Other Cements

OTHER CEMENTS Rapid Hardening Cements.—The necessity for overcoming the delay in urgent work caused by the comparatively slow harden ing of Portland cement has resulted in a demand for rapid harden ing cements. Such cements must be sufficiently slow setting to allow of the concrete being placed in position before the initial set of the cement has taken place, and should harden sufficiently rapidly for the shuttering to be struck or removed by the following day. Rapid hardening cement is also in great demand for pre-cast work, as the moulds can be removed and used again more fre quently, and it is particularly useful for road work where it is possible to lay the wood blocks, etc., on the day following the lay ing of the concrete, instead of waiting several days for the con crete to harden. The demand for such rapid hardening cement has led to the development of two different types of cement, one an improved Portland cement and the other an aluminous cement. The former is made in the same manner as ordinary Portland cement, but the variation in composition is much more restricted and great attention has to be given to correct burning and to very fine grinding of the raw materials and the finished product. The results of the tests and analyses of a cement of this type are as follows Fineness: Residue on i8oX 18o mesh per in. sieve . . • 0.4% Residue on 76X 76 mesh per in. sieve . . . . . Nil Soundness: Expansion (Le Chatelier test) . . . . . . o•5mm.

Setting time: Initial set . . . . . . . . . . ihr. 25m.

Final set . . . . . . . . . . 2hr. ism.

Tensile strength: Breaking strain in lb. per sq.in. of briquettes Iin.X Iin. in section. Neat (7 days) . . . . 8851b. 3 Sand to I Cement.

(24 hours) . . . . . . . . . . 6o21b.

(7 days) . . . . . . . . . . . 7051b.

(28 days) . . . . . . . . . . 7 6olb.

Compression test: Sand and Cement cubes (3 :1 ) at 24 hours gave a compression strength of S,000lb. per square inch.

Analysis: % Silica (SiO2) . . . . . . . . . . . 20.05 Insoluble residue . . . . . . . . . . Alumina (Al203) . . . . . . . . . . 6.52 Ferric oxide (Fe203) . . . . . . . . . 2.52 Lime (Ca0) . . 64.68 Magnesia (MgO) . . . . . . . . . . 0•95 Sulphuric anhydride (503) . . . . . . 2.26 Carbonic anhydride (CO2) and combined water (1120) . 2.22 Alkalis and loss on ignition . 0.45 I 00.00 Lime, after deduction for calcium sulphate . . . 63.10% Lime ratio . 2.83 The aluminous type of cement depends largely on mono-calcium aluminate for its rapid hardening properties. There is a very con siderable rise of temperature during, and soon after, setting and this facilitates the hardening of the cement, as well as being of great assistance when work is being carried out in frosty weather. This cement develops great strength in 24 hours, and piles have been made one day and picked up and driven with a pile driver on the next.

The cement is made by the complete fusion of impure bauxite and limestone, thus differing from Portland cement ; and this difference accounts largely for the resistance which it shows under certain corroding conditions which affect Portland cement.

Some of the mechanical tests on concrete made in England with this cement gave remarkable results at short periods, more particularly the compression tests. One of these, which has been published, showed an average compressive strength at 24 hours of 8,000lb, per sq.in. with 6in. cubes made from 4 parts of Thames ballast (din. to +in.), 2 parts of Thames sand (under 4in.) and i part of aluminous cement. The tensile tests do not show up to such advantage, as the ratio of strength in compression and ten sion is much greater with aluminous cement than it is with Port land cement. The tests and analysis of an ordinary sample of aluminous cement are as follow :— Lime Cements.—These include various grades, from the pure "fat" white lime through the grey limes to the hydraulic limes which have already been referred to under Portland cement. White lime consists of oxide of calcium, or, in its slaked form, hydrate of calcium. It is prepared by calcining calcium carbonate (chalk or limestone) until all the carbonic anhydride has been driven off and only the oxide is left. This oxide (quicklime) has great affinity for water, with which it combines readily, forming calcium hydrate, or slaked lime. This action is exothermic, i.e., gives out heat, and is very vigorous unless it is properly con trolled. The dry slaked lime occupies two or three times the volume of the original quicklime, according to the method by which it is slaked. Grey lime is an impure form of white lime, and, according to the quantity and nature of the impurities, its properties vary between white lime and hydraulic lime. The latter differs from white or grey lime in that, owing to the amount of argillaceous material contained in it, the lime slakes comparatively slowly, and when used for making mortar is capable of hardening under water.

Lime, probably on account of its long history, is frequently made in the simplest form of kiln, the bottle kiln, which is ex tremely extravagant with both coal and labour, particularly if white lime is being produced, as this must be kept free from the ash of the fuel. Various other kilns, such as those described under Portland cement, are also used, but the rotary kiln is not in very general use for lime burning, although by firing with producer gas very pure lime can be obtained from this kiln with great economy of labour and fuel, provided that the raw material is sufficiently pure. Another great advantage of this kiln is that it will take "smalls," i.e., material which is so small that it would choke a stationary kiln, and therefore has to be rejected. On the other hand, a rotary kiln is not suitable for burning large lumps of chalk or limestone, and as there is a certain demand for large lumps of lime (probably because of the uncertain qualify of the fine stuff in the earlier days), it is still necessary to use fixed kilns if this demand is to be met.

Lime, before it is used for mortar, must be slaked, and this was formerly done at the place where it was to be used. On ac count of the irregular way in which this was sometimes carried out, portions of the less pure lime used to slake and expand after the mortar had been used and cause the work to "blow." To overcome this trouble, architects and others frequently specify that the mortar should be made up 2 or 3 months before use. This method of overcoming the difficulty was at the expense of the strength of the finished mortar, for the lime was carbonating and deteriorating during this period. The modern method is to slake the lime mechanically at the lime works and remove the slaked lime from the unslaked by air separation. The slaked lime is then stored in bulk for i o to 14 days, after which it is ready for use, yielding a lime which is free from any risk of "blowing," and at the same time retaining its hardening properties unimpaired.

Mortar made from white lime and the purer forms of grey lime works very easily under the trowel, and, if necessary, can be floated off to a very smooth surface for facing walls, etc., and on account of this property the white lime is frequently spoken of as "fat" lime because it is supposed to suggest the smoothness of butter. Experiments have been made to improve the lime for mortar purposes, etc., by the addition of various substances to the water used for slaking, or to the slaked lime.

The setting of white lime and grey lime is largely due to "dry ing out," and to a small extent calcium hydrate recrystallization. The hardening appears to be almost entirely due to the lime com bining with the carbon dioxide in the air and forming calcium carbonate, although this is sometimes assisted by a slight pozzuo lanic effect of the silica in the sand or other material used with the lime. In hydraulic lime the setting and hardening properties are due to a combination of the above with that of those constitu ents which are similar to Portland cement. For most purposes Portland cement has taken the place of hydraulic lime ; but there are still certain conditions where the latter is preferable to the former, particularly for embedding large steel sections, where the hardening effect should not take place until the steel has finally settled into position.

Selenitic Cement.

The addition of 5 to io% of plaster of Paris to lime increases the hardening properties of the latter by 5o to 00%, and this mixture is sometimes known as Selenitic cement.

Pozzuolanic Cement.

Lime, in the presence of water, read ily combines with silica in the active state and forms a calcium silicate similar to that in Portland cement. Various natural and artificial materials, such as possuolana, trass, keiselguhr, pumice, tufa, santorin earth, granulated slag, etc., contain active silica, and where the cost is low they make a useful addition to lime mortar, To obtain the best effect the granulated slag, or other material, should be ground with the lime until both materials are in a fine state of division and intimately mixed. When properly made, poz zuolanic cements will attain a strength approaching that of Port land cement ; but frequently the material is simply mixed with the lime, and the bulk of the pozzuolanic material acts as an aggre gate instead of an active constituent of the cement.

Calcium Sulphate Cements.

This class includes all those cements which primarily depend on the hydration of calcium sul phate for their setting and hardening properties, and includes plaster of Paris, Keene's cement, Parian cement, etc. The raw material is gypsum (q.v.), which may be almost chemically pure, in which case it is suitable for Keene's cement and other special brands, or may contain a small quantity of foreign matter, when it is suitable for ordinary plaster of Paris. The mode of preparation is to calcine the gypsum at a comparatively low temperature, viz., about 205° C for plaster of Paris, at which temperature the gyp sum loses three-fourths of its combined water, and at about C for the Keene's cement class, when the whole of the com bined water is driven off. At a higher temperature the gypsum becomes "dead burnt" and will then only hydrate very slowly, or in some cases not at all. The gypsum for plaster of Paris is usually calcined either in ovens or in kettles, and for Keene's cement in kilns, where the ash of the fuel can be kept away from the finished product. These methods are inferior in economy to the rotary kiln, and this type of kiln will probably be the method of the future, as, if fired with producer gas, the product is not contaminated with ash. The setting of plaster of Paris depends on the fact that when is treated with water it dis solves, forming a super-saturated solution of The excess held temporarily in solution is then deposited in crystals of In the light of this knowledge the mode of setting of plaster of Paris becomes clear. The plaster is mixed with a quantity of water sufficient to make it into a smooth paste; this quantity of water is quite insufficient to dissolve the whole of it, but it dissolves a small part and gives a super-saturated solution of In a few minutes the surplus hydrated calcium sulphate is deposited from the solution and the water is capable again of dissolving 2 which in turn is fully hydrated and deposited as The process goes on until a rela tively small quantity of water has by instalments dissolved and hydrated the and has deposited in felted crystals forming a solid mass well cemented together. The set ting is rapid occupying only a few minutes, and is accompanied by a considerable expansion of the mass. There is reason to suppose that the change described takes place in two stages, the gypsum first forming orthorhombic crystals and then crystallizing in the monosymmetric system. Gypsum thus crystallized in its normal monosymmetric form is more stable under ordinary conditions than the orthorhombic form. Correlatively, in its process of de hydration to form plaster of Paris, monosymmetric gypsum is converted into the orthorhombic form before it begins to be dehy drated. The essential difference between the setting of Keene's cement and that of plaster of Paris is that the former takes place much more slowly, occupying hours instead of minutes, and the considerable heating and expansion which characterize the setting of plaster of Paris are much less marked.

It is the practice in Great Britain to burn pure gypsum at a low temperature so as to convert it into the hydrate to soak the lumps in a solution of alum or of aluminium sulphate, and to recalcine them at about Soo° Centigrade. Instead of alum various other salts—borax, cream of tartar, potassium carbonate, etc.—may be used. On grinding the recalcined lumps they give Keene's cement, Parian cement, Keating's cement, etc. The quan tity of these materials is so small that analyses of Keene's cement show it to be almost pure anhydrous calcium sulphate, and make it difficult to explain what, if any, influence these minute amounts of alum and the like can exert on the setting of the cement.

These cements form excellent decorative plasters on account of their clean white colour and the sharpness of the castings made from them, this latter quality being due to their expansion when setting. Keene's cement is especially adaptable for surfaces where a hard polished finish is required. All cements having calcium sulphate as their base are suitable only for indoor work because of their solubility in water.

Oxychioride Cements.

In 1853 Sorel discovered that zinc chloride solution, when mixed with zinc oxide, formed a very hard cement, and later he found that magnesium chloride, magne sia and various other metallic oxides and chlorides did the same, an oxychloride being formed in each case. Of these the most im portant is the magnesia oxychloride, commonly known as Sorel cement, and on account of its great strength and unusual binding properties it is used for such widely different purposes as uniting carborundum for grindstones, where great strength and rigidity are required, and binding wood sawdust together to form monolithic floors which have a ccrtain amount of spring in them. It is also used for making artificial marble and other ornamental stone, and if cast on glass which has been waxed to prevent adhesion the ma terial, on removal of the glass, will have a highly glazed surface.

The magnesia, which should be freshly calcined and ground, is mixed with several times its volume of carborundum, wood saw dust, sand or other material, and is then moistened with a solu tion of magnesium chloride having a density of Baume, and thoroughly mixed so that each grain of material is covered with the magnesia oxychloride. The plastic mass is then put into moulds, or placed in position and "floated" off. The speed of set ting and hardening is dependent on the freshness of the mag nesia and the strength of the solution of the magnesium chloride, and when the right strength has been found for any particular batch of materials this should be rigidly adhered to for the work.

Adhesive Cements.

Mixtures of animal, mineral and vege table substances are employed in great variety in the arts for making joints, mending broken china, etc. A strong cement for alabaster and marble, which sets in a day, may be prepared by mixing 12 parts of Portland cement, 8 of fine sand and 1 of in f usorial earth, and making them into a thick paste with silicate of soda; the object to be cemented need not be heated. Casein, with some solvent, usually an alkali, forms the basis of many waterproof cements and cold water glues. For stone, marble and earthenware a strong cement, insoluble in water, can be made as follows:—skimmed-milk cheese is boiled in water till of a gluey consistency, washed, kneaded well in cold water, and incorporated with quicklime; the composition is warmed for use. A similar cement is a mixture of dried fresh curd with one-tenth of its weight of quicklime and a little camphor; it is made into a paste with water when employed. A cement for Derbyshire spar and china, etc., is composed of 7 parts of rosin and 1 of wax, with a little plaster of Paris; a small quantity only should be applied to the surfaces to be united, for, as a general rule, the thinner the stratum of cement the more powerful its action. Quicklime mixed with white of egg, hardened Canada balsam, and thick copal or mastic varnish are also used for cementing broken china, which should be warmed before their application. For small articles, shellac dissolved in spirits of wine is a very convenient cement. Cements such as marine glue are solutions of shellac, india-rubber, or asphaltum in benzine or naphtha. For use with wood which is exposed to moisture, as in the case of wooden cisterns, a mixture may be made of 4 parts of linseed oil, boiled with litharge, and 8 parts of melted glue; other strong cements for the same purpose are prepared by softening gelatine in cold water and dissolving it by heat in linseed oil, or by mixing glue with one-fourth of its weight of turpentine, or with a little bichromate of potash. Ma hogany cement, for filling up cracks in wood, consists of 4 parts of beeswax, 1 of Indian red and, yellow ochre to give colour. Cutler's cement, used for fixing knife blades in their hafts, is made of equal parts of brick-dust and rosin, melted, or of 4 parts of rosin with 1 each of beeswax and brick-dust. For covering bottle corks a mixture of pitch, brick-dust and rosin is employed. A cheap cement, sometimes used to fix iron rails in stone-work, is melted brimstone, or brimstone and brick-dust. For pipe-joints a mixture of iron turnings, sulphur and sal ammoniac, moistened with water, is employed. Japanese cement, for uniting surfaces of paper, is made by mixing rice-flour with water and boiling it. Jewellers' or Armenian cement consists of isinglass with mastic and gum ammoniac dissolved in spirit. Gold and silver chasers keep their work firm by means of a cement of pitch and rosin, a little tallow and brick-dust to thicken. Temporary cement, for lathe-work, such as the polishing and grinding of jewelry and optical glasses, is compounded thus :—rosin, 4oz., whitening pre viously made red hot, 4oz., wax, 4oz.

D. Desch, The Chemistry and Testing of Cement (191I) ; D. B. Butler, Portland Cement (3rd ed., 1913) ; B. Blount, W. H. Woodcock and H. J. Gillett, Cement (192o) ; E. A. Dancaster, Limes and Cements (2nd ed., 192o) ; E. C. Eckel, Cements, Limes and Plasters (2nd ed., New York, 1922) ; A. C. Davis, Manufacture of Portland Cement (3rd ed., Dublin, 1922) and A Hundred Years of Portland Cement (1924) : G. R. Redgrave and C. Spackman, Calcareous Cements (3rd ed., 1924) ; R. W. Lesley, J. B. Loker and G. S. Bartlett, History of Portland Cement in the United States (Chicago, 1925) ; R. W. Meade, Portland Cement (Easton, Pa., U.S.A., 1926) ; H. L. Childe, Manufacture and Uses of Concrete Products and Cast Stone (1927) ; Everyday Uses of Cement (Cement Marketing Co., London, 4th ed., 1921, rev. ed., 1928). See also Concrete and Constructional Engineering, London ; Rock Products, Cement and Engineering News, Chicago ; Le Ciment, Paris ; and various publications of the Building Research station, London ; the Mineral Industry, New York ; and the Bureau of Standards, New York. (W. H. Wo.)