Table 87. Slaked Lime In Portland Cement Mortars

Cement

Portland, Brand R.

Sand

Crushed Quartz, 20-30, or "Standard." Age of briquets, 6 months.

375. In Table 88 only lime paste is used, but both Portland and natural cement are tested, and the specimens are hardened in dry air and damp sand. In the first column of results are given the strengths attained by Portland cement mortar containing three parts sand to one of cement without lime. In the second column, ten per cent. CaO in form of paste is added to the cement. In the third, fourth and fifth columns, respectively, ten, twenty-five and fifty per cent, of the cement is replaced by CaO.

It appears that ten per cent, of the cement in a one-to-three Portland mortar may be replaced by lime made into paste without diminishing the strength, if the mortar hardens in damp sand. Even in dry air exposure, it is only at one year that the lime shows any deleterious effect. To replace twenty-five per cent, or more of the cement with lime, however, diminishes the strength of the mortar in a marked degree.

In the case of natural cement, replacing ten per cent, of the cement with lime is decidedly beneficial, and even twenty-five per cent, lime gives enhanced strength, except for specimens hardened in dry air.

Table 89 gives similar results for one-to-four mortars and different percentages of lime, the briquets being hardened in dry air and damp sand.

Table 88. Use Of Lime Paste In Cement Mortars Containing Three Parts Sand

In all of the above tests the mortars containing much lime paste were not only more plastic, but somewhat wetter than the corresponding mortars of cement and sand alone, on account of the water contained in the paste.

376. The conclusion to be drawn from these tests appears to be that the addition of a small amount, ten to twenty per cent., of slaked lime to cement mortars containing as much as three parts sand, not only renders them more plastic, but actually increases the tensile strength, especially if the mortars are kept damp during the hardening. It also appears that for mortars exposed to the open air the lime should be in the form of slaked powder rather than paste. It may be added, that in all cases care should be taken that the lime is thoroughly slaked before use, and all lumps should be removed by straining or sifting. Further results on this subject are given in connection with the tests on adhesion of cement mortar to brick (Art. 5).

Table 89. Use Of Lime Paste In Cement Mortars Containing Pour Parts Sand To One Cement

Note.—All briquets three months old when broken.

377. Effect Of Plaster Of Paris On The Cohesive Strength Of Mortars

The use of plaster of Paris, or calcium sulphate, in the manufacture of cement to regulate the time of setting, has already been mentioned. The amount of such additions at the factory are usually small, the German Cement Makers' Association limiting it to two per cent.

Tests on three brands of Portland cement, showing the effect of small additions of plaster Paris, are given in Table 90. All of these mortars hardened in water. It is not known whether any of the cements had received additions of plaster Paris before leaving the factory. It is probable that brands R and X had been so treated, since they are German cements, but it is not probable that the other brands of Portland had received any addition of plaster.

It appears that with these brands the addition of from one to three per cent, of plaster Paris hastens the hardening and increases the strength of the mortar at ages of six months to two years. Six per cent, plaster sensibly retards the hardening, but, in all cases except one, Brand S, neat, six months, the mortars containing six per cent, plaster, gave higher results on long time tests than did the corresponding mortars to which no plaster had been added.

Table 90. Plaster Of Paris In Portland Cement Mortars, Hardening In Water

Notes. — Sand, Point aux Pins (river sand) passing No. 10 sieve, except for hot tests, where standard sand was used. Cement and plaster of Paris passed through No. 50 sieve before using. Plaster Paris had no apparent effect on consistency mortar at first, but after making first three briquets of batch of five, the mortar containing plaster Paris dried out somewhat. Each result, mean of five briquets.

Similar tests of natural cement mortars hardening in water are given in Table 91. One of the brands is not much affected by additions of one to three per cent., but the other brand is practically ruined by the addition of more than one or two per cent., and both brands are rendered quite unsound by six per cent, plaster.

Table 91. Plaster Of Paris In Natural Cement Mortars, Hardening In Water

Note. — Sand, Point aux Pins (river sand) passing No. 10 sieve, except for hot tests, where standard sand was used. a — Found badly swelled and nearly disintegrated after a few days in tank.

b — Surface cracks, 1 inch section swelled to 1 1/16 inches.

c — Surface cracks, 1 inch section swelled to 1 1/12 inches. Had nearly disintegrated after 2 days. d — Surface cracks. e — Badly cracked on surface.

f — Badly cracked on surface, and 1 inch section swelled to about 1 1/16 inches.

378. The briquets reported in the preceding tables were hardened in water, as usual. Table 92 gives some of the results obtained by adding plaster Paris to mortars that are hardened in dry air. The effects on the two samples of the same brand of Portland, one quick setting and one slow setting, are quite different. The strength of the quick setting sample is increased, two per cent, giving the best results, while that of the slow setting sample is diminished by the addition of plaster. Both brands of natural cement appear to be notably improved by the plaster, the best result being given by three per cent. Such an addition to one brand results in a remarkable increase in strength of 250 per cent.