement

In general, each result is the mean of five briquets.

359. Table 72 shows in some detail the rate of increase in strength of a sample of natural cement when the specimens are maintained in air and in water. This table has several points of interest. When hardened in water, the cement gained steadily in strength up to six months, when it began to fall off, and at two years this cement failed, as is shown in Table 71. The neat cement specimens hardened in air are very irregular, as usual. These specimens showed high strength at three months, suffered a marked falling off at six months and one year, but showed a remarkable strength, equal to neat Portland cement, at two years. The strength developed at one year by specimens of this sample containing two parts sand to one cement and hardened in air, is also equal to that shown by similar mortars of Portland cement.

Table 72. Rate Of Increase In Strength In Water And Air

All cement, Brand Hn, Sample 26 S, which fails in water after two years, see Table 71.