398. The Adhesion Between Portland And Natural Cements

The question sometimes arises as to whether Portland cement will adhere to natural .cement already set, and whether fresh natural and Portland cement mortars may be used together, as in the case of a Portland facing mortar used with natural cement concrete. Tests bearing on these points are given in Tables 111, 112 and 113.

In the tests in Table 111 fresh Portland cement mortar was applied to natural cement mortar that had set seven days. Natural cement briquets, made neat and with one to four parts sand, as seen in the headings of the columns of the table, were broken at the age of seven days. The fresh Portland mortar was applied to the half briquets on the same day that the latter were broken, by placing the half briquet in one end of the mold, and filling the other half of the mold with fresh Portland mortar of the composition shown in the second column.

Table 111. The Adhesion Of Portland Cement To Hardened Natural Cement Mortar

Notes. — Briquets of natural cement, containing parts sand indicated at top of columns, were broken at seven days. The half briquets were then placed in one end of briquet mold and the other end of mold was filled with fresh Portland mortar.

Fresh mortar made of Portland cement, Brand R.

Sand, " Point aux Pins," passing No. 10 sieve.

In general, each result is mean of ten briquets.

Nearly all briquets broke at juncture of Portland and natural mortars.

It is seen that the neat Portland gave the highest results in adhesion, the one-to-one mortar giving a comparatively low adhesive strength. It is also seen that the neat and one-to-one-mortars adhered best to the richer natural cement briquets, but the one-to-two Portland gave the greatest adhesi ve strength with the poorer natural cement mortars. All of the tests gave very irregular results.

399. To make the briquets the results of which are recorded in Table 112, a plate was placed in the center of the mold, one-half of the mold was filled with fresh natural cement mortar, the plate was then withdrawn and the other half of the mold filled with fresh Portland mortar. Briquets in line 1 were made with Portland cement alone, while those in line 2 contained only natural cement, these briquets being made for purposes of comparison. The briquets containing both Portland and natural were made neat and with from one to three parts sand. By noting the number of briquets that broke at the juncture between Portland and natural, it was found that, in general, the adhesion of rich Portland mortar to rich natural cement mortar is greater than the strength of the natural, but that with the poorer mortars the adhesion is less than the strength of the natural.

Table 112. Adhesion Between Fresh Mortars Of Portland And Natural Cement

Notes. — Portland Cement, Brand R.

Natural Cement, Brand An.

Sand, " Point aux Pins," passing No. 10 sieve.

Both mortars mixed fresh and filled in opposite ends of mold.

400. In Table 113 the natural cement mortar contained three parts sand to one of cement, while the richness of the Portland mortars varied from neat to four parts sand. Four combinations of different brands were used. Brand R, Portland, and brand An, natural, appear to give the best results together. It is also seen from this table that the adhesion of the rich Portland mortar is greater than the cohesive strength of the natural cement, but when the Portland mortar contains three or four parts sand to one cement, the adhesion is less than the strength of the natural cement mortar.

401. The Adhesion To Stone And Other Materials

Since cement mortars are usually employed to bind other materials together, it follows that the adhesive strength is of the greatest importance. On account of the difficulty of making tests of adhesive strength, however, the data concerning it are very meager. Two methods have been employed by the author in making such tests. One method, used for brick, is to cement two bricks together in a cruciform shape. The other method consists in placing small blocks of the substance to be used in the center of a briquet mold, and filling the ends of the mold with the desired mortar.

Table 113. Adhesion Between Fresh Mortars Of Portland And Natural Cement

Notes : — In general, each result is mean of ten briquets.

Results marked N, briquets broke through the natural cement. Results marked J, briquets broke at juncture of Portland and natural.

The small blocks were made one inch square and about one-fourth inch thick, two opposite edges of each .piece being very slightly hollowed to fit, approximately, the side of the mold. These blocks being placed transversely in the center of the mold, and the ends of the latter filled with the mortar to be tested, formed two joints between the mortar and the block.

402. Table 114 shows the adhesion of a rich Portland cement mortar to various materials. The mortar adheres most strongly to brick, the adhesion exceeding the strength of the brick itself. A very high result is also obtained with terra cotta, and the adhesion to Kelleys Island limestone is high. The latter is a dolomitic limestone of the corniferous group, which is soft enough to be worked quite easily. The adhesion to Drummond Island limestone, which is a much harder stone belonging to the Niagara group, is considerably less, and the adhesion to the Potsdam sandstone is very low. A higher result than would be expected is obtained with ground plate glass, but the hammered bar iron gives the lowest result of any of the substances tried.

Table 114. Adhesion Of Portland Cement Mortar To Various Materials

Notes: —Cement, Portland, Brand R.

Adhesion Blocks,. 1 in. X 1 in. X 1/4 in. inserted in center mold.