HOW TO USE PORTLAND CEMENT. From THE German OF L. Golinelli.

Progress, Volume I, Issue II, 1 September 1906, Page 307

HOW TO USE PORTLAND CEMENT. From THE German OF L. Golinelli.

SECOND PAPER. RESISTANCE TO WEATHER AND BEHAVIOUR UNDER EXTREME HFAT AND COLD Mortar made from pure cement is, strictly speaking, not weatherproof, owing to its tendency to form shrinkage cracks and hair cracks. Repeated expansion and shrinkage increases the number and size of these cracks, until finally under the action of water and frost the cement breaks to pieces. Complete weather-resisting qualities can be given to cement mortar only by the addition of sand. One part sand to one part cement will be found sufficient. The temperatures which are reached in warm, or even hot, climates have no bad influence on the hardening of Portland cement. The only precaution necessary is to see that the water necessary for hardening is not removed too soon ; or, m other words, that the work is kept sufficiently moist during the earlier stages of hardening. Even the temperature of boiling water is harmless to the strength of Portland cement, and a heat of 400 degrees to 500 degrees F may be borne without injury. At a red heat it becomes soft and friable. Nevertheless, it has been proved by long experience with Portland cement concrete that this material shows a high degree of strength and safety when exposed to fire. Portland cement is less affected by frost than any other hydraulic material. If the cement

is once fully set it may be exposed to strong freezing without any ill effect. It is only during the setting that injury can take place, and this chiefly to be feared m case the mortar was made too wet, so that the freezing of the water forces the mass asunder. If water is sparingly used, masonry and concrete work may be done in extremely cold weather with good results. It must be remembered, however, that the setting and hardening of cement are much delayed by cold. Cement work done in freezing weather shows, therefore, but little strength at first, but finally reaches its normal hardness. Addition of salt to mortar in cold weather is generally to be avoided on account of the unsightly efflorescences which often result. It is better, when work m extreme heat or cold cannot be avoided, to warm the water and sand used and to reduce the water to the smallest possible quantity in order to hasten the setting of the mortar. If care is taken to allow no free water to separate, or that any excess of water is absorbed

by dry stone, there is nothing to be feared even from extreme cold. Surface plastering with cement should, however, not be attempted in freezing weather. On mixing cement with sea water the setting is delayed and decreased strength results. This is chiefly due to the action of the magnesium sulphate and chloride of the sea water on a portion of tbe cement ; the hardening value of this part is therefore lost and the strength attained is less than with the use of fresh water. One might suppose that this action of the sea-water salts would cause the strength of the cement to continue to fall off and that the work would finally fall to pieces. This is, however, not the case, owing to the fact that the penetration of the sea water into the mass is prevented by the great and constantly increasing density of the Portland cement mortar. A deposit of magnesia is also formed in the pores of the mass, and gives further protection against the entrance of the sea water. Portland cement is therefore unequalled for marine constructions. In all cases in which this work has been intelligently done, Portland cement concrete has fulfilled all requirements satisfactorily. In work exposed to sea water it is of the highest importance to give to concrete as dense and close-grained a surface as possible, since only such a surface is capable of permanently resisting the chemical action of the salt water and the mechanical force of the waves. Portland cement is especially suitable for work of this kind, since uniform tests of tensile and compression strength are a guarantee of a uniform material which can safely be relied upon. This is by no means true in the case of puzzuolana cements, so-called slag cements and hydraulic limes, which are often of very variable quality. Substances added to Portland cement may be divided into those which are intended to give it certain valuable qualities, and others which are added for the purpose of fraud. In the first group are found gypsum and colouring matters. The addition of gypsum (sulphate of lime) which

should not exceed 2 per cent, is made for the pur pose of causing naturally quick-setti q cements to set more slowly In this way the quality oJ the cement is improved and its strength increased Additions of a higher proportion of gypsum are not admissable, as it tends to cause swelling. 1 " Colouring matters are sometimes added to cement 111 order to make it suitable for decorative purposes. Mineral colours are practically always used. Te give the cement a somewhat darker tint, especially for use in making cement wares, a few per cent, ot lamp black is added. Nearly all colouring matters reduce the strength of the cement ; ultramarine, however, in small quantities, increases it. The red iron oxide of commerce, often used to produce a red colour, frequently contains a considerable amount of sulphuric acid, and may cause swelling. Care should be taken m the choice of the colouring matters employed. In regard to the production of white Portland cement it may be said that this has not hitherto

proved successful, on account of impurities contained in the raw materials or derived from the . fuel. The so-called white cements of commerce are for the most part inferior products which do not deserve the name of Portland cement. In consequence of its gray colour, Portland cement cannot be made white by the addition of pigments. From fraudulent and avaricious motives, blast furnace slag, limestone, shale, basalt, ashes, sand, etc., are added to cement. These are simply adulterations, which always injure the quality of the product. These substances may be more or less easily detected, and their use, in consequence of the close watch kept by the association upon the product of its members, has of late practically ceased.

USE OF PORTLAND CEMENT. Packing and Weight. Portland cement is packed in barrels and sacks. The barrel is of 400 lbs. weight and contains 380 lbs. (about 3^- cubic ft.) of cement. Sacks are generally one-fourth barrel, or 95 lbs. Since empty barrels and sacks are received by the manufacturer at a certain price, care should be taken to preserve them m an orderly manner. Empty sacks are most conveniently returned m bundles of ten each. Barrels and sacks should be marked by the manufacturer with name, trade-mark and gross weight of package. Loss by sifting out and variations from standard weight to the amount of 2 per cent are allowable.

Storage (Seasoning) of Cement. Storage of cement improves its quality. If well protected and kept dry the cement gams in strength and becomes more slow-setting and more constant m volume. This so-called seasoning results from the action of the moisture and carbonic acid of the air. At the same time, owing to the disintegration of the coarser grains, the cement increases in fineness. If, howeyer, cement is stored in a damp place it becomes caked, lumpy,

partially set, and finally worthless. Dealers should keep this m mind and give close attention to the choice of a suitable place for storage. Portland cement bears long storage well, and even if packed in sacks may be safely kept in a dry place for many months. If fresh, insufficiently seasoned cement is packed, an increase of volume takes place on long storage, and if the barrels are too completely filled or the hoops too strongly driven the hoops may be broken or the staves bent out. This may occur with faultless cements, and has nothing to do with so-called swelling or cracking.

Choice of Cement and Concrete Materials. Quick-setting cements are used for work exposed to leaching water, also for plastering and casts. For all other purposes slow-setting cement is preferable. Neat Portland cement is rarely used for mortar, and only in case the work is to remain constantly under water or in damp earth. In other cases more or less sand must be used to prevent shrinkage cracks and produce a weatherproof mortar. The character of the sand, gravel and stone used has great influence on the strength of the work. The sand must be sharp and puie ; if it contains clay this must be removed by repeated washing. Very fine sand is generally objectionable ; the best results are obtained with a graded sand, consisting of grains of various sizes, from fine to coarse. Stone and gravel for concrete must also be naturally

clean or well washed, and must not be soft or chalky. Only clean water, free from mud, should be used for mixing cement mortar.

Preparation and Use of Mortar. If good results are to be obtained with Portland cement great care must be taken m the preparation of the mortar. If vessels which have been used for lime or contain set cement are employed these must be well cleaned before use. As cement and sand are mixed m proportions by measure, it is a good plan to provide measuring vessels corresponding to the volume of the cement packages. The necessary figures are given in a previous paragraph under " Packing and Weight.' Guess-work mixtures should never be made, for such methods not only give mortar of varying composition, but also cause unnecessary waste of cement. The proportion of sand to be used varies with the character of the work ; i to 2 parts sand to 1 cement are used only for work requiring extraordinary strength, great resistance to wear, or impermeability to water ; 3 to 4 parts sand to 1 part cement are employed for ordinary weatherproof plastering, building mortar, foundations, artificial stone, etc. More or less water should be taken according to the rate of setting of the cement, conditions of temperature, and kind of work 111 view. As a general rule as little water as possible should be used, and the mortar made plastic by prolonged, vigorous mixing. For the preparation of mortar the measured quantity of sand is spread out and the correct amount of cement scattered evenly over it ; the two materials are then thoroughly mixed together. The necessary quantity of water is then strongly and thoroughly worked into the mass. One of the chief rules in preparing mortar is that cement which has become set should never be worked up anew with water. For this reason no more mortar should be mixed at one time than can be used before the setting begins. Mortar which has become set and is again made plastic with water will never gain its normal hardness.

Another important point is that the brick or stone used for building must be thoroughly wetted before being laid up with cement mortar. The work should also be kept damp for a considerable time after it is finished, in order that the mortar may reach its full, stone-hke hardness. The common practice of wetting bricks just before use by sprinkling water from the mason's brush is quite insufficient. The bricks should be kept under water, in a vessel, until thoroughly saturated, and a stiff mortar used, m order that the brick should not be displaced after laying.

Cement-Lime Mortar. There are many kinds of work which require a quick-hardening mortar, but for which the great strength of a mixture of i cement with i to 4 of sand is unnecessary. The cost of such mortar is also for many purposes, too high. A mixture of cement with 5 or more parts sand would give abundant strength, but such mortar works too " short " and adheres too imperfectly to the brick or stone ; it cannot, therefore, safely be used. In such cases the addition of slaked lime or hydraulic lime will correct the faults of poor mixtures of cement and sand, and will produce a cheap mortar, suitable for a great variety of uses. The addition of slaked lime allows the full advantage to be obtained from the use of good Portland cement, and makes it possible for this material to compete in price with cheaper hydraulic materials. Used in this manner, Portland cement may be employed

with economy for the most ordinary purposes. The advantages of Portland cement-lime mortar are its cheapness in comparison with other hydraulic materials, its rapid hardening, marked hydraulic properties, great strength on exposure to air, and remarkable resistance to weather. The following mixtures for cement-lime mortar have been found by experience to be most suitable •

The above proportions are to be taken by measure. Hydraulic lime may be used in place of ordinary slaked lime. Cement-lime mortar is prepared by making a dry mixture of the required quantities of cement and sand ; milk of lime is then made with the necessary quantities of lime paste and water, and this milk of lime thoroughly mixed and worked in with the mixture of cement and sand The great advantages of cement-lime mortar for a multitude of purposes deserve to be more widely recognised than they are at present.

Portland Cement Mortar in Water and in Air. On account of its remarkable hydraulic properties, great strength, and durability, Portland cement mortar gives excellent results both in water and in air. The first essential to success is, however, that the mortar should be prepared and used suitably and intelligently for the special purpose m view. For work to be exposed to water, care must be taken to produce as dense and impenetrable a surface as possible ; this may be accomplished by mixing the materials in suitable proportions. Ornaments, casts, etc., which are to be exposed to weather must never be made from pure cement ; the same may be said of architectural artificial stone work. A certain proportion of sand must always be added.

In preparation of water-tight mortar it should be remembered that the richer the mixture is in cement, and the longer the mortar hardens the greater will be its impermeability to water. The reason of this is that in the process of hardening the mortar becomes constantly more dense, and the pores gradually close. The porosity of mortar is the greater, the thinner the layer employed and the higher the proportion of sand used. Coarse sand, also, gives a more porous mortar than fine sand. For a coating 3-5 to 4-5 of an inch m thickness which requires to be immediately water-proof, the following mixtures are recommended :

Which of these mixtures is to be employed depends on the nature of the work. By the addition of gravel or broken stone to the above mixtures water-tight concrete is obtained. From motives of economy, however, it is customary to make only the surface of the concrete of water-tight composition.

Cement Plastering and Painting. Experience has shown that cement plastering on walls and cornices, even when made with faultless material, is not always permanent. Some-

times the surface scales off in thin layers ; in other cases blisters form and crack, or the whole layer of plastering may separate from the wall and fall off. In all such cases the work, and not the cement, is at fault. If walls are to be plastered with cement mortar it is indispensable that the surface be previously thoroughly cleaned from dust, dirt or hmemortar, washed and repeatedly wetted. If the wetting is insufficient the dry stone or brick takes away from the cement the water necessary for hardening , if this happens the mortar adheres badly and never reaches its proper hardness. Wallplastering which is exposed to weather should never be made very rich m cement for fear of the formation of shrinkage cracks. The more sand used, provided the necessary strength is obtained, the better the plastering will resist the weather. Fine sand should be used, and for the purpose of avoiding hair cracks and shrinkage cracks it is best to finish the surface with a felt polisher instead of a trowel or steel tool.

The accompanying illustration shows the beacon erected on Rangitoto Island, at the entrance to Auckland harbour, 20 years ago. The beacon is constructed of Portland cement, and is a familiar sight to those who journey m and out of the northern port. It is 84 ft. high, with a depth of 13 ft. below high-water mark. [ Photos supplied by Messrs. John Wihon & Co. Ltd.

Cement plastering must be kept moist and protected from wind and sun. This work can be done most advantageously in spring ; frost should be especially avoided. Some sands contain little particles of coal which are scarcely visible to the eye ; these may have a very bad effect on the appearance of the work. Cement work which is to be painted, either on account of hair cracks or efflorescences derived from the alkalies and lime of the cement, or from any other cause, must be fully hardened and thoroughly dry. It is prudent to let the work stand a year before oil paint is applied. To insure the durability of the painting several methods may be used. 1. The surfaces are repeatedly brushed over with diluted sulphuric acid (i part strong acid to ioo parts water), and allowed to dry before the paint is applied. 2. The surfaces are repeatedly and carefully washed with water, then after 8 days, saturated twice with linoleic acid (to be had from any druggist). After a few days, when the surface has grown hard, the oil paint is applied. 3. An excellent preparatory coating for oil paint is a solution of common water glass in 3 or 4 parts water. After two applications the surface is washed with water ; after a short time the waterglass is again applied. When dry the paint can be used. Even oil paint is, however, often of slight durability when exposed to weather. For work of great permanence the patent process of Drs. Gblinelli, Koch and Adamy of Darmstadt, for preparation of cement work for stereochromatic painting may here be mentioned.

Concrete. One of the most important uses of Portland cement is in making of concrete. This is a mixture of cement with sand and gravel or broken stone, with the addition of the necessary water. To obtain good adhesion of the materials, the water, sand and stone must all be clean ; in some cases washing of these materials may be necessary. The gravel or stone used must be at least equal in hardness to the cement mortar. Good gravel, basalt or hard lime stone are most suitable ; soft sandstone or broken brick are to be avoided. The stone should not be above egg size. The proportions to be used depend on the nature of the work. When once determined they should be rigidly adhered to. For the preparation of concrete, the cement and sand are well mixed together on a dry platform ; enough water is then mixed in to make the mortar about as moist as damp gardenearth ; the gravel or stone, previously well wetted, is then added, and the whole thoroughly mixed by repeatedly turning over with shovels. The proportion of water must be so regulated that after prolonged and vigorous stamping the mass will become elastic and show a little water on the surface. The use of more water is to be avoided, since it makes the mass less dense and lowers its strength. Too great stress cannot be laid upon thorough and careful stamping of the concrete into place, since in no other way can great strength and density be obtained. With good Portland cement the proportion of i part cement and 3 parts sand, with varying amounts of gravel or stone up to 9 parts, will be found suitable ; for some purposes poorer mixtures may be used. Portland cement concrete finds many useful applications in constructions both above and below ground ; for example, in foundations of all kinds, sidewalks, ceilings, walls, arches, cement wares, etc.

Preparation of Concrete Floors, Sidewalks, Steps and Ceilings. For the construction of durable cement floors or sidewalks the foundation must be suitably prepared. For outdoor work on yielding ground a porous layer, at least 10 inches thick, of coarse gravel or slag should be laid, well rammed down and levelled. For indoor work on dry ground it is sufficient to level the surface and stamp it down firmly. Floors and sidewalks are generally built in two layers ; a lower bed of concrete 2\ to 4 inches thick, and a surface coat of richer mortar of thickness of 3-5 to 4-5 of an inch. The concrete layer may be made richer or poorer according to' the service which the work must undergo. For heavy duty a mixtuie of cement 1, sand 3 and gravel 6is recommended. In less important work cement 1, sand 5 and gravel 10 will answer. A suitable mixture for ordinary requirements is, cement 1, sand 4 and gravel 8 or broken stone 6. The surface layer consists of cement 1, sand 1, and must be spread over the concrete before the latter has set. Before spreading the top layer the concrete should be freed from loose material and its surface roughed up. The mortar is spread with a straight-edge and when sufficiently hardened is finished with a wooden tool. A grooved roller is used to produce a ribbed surface, especially in side-walk work. When the work is finished and the cement is well set, the surface is carefully covered with a layer of sand 4 inches in thickness.

This is moistened and kept in place for several weeks if possible. Only in this way can the formation of hair cracks be prevented and a well hardened surface obtained. Cement expands and contracts with changes of temperature, in the same way as iron, wood, sandstone and other materials. From this cause, if the necessary care has not been taken in the work, cracks will result, especially in wide surfaces. These may be avoided by dividing the flooring into smaller blocks, which should not exceed 4 to 5 square yards in area, and should be separated by strips of tar paper or by sand joints 2-5 inch in width. The joints m the concrete must correspond with those cut in the surface layer. The division of the work into blocks is also to be recommended m concrete walls and curbs. One of the most strikingly successful applications of Portland cement in building is its use for ceilings and staircases. Safety against fire, freedom from dry-rot, quickness of construction, and cheapness are some of the many advnatages presented by this type of construction. A description of this and other similar applications of cement would, however, extend these articles beyond desirable limits. Reference must, therefore, be made to special works on this subject. As already stated, cement work must be protected from too rapid drying out, since from this cause shrinkage cracks and lack of hardness and strength may result. A certain amount of water is absolutely necessary for the proper hardening of cement. In conclusion it may be noted that the remarkable properties of Portland cement make it suitable for an endless variety of uses. It is hoped that this brief sketch may serve to bring this valuable material into more extensive notice, and to contribute to a wider appreciation of its qualities and capabilities.

•ement t, i, sand (not too coarse) lime paste ), I i i, „ 3. i. >. 5. i, „ 6, i 2

dement i, i, i, sand 5, lime paste 6 to 7, 8, i I, „ io, 2