Table 98. Effect Of Clay On The Tensile Strength Of One-To-Three Mortars

Note. —Tests by Messrs. J. J. Richey and B. H. Prater.

385. Conclusions

Always keeping in mind the limitations to be observed in drawing general conclusions from experiments having a limited range, it may be said that the indications are as follows: Neat cement and rich mortars are injured by the addition of clay, the rate of hardening and the ultimate strength being diminished. Lean mortars containing three to four parts sand to one cement are usually improved by the addition of clay to the amount of 40 to 100 per cent, of the cement, or 10 to 25 per cent, of the combined weight of cement and sand, and the ability of such mortars to withstand early immersion may be greatly enhanced by such additions. It is evident from the above tests that the expense which should be incurred in washing sand to remove a small percentage of clay is limited, and for certain uses there is no question that mortar may be improved by the addition of clay.

1 Messrs. J. J. Richey and B. H. Prater, Technograph, 1902-3.

(For the effect of clay on the compressive strength of concrete, see Art. 55).

386. Powdered Limestone, Brick, Etc

Various foreign substances are sometimes used with cement, either in lieu of sand, or to make the mortar more plastic. Such foreign ingredients may also occur in mortar as impurities in the sand used. Powdered limestone, slaked lime, powdered brick and clay are some of the materials experimented with in this connection. A few tests of the effects of such mixtures on the setting time of cemerit indicated that the rate of setting of Portland cement was not appreciably affected by the addition of twenty-five per cent, of any of these substances, but the setting time of natural cement appeared to be sensibly hastened by such additions. None of these materials had any appreciable effect on the soundness of either Portland or natural.

Table 99. Foreign Substances In Cement Mortar

Notes. —Sand, "Standard." Materials added to mortar were first pulverized and passed through No. 80 sieve, holes .007 inch square.

5-day results, H = immersed in hot water, 80° C. 5-day results, h = immersed in hot water, 60° C. Composition of mortars: — a — No foreign substance.

b — No foreign substance, but additional amount cement added, making mortar 1 to 3 instead of 1 to 3.75.

c — Kelleys Isd. Limestone, equal to 25 per cent, weight of cement added to mortar.

d — Slaked lime powder, equal to 25 per cent, weight of cement added to mortar.

c — Red clay, equal to 25 per cent, weight of cement added to mortar.

f — Red brick, equal to 25 per cent, weight of cement added to mortar.

Table 99 shows the effect on the tensile strength of mortar of adding twenty-five per cent, of each of the four substances mentioned. It appears that the strength of neat cement mortar, either Portland or natural, is usually diminished by the presence of such materials, but in almost every case mortars containing about four parts sand to one cement are improved by the addition of the substances in question to an amount equal to twenty-five per cent, of the cement. Pulverized clay and brick give the best results, the increased strength amounting to from twenty to forty per cent.

387. Sawdust

Where a very light and porous mortar is desired for use in floors and similar purposes, the incorporation of sawdust in the mortar is suggested by a similar use in clay building materials. The results in Table 100 show that the use of sufficient sawdust to materially diminish the weight, practically ruins the cohesion of the mortar, even ten per cent, of sawdust materially diminishing the strength.

Table 100. Sawdust In Cement Mortar

Notes. — Sand, crushed quartz, 20/40. Sawdust from white pine, passed through sieve with one-quarter inch meshes. a — Briquets broken in applying initial strain. b — Briquets disintegrated in tank.

388. Use Of Ground Terra Cotta As Sand

A light weight mortar may also be made by using as sand or aggregate, materials of burned clay, such as brick or terra cotta. The tests in Table 101 were made to determine the value of ground terra cotta for use in place of sand, and it appears that this material gives excellent results. The strength given with one of the brands of natural cement is especially high.

Table 101. Use Of Ground Terra Cotta As Sand In Cement Mortar

Notes. — Terra Cotta tile, of medium burn, ground and passed through No. 20 sieve, and used in place of sand.