How to Make Cement From Blast-Furnace Slag

How to Make Cement From Blast-Furnace Slag

Table of Contents

The successful establishment of iron blast-furnace plants at Newcastle and Lithgow naturally invites attention to the economic utilization of the various products and by-products arising out of the industry.

The general similarity in composition between Portland cement and iron blast-furnace slag very early attracted attention both in the cement and iron-smelting industries.

Probably the first use made of blast-furnace slag in the cement industry was not of an honourable nature, to use an almost worthless and cumbersome by-product as a not readily detectable adulterant in Portland cement, which was at that time very costly to manufacture.

The astonishing discovery was made that in many cases the adulterated cement was stronger than the original unadulterated article. It was found that chilled blast-furnace slag introduced in moderate quantity and finely ground with the cement clinker did not injure, but usually improved, the quality of the resulting cement. This mode of manufacture is recognized, especially in Germany, as a legitimate branch of the cement-making industry.

The use of slag for the manufacture of both slag cement and Portland cement has assumed very large proportions both in U.S.A. and Europe—so much so that in the former country plants not well situated and equipped for cheaply making cement from clay and limestone are being forced to close down.

Slags from other than iron blast-furnaces, together with some of the latter, are not suitable for cement-making. All slags which disintegrate and fall into powder are wholly unfitted for cement-making. Those highly charged with mineral oxide compounds of sulphur, phosphorus, and large proportions of magnesia should also be at once rejected. Throughout the rest of the article iron blast-furnace slag will be simply referred to as slag.

Granulating Slag

It is necessary to suddenly chill hot molten slag in order to develop its hydraulicity and cementing power. It is well known that suddenly chilling any hot slag gives it physical and chemical properties materially differing from those which develop in unchilled slag—viz., brittle and soluble in acids instead of very tough and insoluble, or almost so, in acids. Unchilled slag is almost devoid of any hydraulicity or cementing power, besides being very difficult to pulverize. Chilling slag for use in cement-making is undertaken in different ways. In all cases it is essential that the slag shall be very hot, therefore the chilling must be done as near to the furnace as practicable.

The chief method of chilling molten slag is granulation in cold water. In this method the stream of hot slag is allowed to fall into a trough containing a rapid stream of cold water, preferably introduced as a jet directed with considerable force against the stream of molten slag. The physical effect is to cause the slag to break up into porous particles, usually called slag sand. This slag sand, as it leaves the vats, contains 15 % to 45 % (usually 30 % to 40 %) of adhering water. The expense of drying the slag sand is the chief disadvantage of water-granulated slag. This method is simple, cheap, and very effective, and is the one almost universally used. It also has a great chemical advantage— viz., that a very large proportion of.the sulphur and alkalis contained in the slag are eliminated from it and carried off in the water.

Another method of granulation is that the stream of slag as it issues from the furnace is struck by a jet of high-pressure steam or air, which has the effect of blowing the slag into fine threads and globules, much the same as slag wool. In this form it has the advantage of being easily pulverized by grinding machinery, but has certain inconveniences, and has fallen into disuse.

Molten slag is also granulated, by allowing the stream to fall upon inclined plates or revolving drums, which are kept cool. This method has been used to a moderate extent in Germany.

It is necessary to pass the granulated slag under a strong magnet in order to remove any particles of metallic iron.

Blast-furnace slag is used in making-four classes of cement—

  1. Puzzolan, or ordinary slag cement, usually simply called slag cement.
  2. Slag Portland cement, which may be a true Portland cement.
  3. Iron cement or Eisen cement, which is a mixture of slag and ordinary Portland cement.
  4. Several special varieties of cement.

Slag Cement

This is essentially a cement of the puzzolan type. A puzzolan material is one capable of forming a hydraulic cement on being simply mixed with lime. Puzzolan materials are largely made up of silica and alumina. Most puzzolan materials possess hydraulicity to a greater or less degree, but the addition of lime usually greatly increases their hydraulic power. Undoubtedly the most important puzzolan material is granulated slag. Some granulated slags high in lime possess, after fine-grinding, a high power of hydraulicity without the addition of slaked lime. The process of manufacture is very simple, and, in brief, is as follows:—

The dried granulated slag is mixed with a predetermined quantity of dry slaked lime and ground to an impalpable powder. It is then ready for use as cement. The slag sand, after being drained, still contains a large percentage of adhering water. This adhering water must be driven off until that remaining is less than 1 %. This drying is necessary for two reasons :—

(a) To prevent the water causing the mixed pure slag and slaked lime to combine and set and thus spoil it for use.

(b) To admit of the material being properly ground. Either dry or wet material (i.e., wet with plenty of water) may be readily ground in suitable machines, but damp material only clogs a fine dry-grinding machine.

Properly drying the material is therefore an essential feature of the process. Rotary driers are most commonly used, and it is found in good types that one pound of coal burned as fuel will evaporate 7 lb. of adhering water.

The limestone used for making the slaked lime is preferably a pure calcium-carbonate limestone, which, after burning and slaking, falls into a fine powder. The amount of magnesia present in the limestone must be very small, the admissible limit depending on the amount of magnesia present in the slag sand. The limestone is carefully burned in a kiln, drawn, cooled, and then slaked with water. The quantity of water to be added in slaking must be carefully gauged to slake the whole of the lime, but at the same time not to leave the slaked lime in a damp condition. Damp or wet slaked lime is not admissible for the same reasons as damp or wet slag sand. The slaked lime is then screened to remove any hard lumps. The latter may be of three types—-viz., under-burned, over-turned, and properly burned but not slaked. The two former, though not desirable, are hot harmful, but the last mentioned is very injurious, as it causes “free lime” to be present in the finished cement. Free lime causes cement to “ blow ” after it has been made up into mortar or concrete, and’ thus causes the material to disintegrate.

A preliminary reduction of the dried granulated slag may be made in a ball mill, and the final grinding, after the requisite amount of slake lime has been added, made in a Fuller-Lehigh mill, tube mill, or other suitable fine dry grinder. The introduction of ball-peb and similar tube mills have, however, rendered a preliminary grinding of the slag unnecessary, as the whole reduction may be done in one operation.

A ball-peb tube mill is one divided into two or three separate compartments, the first compartment containing steel balls- 3 to -5 in. in diameter, the second compartment steel balls 1½ in. to 2 in. in diameter, and the finishing compartment having 7/8-in. ball-pebs; or the first compartment may have steel balls, and the finishing compartment be charged with short lengths of 1-in. diam. steel rods. These mills take feed 1 in. to 2 in. gauge, and finish off to a high degree of fineness. The slag sand being already fairly small, a two-compartment mill is sufficient for slag cement: It is essential that slag cement be ground exceedingly fine; the finished product should not contain more than 3% oversize on 100 mesh per linear inch sieve, nor more than 15 % on a 200-mesh sieve.

Lime-burning, slaking, and screening, as usually carried out, are slow, tedious, and expensive. It is essential for economical work that these operations be made as continuous as possible, and that the materials shall throughout be mechanically handled, and these are most readily attained by the use of producer gas-fired rotary limekilns, hydrating machines and mechanical screens, and the necessary bins, conveyers, elevators, &c. It is necessary for the materials fed to hydrating machines to be of small size. Shaft kilns, even of the best type, are of comparatively small capacity, require much attendant labour, have a high working cost, and do not work satisfactorily if charged with small material, as the latter chokes the draught. The rotary lime-kiln, on the other hand, is preferably fed with limestone crushed to 2-in. or smaller gauge, and has the following advantages—-viz., continuous in action, economical in fuel, small amount of attendant labour, large capacity, and low working cost. The general outline of the process, with necessary bins; elevators, conveyers, &c., is as follows:—

The limestone from the quarry is passed through rock-breakers and rolls, and crushed to ½-in. gauge. The broken material is fed continuously to a rotary kiln, the waste heat of which may be utilized for drying the slag sand. The calcined, lime is fed in weighed batches to a hydrating machine, such as the Clyde hydrator, and slaked with a gauged amount of water. The material is thoroughly turned over and mixed until the action is complete, when it is mechanically discharged. A quantity of 1000 lb. of high-grade free lime requires 450 to 500 lb; of water for the operation, producing 1200 to 1225 lb. of hydrated lime, the rest of the water having been driven off as steam by the great heat developed during slaking. The hydrated lime, which should be perfectly dry, is fed to continuous-action screens, such as the Newaygo. The separated fine material is automatically weighed and mixed with the dried slag sand and ground into a finished product as cement.

Slags used in cement-making in Great Britain and Europe vary very considerably from each other in type and composition (see Table I.) American practice, however, aims at using slags which are much more uniform in character. The slags used by the Illinois Steel Company, Chicago, may be taken as typical of American slags used for making slag cement.


The. following analysis may be taken as illustrating the type of slag made at the Broken Hill Proprietary Co. Ltd. works at Newcastle:—


This slag is higher in alumina but lower in lime than the type usually used in the U.S.A., but is very similar to many slags that have been used to make slag cement in Great Britain, France, Belgium, Germany, Spain, and Switzerland.

The amount of slaked lime that is added to the slag for grinding into cement naturally varies considerably with the type of slag used. The latter, being a by-product, its composition depends very largely upon that of the ores, fluxes, and ash of the fuel used in the blast furnaces.

The following are amounts of lime per 100 lb. of slag used at various places :—


Le Chatelier states that the hydraulic properties of granulated slag are due to the presence of a silico-alumino ferrite of calcium corresponding to the formula 3CaO, Al2O3, 2SiO2. This compound appears also in Portland cements, but in them it is entirely inert owing to the slow cooling it has undergone. When, however, as in the case of granulated slags, it is cooled with great suddenness, it becomes an important hydraulic agent. When go cooled it is attacked by weak acids and also by alkalis. It combines particularly with hydrated lime, and in setting gives rise to silicates and aluminates of lime identical with those which are formed by entirely different reactions during the setting of Portland cement. It is upon this property that the manufacture of slag cement, which assumes daily greater importance, is based.

Chemists are still in enthusiastic disagreement as to the precise compounds formed and the reactions which take place both during the manufacture and setting of cement.

Slag cements differ widely in chemical, composition from Portland cements mainly in the high percentage of Al2O3 and in their relatively low percentage of CaO, which, unless a larger proportion than is ordinarily required has been added for some special purpose, need not exceed 46% to 51%, as against 58% to 62% CaO generally present in Portland cements.

The following analyses are given by Redgrave (Proc. Inst. C.E., vol. cv.) as good examples :—


The average composition of a number of well-known British European, and American slag cements is given in Table II.


It will be seen by comparing Tables I. and II. that, despite the apparently great variations in practice, the ultimate, composition of very many finished slag cements falls within quite narrow limits, namely:—


Nevertheless, many excellent British and other European slag cements have an appreciably lower percentage of CaO—for example, Cleveland cements and that quoted by Redgrave; and some are materially higher in alumina—e.g., Cleveland and Seraing. Kidd, in Proc. Inst. C.E., vols. cv. and cvii., states that he had used Cleveland slag cements in marine work with most excellent results. Banks (Iron and Steel Instit., 1905) claimed that slag cements made from Cleveland slags were the strongest cements known.

Slags containing a high percentage of lime after granulation and grinding, but without the addition of slaked lime, have a very high degree of hydraulicity and strength. Attempts have been made to produce slags approaching the Portland cement composition—i.e., to make a slag which, when finely ground, would be a Portland cement. These attempts have not been successful, and were abandoned, very largely because producing, slags abnormally high in lime interfered too much with the main function of the blast furnace—namely, as a producer of pig iron.

Denby and Lewis recently, before the Faraday Society, quoted a slag with a high lime-content as follows :—


This slag is similar in composition to many finished slag cements.

The high lime was due to the necessity to remove sulphur from the metal, but some of the sulphur was removed from the slag during granulation. This slag, when ground, had a tensile strength of 430 lb. per sq. in. at 28 days.

The specific gravity of slag cement usually ranges from 2.7 to 2.9 as compared with 3.15, which may be considered a fair value for good Portland cement.

Slag cements are usually lighter in colour and slightly different in tint from Portland cement, the lightness in colour being largely due to the admixed slaked lime.

Slag cements normally set very slowly compared to Portland cements. The set of slag cements may be hastened by the addition of puzzolanic materials. Of these, burned clay, certain active forms of silica, and slags high in Al2O3 are the cheapest and most generally used. Some slags used in making slag cement already contain a high percentage of Al2O3, and may not require any further regulation.

The most important method of regulation used with the low-percentage Al2O3 slag cements in the U.S.A. is the Whiting process. This process includes the use of caustic soda, caustic potash, sodium chloride, &c., added either as aqueous solution or in a dry state at some stage of the process of cement manufacture. Caustic soda 0.125 to 3% may be added, depending upon the use for which the cement is intended. It is frequently added as an aqueous solution in slaking the freshly-burnt lime.

Slag cements differ from Portland cement in requiring no preliminary seasoning. Owing to the complete hydration of the lime used in slag cement and the inert character of the silicates present, little or no change can take place in the material, even when exposed to the atmosphere for a lengthy period. Slag cement protected from actual moisture undergoes no change whatever.

Slag cements fall below high-grade Portland cements in tensile strength, but good American slag cement develops sufficient strength to pass many American specifications for Portland cement. Tested neat, they do not approach Portland cements so nearly as tested in 2 to 1 and 3 to 1 mortars.

The following is the result of tests of some American slag cements :—


These results would satisfy the strength requirements in. the standard Portland, cement specification of many American public bodies, but would not fulfil the requirements of the latest British standard specifications or the New South Wales Government specification.

Slag cements are deficient in resistance to mechanical wear, and are therefore not suitable for use in the surface of pavements and floors. They are well fitted for foundations and mass concrete generally, in which a high-grade strength is not necessary. They are deemed to be superior to Portland cements for making concrete to be used in or under either fresh or salt water. They are, therefore, specially suitable for use in connection with hydraulic, harbour, and marine works.

Mortar made from “ fat limes ” is, at its best, a very poor material as regards strength and setting. Such mortars, except on the surface, never set, and their hardness is little more than that due to merely drying out.

Hydraulic limes, which, on mixing into mortar or concrete set and harden, are practically not used in Australia.

Slow-setting slag cement could with great advantage as to quality replace “ fat lime ” in making mortar for ordinary building operations, and probably also at no increased cost in districts to which freight charges would be small.

A very large portion of the Portland cement is used in Australia under conditions where its very great strength and cost are quite disproportionate to the moderate requirements of the work. There is thus, at present, a very wide unfilled gap in quality, strength, and cost between the very indifferent “fat lime ” mortar and that made from Portland cement, which could with advantage be filled by the use of a good cheap slag cement.

Portland Cement made from Slag and Limestone

Granulated slag is very extensively used in Britain, Europe, and more so in U.S.A., in the manufacture of Portland cement. The Universal Portland Cement Co. make a cement of this class, and have five mills, whose combined production is 12,000,000 barrels, or 2,000,000 tons, per year.

The mode of manufacture, chemical composition, and requirements for strength, soundness, &c., for Portland cement have been rigidly defined within narrow limits in most countries. The following particulars, from the British Standard Specification (1915), may be taken as closely representing the requirements for Portland cement in most countries. The cement shall be manufactured by intimately mixing together calcareous and argillaceous materials, burning them at a clinkering temperature, and grinding the resulting clinker so as to produce, a cement capable of complying with the other requirements of the specification. No addition of any material shall be made after burning other than calcium sulphate or water, or both (added to control the time of set). The following percentages shall not be exceeded—viz., insoluble residue, 1.5%; magnesia, 3%; sulphuric anhydride, 2.75 %; and loss of ignition, 3%. The proportion of lime to silica and alumina, when calculated (in chemical equivalents) by the formula CaO/SiO2 + Al2O3 shall not be greater than 2.85 or less than 2.0.

It has long been established by law and custom in most countries that a cement made from blast-furnace slag and limestone so as to comply with standard requirements of method of manufacture, chemical composition, fineness, strength, soundness, specific gravity, &c., is legitimately a Portland cement, and may be sold as such.

Most cement makers aim at producing a cement having but a small range of composition for its components. Many blast-furnace slags contain a high percentage of alumina, which, when mixed with lime, would give a cement within the proportions of lime to silica and alumina given above, but considerably higher than they desire to have. The desired proportion is obtained by adding crushed quartz or silica sand to the mixture of slag and limestone. The method of manufacture is, briefly, as follows :—

The limestone is crushed to about 1½-in. gauge and dried in rotary driers, using coal as a fuel, or the waste heat from the kilns may be used for this purpose. The limestone receives a preliminary grinding, and is delivered to hoppers above the scales. The slag is fed direct to driers, given a preliminary grinding, and delivered to hoppers above the scales. The limestone and slag are proportioned at the scales. The scales are preferably of the automatic electrically-operated type, interconnected, so that one cannot dump without the other. The mixed materials are very finely ground in a tube or other fine-grinding mill, then elevated to hoppers above the rotary kilns. In these kilns, using pulverized coal as fuel, the mixture attains a temperature of about 2500° Fahr., and gradually burns to a hard clinker. The clinker is seasoned for about 10 days, given a preliminary grinding, mixed with the amount of gypsum to regulate the setting, and then receives the finishing grinding in a tube or other mill. The resulting product is cement, which is carried to storage bins and bagged for distribution. There is reason to believe that portion of the foreign Portland cements imported before the war was of this origin, and readily came up to New South Wales Government standard tests.

Iron Portland Cement

This cement, called Eisen Portland cement in Germany, is made of a finely-ground mixture of Portland cement and granulated slag, usually in the proportion of about 3 of cement to 1 of slag. As before mentioned, this class of cement probably was at first only a fraudulently adulterated Portland cement, but it is now extensively made and sold as a separate class of cement, for which special advantages are claimed. Its development as a separate branch of the cement industry rests on the theory of Dr. Michaelis, put forward in 1876—namely, that about one-third of the lime of Portland cement separates as crystalline calcium hydrates. This compound has no strengthening effect, and may have a harmful one. If this theory is correct it follows that it is desirable to add some substance which will unite with the free lime and convert it into calcium hydro-silicate or other useful compound, and thus raise the effective quantity of cement.

Ground granulated slag is used for this purpose, and may be added to Portland cement made from slag and limestone or to that made from clay and limestone. The slag is added, to the Portland cement clinker during the final grinding, so that the resulting cement mixture is very fine ground and intimately mixed. Opinion is divided as to whether the addition is a benefit or merely an adulterant. There is no doubt that in many cases it produces an article which, is sounder and stronger than the original cement. The addition of a small amount of suitable granulated slag to high-lime Portland cements would be beneficial as a safeguard against that bugbear of the danger of expansion due to “ free lime.”

This addition should be made after clinkering and before the final grinding, so that the resulting product is very intimately mixed. Standard specifications for Portland cement prohibit any such addition, and it could only be done at the express instruction of the user.

The specific gravity of iron Portland cement—namely about 3.0—is intermediate between that of slag cement and Portland cement. Iron Portland cement is received with a high degree of favour in the Continental countries of Europe.

The following are comparative mortar tests made on ordinary Portland cement and on iron cement made from 75 parts of the foregoing and 25 parts of granulated slag :—


The following are tests of (German) Eisen cement made at a time when the German specifications for Portland cement required the following strengths for 1 to 3 cement-sand mixtures at 28 days—viz., tension 227 lb., and compression 2270 lb. per sq. in. :—


The very high compression strength of iron cement makes it particularly suitable for special work, such as reinforced concrete.

Special Cements

Probably the most important of these is that made by the Colloseus method. This cement is now being made at a number of plants in Germany, Britain, and U.S.A. In this process basic blast-iron slag is granulated in a special device, using a limited amount of aqueous solution of alkaline salts. The special salts are intimately mixed with the slag and the latter chemically and physically changed, being a porous clinker easily powdered. The chief salt is magnesium sulphate (crude Epsom salts), used as a 5% solution.

The stream of slag falls on a horizontal ribbed drum rotating at 650 r.p.m. Between the ribs are slots through which spurts the granulating solution. There is also a jet of solution just below the stream of slag. The slag is granulated as little pellets. Ungranulated slag is separated from the mass on a turn-table. Only so much solution is used that the granulated slag is perfectly dry after the operation. The slag is then treated as in the case of ordinary Portland cement clinker—i.e., no further addition of any material is made except such as is customary to regulate the setting. The sulphur content of the slag should, be low, and the amount of magnesia should be less than 3.5 to 4%.

The cement is ground very fine, specific gravity 2.97 to 3.0. Time, of set—initial, 18 to 20 minutes; final, 35 to 45 minutes. This is a very quick-setting cement when compared to the ordinary slag cement.

The following are results of tension tests of fresh cement and of cement which has been stored one month. Each test is the average of 10 samples :—


This cement is considered superior to Portland cement in that it contains, no free lime, and therefore is free from the danger of expansion.

Another special cement is made by finely grinding granulated slag with a considerable quantity of plaster of Paris. This gives a quick-setting, hard cement.

Slag Bricks

The slag-brick industry may be considered to be a specialized branch of the slag-cement industry, but a wider range of slags may be used. The manufacture includes bricks, pipes, and other special shapes, and the details of practice vary considerably in different works. The most usual method is to finely grind together 100 parts of dry granulated slag and 10 parts of slaked lime, giving what is really a slag cement of lower lime-content. A small amount of water is added to the material and mixed to a stiff pug and passed on to suitable brick or pipe-making machinery. The bricks or pipes are stacked to dry and harden before distributing for use. The hardening is sometimes hastened by treatment under the heat and pressure of steam. As before mentioned, some finely-ground granulated slags possess a high power of
hydraulicity without the addition of slaked lime, and may be made directly into bricks.

Another important variation of process is to mix slag cement with fine slag sand which has been drained but not dried, there, being sufficient water to pug the mixture before passing to the brick machines. Well-made slag bricks are stronger than clay bricks. Slag bricks are used for ordinary building purposes, but, as they are more refractory than red-clay bricks, they are of special service in the outer walls of furnaces and for chimney stacks.

It will be seen from the foregoing brief survey of the subject that a large variety of cements may be made from suitable basic granulated iron blast-furnace slags. It has been prophesied that in a few years nearly all cement will be made by some process from blast-furnace slag.

The writer is indebted to Mr. G. D. Delprat, general manager Broken Hill Proprietary Company Limited, for kind permission to use data in connection with the blast-furnace slags made at the Newcastle iron and steel works.


Mr. G. Stephen Hart said, as Mr. Poole mentioned, chemists were still in “ enthusiastic disagreement ” as to why cements set and the chemical compounds formed, but microscopical examination showed optically different compounds named alit, belit, celit, and felit. A chilled slag differed physically from a slowly-cooled one, presumably on account of a different grouping of its atoms, as with steels cooled quickly and slowly; but to assert that slag cements on setting formed the same compounds as Portland cements seemed rather daring. It was not supported by their properties when set. The tests quoted illustrated the curious fact that in both neat and 1 to 3 mixtures slag cements were only five times as strong in compression as in tension, whilst Portland cements were ten times as strong. The neat slag cement quoted gave a strength in compression of 2830 lb. in 28 days. The New South Wales standard for Portland cement mixed with three times its weight of sand was 2250 lb. after 28 days, and the best cements gave more than double that strength in a 1 to 3 mixture ; still, there should be a use for slag cement where great strength was not needed—for example, in taking the place of mortar, as suggested by Mr. Poole. It would have a more local market, as bagging, freight, etc., would be as great as on stronger Portland cement, which could be mixed with sand where used. For the manufacture of a true Portland cement the analysis of the Newcastle slag showed it to be too high in Al2O3 compared with SiO2 for rotary-kiln practice, and mixing with siliceous material, as suggested, would tend to complicate the process. It was, therefore, mostly a question of costs. The danger of free lime in Portland cement was far less with modern rotary kilns than when old-fashioned stationary kilns were used. In those the whole charge was never properly burnt, and a gang of men had to be employed on the clinker heaps to sort out the good clinker from the bad. In cement one part of CaO could combine with 2.8 of SiO2, but only with 1 of Al2O3; therefore a siliceous cement needed a much higher percentage of lime. The old stationary kilns could not burn a highly siliceous cement, but present-day rotary-kiln practice favoured that, and he (Mr. Hart) had no doubt that every Australasian cement carried a higher percentage of lime than the example quoted in the paper. Mr W. A. Brown, in “ The Portland Cement Industry,” published in 1916, stated that in a good Portland cement the lime should be from 60 to 67% and the silica 20 to 25%. If one part MgO was taken as equal to 1.4 parts CaO, a New Zealand cement described in a paper presented to the Institute contained CaO and MgO equivalent to 65% of CaO and 25.9% of SiO2. With reference to the strength tests in that paper, it should be remembered that briquettes made differently would give different results. The British standard specifications insisted upon briquettes being patted down with a spatula weighing 11 oz. They also demanded an increase of strength of about 10% between 7 and 28 days. New South Wales specifications used the Boehme hammer—a trip hammer weighing about 5 lb.— which, after 150 blows, was automatically stopped. Thus, any personal factor was minimized, but briquettes so made were relatively stronger after 7 days, and, to comply with B.S.S. requirements, briquettes should be made as the B.S.S. specified.

Different cement tests were required by different Australian States, and even by different public departments in one State. That was most undesirable, as there was always the chance that one body would devise an unusual test which no cement could pass unless too bad for use, if judged by some other department’s specifications.

MR, A. S. Kenyon said with regard to Portland cement he did not think that engineers were going to relax their specifications, but rather the other way. Mr. Hart’s candid admissions, that his company had always proved able to more than meet requirements made one inclined to stiffen them up. It was not a 10% increase, on the seven days test, that was asked for ; but that only 90% of the set should occur during the first seven days, which was a very different thing. It was not that the engineer worried about the seven days set, but that he wanted a 28 days set. With regard to slag cement, he would like to ask Mr. Poole to add a word as to the possible amount of cement they were likely to obtain from those works, whether it would equal the compression test, and whether the quality would equal that of the ordinary quality of cement in use. In other words—What was the amount of commercial cement that might be made in that way in Australia ?

Mr. Poole, in reply, said he was not aware that the Commonwealth Portland cement was as high as 64% in lime. But, in speaking of free lime, he certainly did not include hydrated lime as free lime. Free lime was anhydrous calcium oxide (CaO). It was not contended that a slag cement was as good as high-class Portland cement; but it must not be forgotten that the cement made in New South Wales had grown up under, as far as he knew, the stiffest set of tests in the world. Those tests had been in vogue for quite a number of years, and cements that had grown up under them had to be very high-grade to pass. The result had been that public bodies, private engineers, and architects merely stipulated that the cement to be supplied must, come up to the New South Wales Government specification. He thought the majority of engineers did not like a very high-lime cement, because there was always a fear that it would be unsafe. No man wished to undertake an important piece of work with cement, even though it showed up well in laboratory tests, if it afterwards cracked or disintegration took place. He would prefer a cement of comparatively low-strength test, but absolutely sound in quality. As far as free lime was concerned, he did not think it was usual to test it at works. Yet certain well-known tests could be carried out at works to show the general soundness of a cement. With regard to the carrying of cement in bulk, he believed it was coming into vogue in the United States, where it was being carried, like other materials, in bulk, in proper trucks. In and around Newcastle it could be carried in bulk; but under the present system of railway trucks it would not be possible to ship it in bulk to other parts of New South Wales. They had one of the most antiquated systems of trucking in the world. When two such world-wide authorities as Michaelis and Le Chatelier disagreed as to the chemical reactions which took place during the setting of cement, it was not for those who were not expert chemists to enter into the fray. He thought that Newcastle slag could be made into a good straight-out Portland cement. He was not giving secrets away when he said that it had been examined for that purpose by one of the most progressive cement companies in Australia, and that they were quite satisfied. As to the quantity of slag cement that may be available, he could inform Mr. Kenyon that there was a company in formation ; but whether they would make a straight-out slag cement was another question. The Portland cement man did not look favourably on slag cement. It was only rarely that the full strength of Portland cement was fully utilized. As a matter of fact, outside of reinforced concrete, the great strength of Portland cement was rarely utilized. A cement of much less strength would suffice for most purposes. The output of slag cement would depend upon the output of slag from the furnace at Newcastle. There was one blast furnace now in use, and shortly a second would come into commission. The amount of slag available would be practically the quantity the furnaces produced, less what the company may require for their own particular purposes in the steel works.