380. Effect Of Clay On Cement Mortar And Concrete

Clay may occur in cement mortar or concrete due to the use of sand or aggregate that is not clean. As the plasticity of cement mortar is increased by the presence of clay, small amounts are sometimes added to produce this effect, and clay is also sometimes used to render mortar stiff enough to withstand immediate immersion in water. In the case of concrete, the presence of a certain percentage of clay renders it easier to compact the mass by tamping, though if too much clay is present, the mass becomes sticky.

A number of tests have been made to determine the behavior of such mixtures of clay and cement. In all of these tests the clay was first dried, pulverized and sifted, and then a weighed quantity equal to a given per cent, of the weight of the cement was added to the latter. In the writer's first tests of this kind small percentages of clay were used, less than ten per cent., but it was found that with lean mortars much larger percentages must be used to determine the point where clay began to be injurious.

381. Table 93 shows the effect of clay on the time of setting and soundness of neat cement. The effect of small percentages of clay on the time of setting of Portland cement is not very marked, but with natural cement even ten per cent, of clay retards the setting in a marked degree. As to the effect on soundness, Portland cement pats disintegrate with more than twenty-five per cent, of clay added, while the natural cement is affected if more than ten per cent, of clay is present.

382. Table 94 shows the tensile strength of neat cement mortars to which clay to the amount of 10 to 100 per cent, of the cement has been added. Some of the Portland briquets were immersed as soon as molded, while others were left the customary twenty-four hours in moist air before immersion.

It is seen that to mix clay with neat Portland cement results in a decided decrease in strength, the results obtained with twenty-five per cent, clay being only about sixty or seventy per cent, of the strength of the mortar without clay. With natural cement the presence of clay seriously retards the hardening and results in decreased strength, though it does not have as deleterious an effect as it does with Portland. The mixing of clay with neat cement is of course very severe treatment.

Table 93. Effect Of Pulverized Clay On The Time Of Setting And Soundness Of Cement

Note. — Results marked a, pats cracked badly in air and were not immersed.

Table 94. Effect Of Clay On Tensile Strength; Neat Cement Paste

In Table 95 the mortars contain equal parts cement and sand, and the clay is from 50 per cent, to 200 per cent, of the weight of cement. It appears from this table that clay in as large amounts as 50 per cent, of the cement is injurious to one-to-one mortars of either Portland or natural cement.

383. The mortars in Table 96 are all of Portland, and contain three parts sand to one cement. Smaller percentages' of clay are used, namely, 10 to 40 per cent. The mortars hardening in water show a decided improvement due to the presence of clay, but the briquets hardening in the open air indicate that at two years the mortar without clay is stronger. It may be noted in passing that these results, obtained at two years, with one-to-three mortars hardened in open air, are very high.

Table 95. Effect Of Large Amounts Of Clay In Mortars Containing Equal Parts Cement And Sand

Table 96. Effect Of Clay In Portland Cement Mortar Containing Three Parts. Sand To One Cement

Notes. — Cement, Portland, Brand R, Sample 83 T.

Sand, three parts crushed quartz 20/30 to one cement by weight. Clay, red clay dried, pulverized, and passed through No. 100 sieve.

Clay added to mortar, amount cement and sand remaining constant.

The effect of clay on mortars containing four parts sand to one cement is shown in Table 97. In this case the addition of clay equal to the weight of the cement almost invariably results in increasing the strength of the mortar. Briquets immersed as soon as made were especially benefited by the presence of clay, except in one case, the red clay did not appear to increase the ability of the natural cement Gn to withstand early immersion. The red clay appears to give better results than the blue with Portland, while the reverse is true with at least one brand of natural. Whether this difference is a chemical or physical one is not known; the red clay is a good puddling clay, while the blue clay is not, but appears to contain some very fine sand.

Table 97. Effect Of Large Amounts Of Clay In Cement Mortars Containing Four Parts Sand To One Cement

Notes.—Sand, crushed quartz 20/30, ("Standard"), four parts to one cement by weight. Clay, dried, pulverized and passed through sieve before using. All briquets immersed in tank in laboratory as usual. Each result, mean of five briquets.

Results marked "a," briquets disintegrated some on face from early immersion.

384. Table 98 gives the results of tests by other experimenters, showing the effect of clay on one-to-three mortars of Portland and natural cement.1 The amount of clay used in these tests appears to be stated as percentage of the total ingredients instead of as a percentage of the cement as in the preceding tables. The mortars were mixed quite dry for these experiments. The Portland cement mortar seems to be improved by the addition of clay to the amount of twelve per cent, of the mortar. The hardening of natural cement mortar is somewhat slower with twelve per cent, clay than with three to six per cent., but at the age of twelve weeks the mortars containing clay were all stronger than that without clay.