The Effect of Clays on the Properties of Masonry Cement

mortar plasticsier bricks and mortar


Portland cement is not a material of great antiquity. It was discovered only a little over a century ago and  quite by accident. It has been the custom in the manu- facture of hydraulic limes to argillaceous limestones to temperatures not greatly above that at which the carbon dioxide would be liberated, but this process was carried out in a stack furnace under conditions such that some parts of the material were heated to temperatures which produced sintering. These hard lumps of sintered material were discarded because they were so much more difficult to grind than the unsintered material. However, in Aspdin ground some of these hard lumps and, in confirmation of a report of Vicat, found that they produced a cementing ma-terial far superior to any of those formerly produced by the old process. To this the name "portland cement" was given because of a similarity in appearance, when made into concrete, to a natural rock quarried on the Isle of Portland in England.

Cement manufacturers are naturally interested in new developments and one of these is use of clay as a plasticiser in cements to be used by masons. The object of this investigation is to study the effects of clay admixtures to masonry cement.


Masonry cement usually consists of a mixture of Portland cement and hydrated lime. At present, the manufacturers of cement have gone into the production of prepared masonry cements by adding such materials as finely ground limestone, finely Fround shale, or clay refuse from common clay ware manufacturing plants. In the last few years, efforts have been made to correlate properties of the admixed material with the properties of the cement. However Grant, in 1881, reported the results of tests on a sample of well washed sand compared with one containing 18 per cent of clay in a mortar composed of one fert of cement and two parts of sand. The most important function of a brick masonry mortar is to bind the masonry units into a strong durable structure. Insofar as strength is concerned, Portland cement mortar is more than adequate, but number of other properties are concerned in establishing their utility.

 The lime technology Iiterature is replete with information on mortars. Some differences of opinion exist in regard to matters of detail, but there is a general agreement that the following properties are determining factors in the quality of brick masonry mortar;

1. The should have excellent adhesion to the brick. This is especially important for the vertical joints which are not under pressure and hence may be subjected to ten- sile stresses.

2. It must produce maximum waterproofness of the wall by reason of its durability of adhesion under conditions of service.

3. The mortar should have such body character- istlcs that it will not fall away when the joints are formed, thereby saving mortar.

4. It must have good plasticity and workability, thereby making it possible to lay more wall per day and permitting better trowelling which is conducive to good wokmanship.

5. It shouldn't be an attraction for moisture and the sealing of the pores in the mortar, retain water long enough and in an  for the cement to set properly.

6. It should not dry out on the mason's boards.

7. The mortar must not burn or bleach mortar colors, nor should it be injurious to the skin on hands.

8. It must be of superior quality in its resistance to deterioration by heat.

9. It should be made of ingredients which do not have to be added in the form of a putty and of such a character that tender's labor is reduced to a minimum in making the mortar.

10. It must not cause efflorescence on the brick work.

11. It should remain fairly constant in volume under conditions of service.

12. The mortars should be resistant to freezing to insure a durable bond in freezing climates.

Some plasticizers function effectively in one or more respects but are deficient in others. The most satisfactory plasticizer is the one that performs all these functions in maximum degree at minimum cost.

It was not until quite recently that the quality of clay as a masonry mortar plasticizer was demonstrated. Probably the first use of clay as a plasticizer in mortar was about 1884 when a cement mortar containing clay was used in constructing some kilns in Brazi, Indiana. It was claimed that these mortars proved to be more resistant to weathering than those in which other plasticizers were used. In 1888 a building was erected by McCorel, Regan Co. near Brazil with mortar containing fire clay substituted for one half of the cement. Thirty-five years later, an inspection of this building showed the mortar to be in an exceptionally good condition.

According to a fire clay stratum in western Pennsylvania at Kittanning yields, on hard burning, an extremely vitreous brick of very low absorption, pure white in color. For many years the brick makers themselves have bean using a mortar made of portland cement With about 25 per cent of this clay admixture. The writer has examined such mortar after it had been exposed for ten years and could detect no evidence of weather. He then made some similar mortar and placed in a very exposed position and has been able in five years to observe no evidence of weathering action. Sharp points purposely left have remained apparently as sharp as ever.

In pioneer days clay was often the only material for mortar and had to be renewed every year or two. When lime became available and has been used for a great many years, but its durablility in chimneys and other exposed positions was unsatisfactory so that when portland cement came into use many went too far to secure strength and durability.

By proper proportioning of portland cement and lime, with special regard to the absorption rate of the masonry unit, joint thickness and season of the year, satis- factory workability and durability are secured. Valuable and well balanced results have been obtained by replacing part of the lime with properly ground limestone, the end product of the lime. Others have used various clays and shales, and when their choice has been fortunate, often very excellent results have been obtained.

As a resuIt of the manufacturers experiences, and the many engineering tests which Iowa state College at Ames has made, it is now believed that any clay plant having a clay not too high in alumina and organic matter can prepare a  mortar mix which will pass specifications.

In general, the presence of clays in sands for concrete and mortar has been rightly regarded with suspicion, because most clays in sands are contaminated with organic matter which interferes with the setting and hardening of the portland cement. In addition, many clays contain appreciable quantities of alkali metal compounds which are set free in the mortar or concrete by base exchange with lime. These alkalis then are apt to appear as un- sightly efflorescence on the surface of the wall. The sodium hydroxide test used on sands for concrete will
a satisfactory estimate of deleterious organic matter, while an efflorescence experiment provides information concerning alkali compounds. Moisture is drawn up through the specimens and on evaporating from the sides leaves an encrustation of alkali salts if present. If practically none appear within two weeks the clay may be regarded as satisfactory.

1924, Johnston Bros. Company of Fort Dodge, Iowa, developed a market among silo builders for mortars containing fire clay which they called Mortar Mix.

In 1930 and 1903, Spangler reported the results of tests on mortars containlng clay and found that when ground clay was used to 35-50%, (by volume) in place of the cement, mortars were produced which were at least as strong and durable as those with an equal percentage of hydrated lime. In 1935, Withey and Wendt reported the results of some tests on mortars containing clay and concluded that a 1 cement and 3 sand mortar with to 25% of cement replaced by clay may be used where high strength and weather resistance are required.

In 1936, Collins reported the results of some tests on mortars containing clay and found that they were of high quality. Among other things, he also found that with the exception of the straight cement mortar, the mortars containing rock dust, which may be comparable with the ground limestone in the masonry cement used in the investigation, ranked first in weather resistance, tensile and compressive strengths, and absorption. However, the plasticity and bond strengths are not satisfactory.

 A more recent investigation on the subject was carried on at the New York state College of Ceramics, N. Y. These tests were conducted to demonstrate the utility of fine ground New York state clays as plasticizers in masonry mortars. It was found that the substitution of finely ground New York state clays for lime in mortars raised the average bond or adhesive strength about 5.5%. The average compressive strength was increased 36%, and the average tensile strength was raised 40% by the substitution of clay. Greater resistance to freezing and thawing was obtained with the clay mortars and the shrinkage was less than corresponding mortars containing lime. Water retentivity was higher. In general, the mortars made with the New York state clays compared favorably with those made in Iowa.

The recuirements of the clay admixtures which are to serve as plasticizers and pozzolanic materials are of high elasticity and 14 to per cent solution of silica. This latter property is accomplished by carefully heating a suitable plastic clay at 500 to 600 C until the desired silica is liberated in the amorphous state by the expulsion of part of the chemically heId water.


From the tests made, it was found that of all the clays used, Kentucky Ball clay and Zadoc stoneware clay gave the best results, considering the water retention as one of the main criteria. to the Missouri Fortland research laboratory, IlThe mortar made with Kentucky Ball c1 ay was the best mortar of this series of tests. It was better than the straight Red Diamond mortar in all respects. The mortar containing Zndoc was not as good as the Kentucky Ball clay, but was better than all the other materials It is suggested that further work be done on the theory concerning the possible stabilising influence of the clay in the oil industry, clay is used as a stabiliser but this is in a different sense of the word.



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