How to Use Portland Cement From THE German OF L. Golinelli.

Progress, Volume I, Issue 10, 1 August 1906, Page 268

How to Use Portland Cement From THE German OF L. Golinelli.

FIRST PAPER. It is an established fact that Portland cement is superior to all other hydraulic materials, natural or artificial, and for this reason it is widely distributed throughout the world. Its use would, however, be far greater if the knowledge of the applications and methods of testing of cement had kept pace, during the past ten years, with the improvements which have been made in quality and methods of manufacture. Even in sections where the manufacture of Portland cement has been extensively and successfully developed, and where one would consequently expect to find a certain amount of knowledge of the subject, a correct understanding of methods of testing and intelligent use of cement is often painfully lacking. The preparation and use of cement mortar, as practised in many cases by masons, or their helpers, is not only imperfect, but wasteful. Better work could often be done with less material if careful methods were used. In the case of Portland cement such careful methods are especially necessary and profitable, and if they were followed by the common complaint that Portland cement is c too expensive would soon be no longer heard. Those who do intelligent and careful work have for a long time recognised the injustice of this charge. On the other hand, it is undoubtedly true that a highgrade material like Portland cement, which in skilful hands may be usefully and economically employed for an immense variety of purposes, is especially liable to suffer from ignorance and misuse.

I. PROPERTIES OF PORTLAND CEMENT. DEFINITION AND MANUFACTURE. Portland cement is a material which hardens in the presence of water, prepared by burning at a sintering temperature an intimate mixture consisting essentially of lime (or carbonate of lime) and clay m certain approximate proportions. The raw materials, clay and carbonate of lime, are ground and mixed according to their character in either the wet or dry way. If the dry process is used, the mixed materials are moistened with water and moulded into blocks. In the wet process the bricks are made from the wet material after it has been reduced to the proper consistency. After drying the bricks of cement material are burned in suitable kilns to the point of sintering. The resulting " clinker '" is ground to a fine powder ; this is the finished cement. In the most modern or rotary system, the raw materials are reduced to a very fine state by grinding in a dry or wet condition, and then burning, in that fine condition, m rotary furnaces lined with firebricks. The furnaces are on a slight angle ; the finely-pulverised raw materials enter at the upper end and travel slowly forward. The coal is ground almost as fine as cement, and is forced in at the lower end of the furnace, and explodes like gas, producing an intense heat, ranging from 2500 0 to 3000 0 Fahrenheit.

CHEMICAL COMPOSITION. The raw materials indicate the nature of the constituents of Portland cement. These are silica, alumina, iron oxide, lime and a small amount of magnesia. Alkalies and sulphates are also always present, and are derived from the rawmaterials, which are never found pure in nature. The presence of sulphuric acid (sulphate of lime) is also due to the sulphur in the fuel employed and to the addition of a small amount of gypsum (sulphate of lime) for the purpose of making the cement slow-setting. The composition of good Portland cement usually varies between the following limits : Lime . . . . . . 62 to 65 per cent. Silica . . . . 20 to 26 Alumina . . . . 7 to 14 Magnesia . . . . 1 to 3 Alkalies . . . . traces to 3 Sulphuric Acid . . traces to 2 According to the character of the raw material used, each manufacturer determines the correct composition of his product within the above limits, and this composition must be kept uniform by constant chemical analysis. The widespread belief that defective quality of cement is due to bad raw materials is seldom well founded ; the fault is generally due to incorrect proportions and careless manufacture. In studying the qualities of Portland cement, the following points are to be especially noted • 1. Form and fineness of gram. 2. Colour and specific gravity. 3. Time of setting.

4- Hardening. 5. Strength. 6. Constancy of volume. 7. Hair cracks and shrinkage cracks. 8. Behaviour under extreme heat and cold. 9. Additions and adulterations.

FORM AND FINENESS OF GRAIN. When examined under the microscope, particles of hydraulic lime have a more or less, rounded form. Portland cement, on the other hand, shows thm leaflets of shale-like structure, like pounded glass. The high quality of Portland cement is in part due to this shale-like character since greater density of mortar results from the greater surface of contact and smaller proportion of voids between the particles. As to fineness of grinding, it may be mentioned that the coarser particles of cement act practically like sand. It is therefore important that the grinding be not too coarse. A residue of not more than 5 per cent, on a sieve of 75 meshes to the linear inch may fairly be demanded. The finer the grinding, the more sand can be used with the cement. It should be remembered, however, that poor cements, especially those too high in clay or imperfectly burned, are especially easy to grind to great fineness. Such cements may be generally recognised by their yellowish colour and the spotty appearance of the work.

COLOUR AND SPECIFIC GRAVITY. The colour of Portland cement should be a greenish gray ; a yellowish or reddish gray tint indicates

generally an inferior, light-burned product, or one to which under-burned or " dusted " material has been added. Portland cement has a high specific gravity, a quality which contributes to its high value. No other hydraulic material has so high a specific gravity, or yields so dense and resistant a mortar. The specific gravity of Portland cement is from 3.12 to 3.25.

TIME OF SETTING. When Portland cement is mixed with a suitable quality of water, a plastic paste is produced, which after a time becomes hard. The change from a semi-liquid to a solid mass is called setting and the time required for this change, the time of setting. Cement is said to be set when it resists a light pressure of the finger nail on the surface. It is of the greatest importance to know the time of setting of a cement which it is proposed to use, since a cement which has become set and has been again mixed up with water, possesses little or no hydraulic energy. The mistake of mixing " seL " mortar anew with water occurs only too often, and gives rise to the unjust complaint that the cement does not harden or possesses no strength. Great care should therefore be taken to mix only such a quantity of mortar as can be conveniently used up in the time available ; this can be easily managed when the time of setting is known. Remnants of mortar which have become set should be discarded and must under no circumstances be again worked up with water. The determination of the time of setting is extremely

simple, and may be made by any mason at the place where the cement is used. The best method is as follows : The cement is mixed with water to a thick paste, worked one to two minutes with a spoon or trowel and spread out in the form of a pat on a glass plate. This pat should be about one-half inch thick in the middle and thin at the edges. As soon as the surface resists a light pressure of the finger-nail the cement is set. Since the temperature and the proportion of water used are of great influence on the result, it is best to have the cement and water at the ordinary temperature of 60 to 70 degrees, and to use not more than 30 to 32 per cent, of water. The water must be clean. The pat should be protected from sunshine and drafts of air. It is in the power of the manufacturer to produce either quick-setting or slow-setting cement, as may be required. A cement which requires two hours or longer to set is called slow-settmg. Such cement is preferable to that which sets quickly, on account of its greater strength. Quick-setting cement is used only for certain purposes. Slowsetting cement can be made to set more quickly by using warm water, and also by limiting the water used to the smallest possible quantity. Among the substances which modify the time setting may be mentioned : Potash and soda, which hasten the setting. Sulphates a.nd calcium chloride, which retard the setting. In all cases the consumer will do well to notify the manufacturer what time of setting is desired, also for what purpose the cement is to be used.

In this way many unjust complaints in regard to quality, and many unnecessary expenses, may be avoided.

HARDENING. The set cement is capable of hardening, either in water or m air, and in a short time will acquire a high degree of strength. The processes of setting and hardening should not be confused. The latter begins at the point where the former ceases, and continues up to the highest strength which the cement attains after the lapse of many years. As to the nature of the hardening process, to account for which various theories have been advanced, it need only be here stated that the hardening consists in chemical combination with water taking place under certain physical conditions. Among the most important of these conditions are rest durmg the setting, and protection from too rapid drying out. The latter point can not be too strongly insisted upon. If the cement is deprived of the necessary water it can never reach its full hardness.

STRENGTH. Portland cement attains within a few days a high degree of strength. In the use of cement for building purposes compression strength is the quality generally required. Cement is, however, generally tested only for tensile strength, owing to the fact that the tests of tensile strength can be made much more quickly, simply and cheaply than those of compression. There is also a definite (though by no means exact) relation between the two tests,

the compression strength being generally from 8 to 12 times the tensile strength. The German official standards for Portland cement require that a mixture of one part cement with three parts normal sand shall show a tensile strength at 28 days of at least 2271D5. per sq. in. It is by no means simple, however, to make tensile strength tests m such a manner as to give reliable results. Complaints m regard to the quality of cement are often due entirely to faulty testing. In the preparation of briquettes, the temperature and quantity of the water used, the character of the sand employed, and the thoroughness with which the mortar is worked, are of immense influence on the results. The strength will generally be greater the less water used ; nevertheless it is always necessary to use such a quantity of water that it shall show itself on the surface of the briquette on tamping it into the mould. Long and vigorous working of the mortar increases its strength. In extensive building operations the use of mixing machines, especially pans with edge runners, is therefore highly advantageous.

CONSTANCY OF VOLUME AND CRACKING. Strictly speaking, there is no such thing as constancy of volume, either in the case of mortar or stone, since heat and cold, wetting or drying, modify the volume more or less. Portland cement also suffers changes of volume on hardening in water or in air. In the case of good Portland cement, however, these changes are extremely small and much less than those which occur in different kinds of stone. Bad cements, on the other hand, may show the dangerous quality of cracking or swelling. This shows itself in a strong expansion, which destroys the cohesion of the mortar and may cause its total destruction. Cement which swells badly, if laid between retaining walls, shows an immense power of expansion, even to the extent of forcing out the stones of the masonry. The swelling does not show itself until after the setting. The worse the fault is the sooner it will appear. It shows itself, also, sooner m water than m air. In pats of cement kept under water this defect is to be noticed in the appearance of fine net-like cracks, or in worse cases in curving Df the pats and the appearance of cracks around the edges. It is characteristic of expansion cracks that they run from the edges toward the centre Df the pat and are widest at the edges and narrower toward the centre. These expansion cracks should

not be confused with shrinkage cracks, mention of which will be made later. The swelling of cement is always due to defects in manufacture. These are : 1. Faulty composition of the raw material, especially too high a proportion of lime. 2. Imperfect preparation of the raw material. 3. Imperfect burning of the clinker. 4. Too high proportion of sulphate or magnesia. According to the German official requirements, a cement is considered to be constant in volume if a pat, kept 28 days under water, remains perfectly flat and free from cracks. Swelling, due to too much lime, shows itself in this test with certainty within a few days or weeks. Cement containing too much magnesia, however, and burned to the point of sintering, shows noticeable expansion only after the lapse of long periods, extending even to several years. Only chemical analysis, or the guarantee of the manufacturer, can afford protection against the danger of expansion from excess of magnesia. Experience has shown that the presence of magnesia up to 3 per cent, is entirely harmless. In conclusion, two other peculiar appearances may be mentioned which are often erroneously considered to indicate swelling of the cement. It is sometimes noticed that pats of neat cement, left m air, lose considerably m strength, and after a certain time become soft or friable, while similar pats kept in water are faultless in all respects. This is especially liable to occur in the case of pats made very wet and allowed to dry out immediately after setting If, on the other hand, the pats are kept moist during the first stages of hardening, this defect is not developed. Cracks, similar to those produced by swelling, are also produced when placed in water too soon, or before the setting is complete. To prevent this the official requirements specify that test pieces shall be kept 24 hours in moist air before placing in water.

SHRINKAGE CRACKS AND HATR CRACKS. Portland cement mortar without sand, exposed to the air, diminishes in volume. If the diying takes place gradually and uniformly, as in a closed room, the cement shows no defects. Too rapid drying, in draughts of air or in sunshine, without the precaution of keeping the cement moist, causes so-called shrinkage cracks. These may be distinguished, in pats of cement, from expansioncracks by the fact that they appear during the

setting and show themselves as irregular curved lines extending over the middle of the pat. As already stated, the formation of shrinkage cracks is due to faulty use of the cement, and has practically nothing to do with its quality. Very finely ground cements are, moreover, more likely to show hair cracks than those which are more coaisely ground. Hair cracks appear as fine lines on cement work which has stood some time. They are especially to be noticed on cement which has lam in the open air, and are dtfe to frequent changes between wet and dry conditions. Hair cracks and shrinkage cracks occur chiefly when pure cement or mortar too rich m cement is used. They may be certainly avoided by the addition of sufficient sand and suitable treatment of the work. (To be continued.)