369. Mixture Of Portland And Natural Cements

For certain uses mortar is sometimes made from a mixture of Portland and natural cement, with the idea of retaining some of the properties of the Portland without involving the expense of using a clear Portland mortar. Several tests have been made to determine the rate of hardening and the ultimate strength of such mixtures.

The mortars used in the tests given in Table 84 contained two parts sand to one of cement, and the cement was composed of one-eighth, one-quarter and one-half Portland to seven-eighths, three-quarters, and one-half natural. Mortars made with Portland alone and with natural alone are included for comparison. It is seen that the mortars containing some Portland harden more rapidly than the natural cement mortar, so that the increased strength developed at short periods is more than proportional to the per cent, of Portland used. The results obtained at two and three years, however, indicate that mortars containing only a small proportion of Portland, as one-eighth or one-quarter, do not give a higher ultimate strength than is obtained with clear natural cement mortar.

Table 84. Tensile Strength Of Mortars Made With Mixture Of Portland And Natural

Notes. — Portland cement, Brand R, Sample 42 M.

Natural cement, Brand Gn, Sample 54 R.

Sand, two parts of " Point aux Pins," pass No. 10 sieve, to one part cement by weight. All briquets made by one molder and immersed in one tank. Each result, mean of ten briquets.

370. In Table 85 four kinds or mixtures of cement are used, Portland, natural, an "Improved cement" or a cement sold as a mixed cement, and a sample made by mixing twenty per cent, of the Portland with eighty per cent, of the natural. The first point noticed is that the "Improved" cement does not exhibit the early hardening properties due to the Portland cement in its composition (if any), as strongly as the sample containing twenty per cent. Portland. In only two tests did the "Improved" cement give a higher strength than the clear natural. The results of the two-year tests are of interest as showing how nearly the same ultimate strength is shown by the four samples. The sample of natural cement is of exceptional quality.

Table 85. Comparisons Of Portland, Natural, And "Improved" Cements

371. Conclusions

It appears from these tests on the effect of mixing Portland and natural cements that, in general, the full strength of both cements is developed in the mixture; that in the early stages of hardening, the mixture sometimes exhibits more nearly the properties of the Portland, gaining strength quite rapidly, but that the ultimate strength of mixtures containing small amounts of Portland are sometimes as low as mortars made with natural cement alone. It cannot be stated that all samples of Portland and natural cement will give as good results in combination as those obtained in the above tests, and any extended use of such mixtures should be based on full tests of mixtures of the brands that are to be used in combination.

372. Free Lime In Cement

The presence of free lime in cement is known to be a serious defect. Table 86 gives the results obtained by adding ground quicklime to Portland cement in one-to-two mortars. It appears that eight per cent, quicklime reduces the strength at six months about twenty-five per cent., and smaller amounts of lime produce approximately proportional decrements. The seven-day results, both hot and cold, show greater proportional effects. The free lime occurring in cements as a result of defects of manufacture is likely to be much more dangerous in character than the lime used in these tests.

373. The Use Of Slaked Lime With Cement

A small quantity of Portland cement is frequently added to lime mortar to hasten the hardening and improve the strength. The addition of a small amount of slaked lime to Portland cement mortar is also practiced. This not only cheapens the mortar but renders it much more plastic, or less " brash," in mason's parlance. It is very difficult to lay bricks in a full mortar bed with Portland cement mortar containing two or three parts sand to one cement, and to use a richer mortar is usually too expensive. The work is very much facilitated by mixing a little slaked lime paste or powder with the mortar.

Table 86. Mixture Of Ground Quicklime With Portland Cement

Notes.— Cement: Portland, Brand r, Sample 83 t.

Lime

Quicklime ground to pass No. 100 sieve (holes .0065 in. sq.).

Sand

Standard crushed quartz, 600 grams, to 300 grams of cement plus lime. Per cent, of lime given replaced the same weight of cement; thus: for "4 per cent, lime " the mortar contained 288 grams cement, 12 grams lime and 600 grams sand. All briquets made by one molder; each result, mean of five briquets.

374. The tensile strength of such mixtures is shown by the tests in Tables 87 to 89. In the mortars of Table 87 a sample of Portland cement is mixed with slaked lime in two forms, paste and powder. When the briquets are hardened in open air the addition of ten to twenty per cent, of CaO in the form of lime paste decreases the strength about twenty-five per cent. ; seven per cent, of lime in the form of slaked, dry powder has, however, no deleterious effect, and even twenty-eight per cent, gives no serious decrease in strength. For water-hardened specimens the addition of twenty to thirty per cent, of lime in the form of paste appears to increase the strength twenty per cent, and no deleterious effect is shown by the addition of forty per cent. Also for water-hardened specimens, seven to twenty-eight per cent, of CaO in the form of slaked powder increases the strength nearly twenty per cent. It thus appears that the addition of lime gives better results in mortars that are to harden in water, and that for air-hardened mortars lime powder should be used in preference to lime paste. Similar tests of seven-day briquets showed the lime paste to retard the hardening of the mortar.