cement plant blast furnace

blast furnace slag cement (application, pros & cons)

Blast Furnace Slag (BFS) cement is the combination of Ordinary Portland Cement (OPC) and fine Granulated Blast Furnace Slag (GBFS) gained as a byproduct in the steel making manufacturer with below 70% to that of cement. Ground Granulated Blast Furnace Slag (GGBFS) is a fine glassy granule which encompasses cementitious possessions.

GGBFS is gained as a derivative in the removal of iron from its ore. The procedure of removal of iron is the blast furnace. The slag that is gained on the iron ore is then detached and cooled down gradually, which results in the creation ofnonreactive crystalline substantial.

how to make cement from blast-furnace slag

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.

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 Europeso 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.

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 slagviz., 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.

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 :

(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 1 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 -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.

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.

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.

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.

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 CaOfor example, Cleveland cements and that quoted by Redgrave; and some are materially higher in aluminae.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 compositioni.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 furnacenamely, as a producer of pig iron.

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.

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.

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.

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.

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 exceededviz., 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 1-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.

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 1876namely, 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 cementnamely about 3.0is 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 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 daysviz., tension 227 lb., and compression 2270 lb. per sq. in. :

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 clinkeri.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 setinitial, 18 to 20 minutes; final, 35 to 45 minutes. This is a very quick-setting cement when compared to the ordinary slag cement.

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 neededfor 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 hammera 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 departments 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. Harts 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 wordsWhat 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.

granulated blast furnace slag - an overview | sciencedirect topics

GBFS is an amorphous, coarse sand-sized material (Fig. 12.1). It exhibits hydraulic cementitious features if it was finely ground form. Although average granule size of GBFS depends on many factors such as its source it is about 11.5mm. Fig. 12.2 shows a gradation curve of GBFS obtained from Ereli Iron and Steel Plant, Zonguldak, Turkey. Particle shape of GBFS is changing from sub angular to sub round. Some typical physical properties of GBFS are shown in Table 12.1.

GGBFS has off-white or near-white color and it exhibits excellent cementitious property, when finely ground and combined with Portland cement (PC). The specific gravity of the GGBFS is about 2.90. The fineness of GGBFS is a very important parameter and it is measured by specific surface area that controls its reactivity (Pal et al., 2003). In general, increased fineness results in better strength development, but in practice, fineness is limited by economic and performance considerations and factors such as setting times and shrinkage. The specific surface area is changing from country to country. A common range varying from 375 to 450m2/kg Blaines could be considered.

The glass content of slag is also an important variable and shall be not less than 67% according to BS 6699:1992. Generally, the glass content of the slag should be in excess of 90% to show satisfactory properties (Siddique and Kaur, 2012). The European Standard BS EN 15167-1:2006 also mentioned minimum glass content by mass as two third in definition of GBFS that the old standard BS 6699:1992 replaced by BS EN 15167-1:2006.

The GBFS, Na2SiO39H2O and water were in the mass ratio of 1:0.11:0.30. In a typical synthesis, 1500g of GBFS was put into a net paste stirrer and then the aqueous solution of alkaline activator was poured into it. The slurry was poured into a triplicate iron mould measuring 160mm40mm40mm to be compacted on the vibrating table, and then put into a standard curing box for 24h. The samples were continuously cured at room temperature for an additional 7days after demoulding, and then the samples, sealed in thin plastic bags respectively, were cured at 65C for 24h in a nitrogen atmosphere. The sample with compressive and flexural strengths of 86MPa and 2MPa respectively was crushed to obtain a Na-alkali-activated slag-based cementitious material (Na-ASCM) with particle size distribution from 0.125 to 0.315mm.

160g of Na-ASCM was placed into a 400mL aqueous solution of 2.0m NH4Ac at room temperature for 24h to carry out the ion exchange of Na+ with NH4+. The sample was filtrated and washed adequately with deionized water, and then dried at 65C for 4h in a nitrogen atmosphere. As described above, the ion exchange was repeated once more to get NH4-ASCM cement.

As described in Chapter 2, granulated blast furnace slag consists of the same major oxides as does Portland cement, which explains why its first application was in the cement industry. However, the relative proportions of the oxides differ between Portland cement and this slag.

The first blast furnace slag cement was produced in Germany, in 1800, by mixing slag and lime [6]. Even before 1800, however, the properties of such cement were studied in France. Since the 1860s, it has been used in commercial production, and in the United States, slag cement production began in 1896 [7].

However, early attempts at using blast furnace slag in cement production were not successful. Poor performance of the initial slag cement was often due to poorly granulated slag and accelerators being added in large quantities without consideration of the long-term effects [7]. This even led to slag being prohibited in cement specifications. The reasons for abandoning production of this first type of cement in most countries were the inability of manufacturers to find suitable supplies of granulated slag, the slags sensitivity to deterioration in storage, and its low strength in comparison to present-day Portland cement. Currently, a wealth of research exists on slag usage in cement production, processes have been developed for production, and the use of slag in cement is more developed. Possibly the most attractive feature of slag usage in cement production is the energy and financial savings in producing such a cement.

Today, blast furnace slag usage in the cement industry is known as slag cement. This term usually refers to either combinations of Portland cement and ground slag or to the ground slag alone. Within the current European cement standards, 14 listed cements may contain slag in different percentages by cement mass [8]. In Tables 4.1 and 4.2, the compositions and designations of common cements covered by EN 197-1:2011 [16] can be seen. All these cements are mostly mixtures of ground Portland cement clinker and blast furnace slag. Granulated blast furnace slag should contain calcium oxide (CaO), magnesium oxide (MgO), and silicon dioxide (SiO2), the sum of which should account for at least two-thirds of the mass. The remaining third contains aluminium oxide (Al2O3), with small amounts of other compounds. In addition, the ratio by mass (CaO + MgO):SiO2 should exceed 1.0. Cement with blast furnace slag is usually intended to be equal to ordinary Portland cement (CEM I) and to meet the same requirements. Adding higher amounts of slag results in a higher sulphate resistance and lower heat of hydration properties. When producing such cement, slag is usually the hardest to grind, and it forms the coarsest portion, while the cement is ground the finest [9]. A potential problem is moisture, which occurs in blast furnace slag during the granulation process. The presence of moisture requires dewatering the blast furnace slag before using it as an additive or as a mineral admixture to Portland cement [10]. According to [1113], this cement increases strength, reduces permeability, improves resistance to chemical attack, and inhibits rebar corrosion. In addition, blast furnace slag has a high resistance to freezing, thawing, chemicals, and seawater [14]. Therefore, it is recommended for concrete structures that require high durability. Concrete made with blast furnace slag cement sets more slowly than concrete made with ordinary Portland cement, depending on the amount of slag in the cementitious material. It also continues to gain strength over a longer period under production conditions. This results in less heat of hydration and lower increases in temperature, making avoiding of cold joints easier, but also possibly affecting construction schedules for which quick setting is required [13].

For a cement of extreme sulphate resistance, EN 197-1:2011 suggests that common cement contains blast furnace slag. Sulphate-resistant common cement is typically used for foundations because the presence of sulphates in the soil can lead to damage of ordinary cement [15].

The blast furnace cement (CEM III) in Table 4.1 is so-called low early-strength cement, indicated by L. Low early-strength CEM III cement conforming to the requirements in Table 4.2 can also be designated sulphate-resistant common cement. Common cement with blast furnace slag often has LH in its designation, which implies low heat of hydration.

In addition to EN 197-1:2011, European legislation includes five more types of cement containing blast furnace slag; four very low heat cements covered by EN 14216:2004 (Table 4.3) and a supersulphated cement covered by EN 15743:2010.

Very low heat cement has been found to be particularly useful in the grout for encapsulating nuclear wastes, and there may be other applications for which it is more important to avoid generating heat than to develop great strength (Table 4.4) [18].

Supersulphated cement is a mixture of ground granulated slag; calcium sulphate gypsum, which can be gypsum calcium sulphate dehydrate (CaSO4H2O), hemydrate (CaSO41/2H2O), anhydrite (CaSO4) or any mixture of these substances; and a small amount of Portland cement clinker [7, 9]. This cement has excellent resistance to aggressive agents (seawater, sulphates, chlorides, alkali hydroxides, and weak acids) and low heat of hydration. Compared with ordinary Portland cement, it is more susceptible to carbonation during storage and requires extra care during the initial curing period to keep the surface moist [7]. Due to its high resistance, this cement is generally used in seawater work, concrete pipes exposed to aggressive groundwater, or in chemical plants.

To justify the widespread application of blast furnace slag as an additive to Portland cement, manufacturers found significantly decreased consumption of natural raw materials, lower energy demands (grinding slag for cement replacement requires only 25% of the energy needed to manufacture Portland cement [10]), and specifically, lower CO2 emissions [20]. Figure 4.3 presents the required energy to produce different types of cement. As can be seen, a possibility exists for a significant reduction in fuel consumption by utilizing pelletized slag. It requires only 15% of the energy of Portland cement clinker because this slag does not need to be dried [21].

For the production of cement or cementitious materials, any type of blast-furnace slag can be used, but the most common is granulated or glassy slag due to its ease in handling and more rapid reactions between the glass and other raw materials [7]. As cement is hydrated, calcium silicate hydrates (C-S-H) and calcium hydroxide (C-H) form. The main influence on the development of strength is C-S-H gel. When slag is added, its silicates react with C-H, forming additional C-S-H gel and creating denser, harder cementitious paste with increased strength [22]. However, the rate of hydraulic reactivity of granulated blast furnace slag is slower than for ordinary Portland cement; thus, slag with greater fineness is usually used [23].

Aside from being used as a cement additive, blast furnace slag can be added to concrete separately, thereby achieving an effect similar to concrete in its fresh and hardened state. In European legislation, the usage of blast furnace slag as an additive to concrete, mortar and grout is covered by EN 15167-1:2006.

GGBFS is obtained by finely grinding the granulated blast furnace slag (BFS), which in turn is obtained by sudden quenching of molten slag removed from the blast furnaces of the iron and steel industry. It mainly consists of oxides of calcium (CaO), silica (SiO2), alumina (Al2O3), and magnesia (MgO), along with some other minor oxides in small quantities. GGBFS is probably the most widely investigated and most effective cement replacement material used in concrete manufacturing. McGannon [19] quantified the hydraulic activity of GGBFS in terms of the basicity coefficient (Kb) which is the ratio between the total content of basic constituents to total content of acidic constituents as given in Eq. (13.4)

Wang et al. [16] and Bakharev et al. [20] further simplified the equation by excluding the minor components such as Fe2O3, K2O, and Na2O (generally less than 1%) in the computation of the basicity coefficient, Kb.

Based on the basicity coefficient (Kb), the GGBFS is classified into three groups: acidic (Kb<0.9), neutral (Kb=0.91.1), and basic (Kb>1.1). Neutral and alkaline slags are preferred as starting materials for activation in AA slag (AAS) binders.

BOF slag exhibits good hydraulic activity when blended with GBFS and gypsum and with or without OPC clinker addition even though the fineness of grind, achieved in a laboratory ball mill, was not as high as that commonly occurring for commercially available OPC clinker.

The stability of SSBC pastes containing relatively high contents of steel slag was satisfactory when a small quantity (10%) of OPC clinker was added. Good stability of SSBC in production practice should be obtained if the steel slag content is not greater than 40% and at least 10% OPC clinker is incorporated in the blend.

No significant differences in compressive strength were exhibited for SSBC pastes containing recently produced steel slag and those containing aged steel slag, with the stability of both being acceptable when 10% OPC clinker was added.

Magnetic reseparation of the steel slag can improve the efficiency of intergrinding steel slag and the OPC clinker by about 50% compared with intergrinding OPC clinker and nonmagnetically reseparated steel slag. The grindability of mixtures of steel slag and OPC clinker depends on the relative content and initial pregrind size of the steel slag. No decrease in grindability was measured when less than 20% of 2.364.75mm steel slag was added to the OPC clinker.

Granulated phosphorus slag is a latent cementitious material but less reactive than granulated blast-furnace slag at early age due to the lower Al2O3 content and the presence of P2O5 and F. A hydraulic index of phosphorus slag is defined as follows (Shi et al., 2006; RCT 5024-83, 1983):

The phase composition of cement hydration products is chiefly represented by tobermorite 1.13nm. With addition of Portland cement, the intensity of tobermorite occurrence is higher and lines (peaks) characteristic of truscottite (Ca,Mn)14Si2 4O58(OH)82H2Oa mineral of the reyerite group and wenkiteBa4Ca6(Si,Al)20 O41(OH)2(SO4)3H2Oa mineral of the cancrinite-sodalite group appears in the X-ray diffraction patterns (Sanserbaev, 1987).

Experience obtained from the use of concrete and reinforced concrete structures from these cements in industrial, hydro engineering, and agricultural construction coincides well with assumption on the higher durability compared with those made from Portland cement concretes (Krivenko et al., 1993).

Supersulfated cement is made by intergrinding a mixture of 8085% of granulated blast furnace slag with 1015% of dead burnt gypsum and up to 5% of Portland cement clinker. This cement is highly resistant to seawater and sulfates. In Europe, its greatest use is in Belgium and France.

This is a very rapid hardening cement produced from bauxite, gaining 80% of its strength in 24h. However, it is no longer used for structural work because it suffers conversion to a much weaker phase under warm damp conditions which has led to a number of structural collapses. It is a good refractory cement suitable for temperatures up to 1600C with suitable refractory aggregates.

Cements for limited markets include oil well cement which is a highly specialized product used for slurry to be pumped for up to 5km into the earths crust to seal the gap in oil well casings against blow-out or leakage of oil. Temperatures up to 150C and pressures of 100MPa may exist and it is essential that the cement does not set until it reaches the required position, after which it should harden rapidly.

Bakharev et al. (2002) investigated resistance to sulphate attack of alkali-activated granulated blast furnace slag using the ASTM C1012 test procedure. Alkali-activated slag concrete specimens were immersed in 5% Na2SO4 and 5% MgSO4 solutions for a period of up to 12months. Ordinary Portland cement (OPC) concrete samples were subjected to the same treatment for comparison. Up to 60days, strength development was the same for both alkali-activated slag and Portland cement concretes in both environments. After that time, strength reduction in Portland cement concrete was higher than in alkali-activated slag samples in both of the sulphate solutions (see Figures14.1 and 14.2). After 12months of exposure to the Na2SO4 solution, the strength decrease was up to 17% for alkali-activated slag concrete and up to 25% for Portland cement concrete. The reduction of concrete compressive strength was more substantial in the MgSO4 solution for both types of binders: after 12months in the MgSO4 solution, the strength decrease was up to 37% for Portland cement concrete and 23% for alkali-activated slag samples. Structural characterization of mortars from the surface of alkali-activated slag and Portland cement samples by X-ray diffraction analysis indicated different degradation products. In Portland cement samples, ettringite was present in the sample exposed to Na2SO4 solution, and considerable amounts of ettringite and gypsum were found in the sample exposed to MgSO4. Meanwhile, no gypsum or ettringite were present in the alkali-activated slag sample exposed to Na2SO4 solution, while a considerable amount of gypsum was present in samples exposed to MgSO4 solution. As a result of gypsum formation, cracks at the corners of alkali-activated slag specimens and some softening of the concrete occurred after the exposure to the MgSO4 solution.

Puertas et al. (2002) investigated sulphate resistance of alkali-activated granulated blast furnace slag using variants of two different testing methods: ASTM C1012 and Koch-Steinegger. Blast furnace slag mortar samples were prepared using both sodium silicate and sodium hydroxide solutions as activator. Additionally, sulphate resistance of 50% slag+50% fly ash mixture activated with 10m NaOH solution was evaluated. Sodium silicate activated slag samples, and slag/fly ash mixture showed good sulphate resistance. However, mortars prepared from slag activated with NaOH showed 1525% lower strength after exposure to the sodium sulphate solution, compared to the reference samples cured in water. Traces of ettringite were detected in these samples.

More recently, resistance to sulphate attack of alkali-activated granulated blast furnace slag concrete was evaluated after five cycles of immersion in saturated sodium sulphate solution for 24h followed by drying of the samples at 1055C for 24h (Chi, 2012). The testing method was adopted from ASTM C88. Alkali-activated slag concrete had lower weight loss compared to OPC concrete. However, no regular trends were observed in the reduction of compressive strength of the concrete samples. Some of the alkali-activated slag samples showed some strength increase after the treatment with the sulphate solution, for example, alkali-activated slag samples cured at relative humidity of 80% and at temperature of 60C prior to testing.

The effects of external sulphate attack on mechanical properties and microstructure of alkali-activated granulated blast furnace slag were recently investigated by Komljenovi et al. (2013). Alkali-activated slag mortar samples were immersed in 5% Na2SO4 solution for up to 90days. At the same time, control samples of alkali-activated slag were cured in humid chamber, and Portland-slag cement (CEM II) was used as a benchmark material. Exposure to the sulphate solution caused a decrease in strength of Portland cement mortars, but not of alkali-activated slag samples. Strength loss of Portland cement mortars in sulphate solution was attributed to the formation of ettringite and gypsum. On the other hand, alkali-activated slag did not show significant structural alteration. Good resistance of alkali-activated slag to sulphate attack was attributed to the absence of portlandite and the unavailability of aluminium, substituted in CSH(I) or present in hydrotalcite gel, for reaction with sulphates.

Sulphate resistance of alkali-activated fly ash/slag (1:1 mass ratio) binder has been studied recently by Ismail et al. (2013). Alkali-activated fly ash/slag binder pastes, prepared with different water/binder ratios, were immersed in 50g/L NaSO4 and 50g/L MgSO4 solutions for 3months. It was found that decreasing the water/binder ratio increases the resistance to sulphate attack. Nevertheless, the cation accompanying the sulphate ion had more pronounced effect on sulphate resistance of the alkali-activated binders. No evident physical changes were observed in the alkali-activated binder samples after the exposure to the NaSO4 solution. Ismail et al. (2013) suggested that alkali-activated binder continued to stabilize and develop in the presence of the NaSO4 solution and noted that NaSO4 is often used as an activator in alkali-activated slag systems. On the other hand, MgSO4 was more aggressive to the alkali-activated binder pastes than NaSO4 solution. The presence of magnesium led to decalcification of the Ca-rich gel phases present in the alkali activated fly ash/slag system, causing degradation of the binder and precipitation of gypsum.

Holcim group and GTZ have defined the potential group classification of alternative raw materials which is presented in Table 3.3.7 [17]. The compounds extracted from various waste materials are listed along with the industrial sources from which the wastes were generated. For these resources to have increased utilization in the cement industry, policies need to be formulated which induce DCs to include them in the manufacturing process of their cement plants either as blending material or alternate fuels.

The cement industries will control fugitive emissions from all the raw material and products storage and transfer points. However, the feasibility for the control of fugitive emissions from limestone and coal storage areas will be decided by the government.

After performance evaluation of various types of continuous monitoring equipment and feedback from the industries and equipment manufacturers, appropriate government body will decide feasible unit operations/sections for installation of continuous monitoring equipment.

Based on mutual consultation and agreement between the state governments, interstate movement of hazardous waste could be permitted, in particular, to take care of difficulties faced by some states in development of treatment, storage, and disposal facilitiessuch as not having viable quantities of hazardous waste.

Field trials conducted by government bodies indicate that use of hazardous waste (such as effluent treatment plant sludge from textile units, tire pieces, paint sludge, tar residue, and refinery sludge) as alternative fuels in cement kilns could be promoted. The use could be in compliance with notified emission norms for hazardous waste incinerators, reuse of hazardous waste, however, for instance paint sludge after reconditioning as primer/coating is a preferable option over incineration. Such alternative options to reuse the wastes and sludge could be explored and encouraged.

The safer alternative to consider is incinerating the high calorific value hazardous waste in cement kiln as compared to conventional incineration. Prior to the incineration, the wastes must be subjected to suitable processing. Given the vast spread of cement plants across the country, the aforementioned option seems attractive. Sludge from petrochemical industry, oil refinery, and paint industry as well as residues from pesticide and drug industries are particularly suitable for this purpose as they possess high calorific value. In the cement kilns, the high flame temperature of around 2000C, high material temperature of around 1400C, and large residence time of around 45s ensure that the material incinerates completely. The noncombustible residue including heavy metals gets mixed into the clinker. To avoid such occurrence, it is advisable to blend and process the metallic waste before introducing them into the cement kiln. It is indicated from the field trials conducted by appropriate government bodies that monitoring hazardous air pollutants followed by a compliance notification of emission norms for hazardous waste use needs to be promoted.

The primary raw materials used to produce cement are limestone, gypsum, and silica. Many Indian companies have their concerns about the diminishing limestone reserves in India. They also mentioned that at this rate, the current reserves may last only for 1520 years. Further, gypsum an important additive is being imported from other countries. On the other hand, natural sand, which is also known as silica is obtained from mining which leads to soil erosion and degradation.

Coal acts as a major source of fuel for manufacturing cement. Therefore, it is of paramount importance that the availability of proper quality of coal exists for a longer period of time. According to the 95th performance of Cement Industry Report, Coal India Limited (CIL) and Singareni Collieries Co. Ltd. (SCCL) are the major suppliers of indigenous coal for the cement industry. It is learnt that in 20082009, against a consumption of 29.58Mt, CIL and SCCL supplied 14.29Mt, meeting only 48% of the total requirement. Around 15.28Mt was procured from other sources like open market purchases, import, use of pet coke, etc., at a higher cost, to meet the requirements. Other reasons for this deficit are delay in signing the fuel supply agreements (FSAs) by the cement and coal companies.

The report also mentioned that as per the notification issued by the New Coal Distribution Policy in 2007, the FSAs would be signed only for the 75% of the requirement based on the norms, resulting in an initial shortage in the allocation of coal.

The associated economics need to be analyzed and avenues for alleviation of finances should be explored by concerned authorities. Systemic evaluation of the components and their linkages will address the shortcomings and identify the areas of improvement.

CO2 intensity of the cement sector is 0.85kg CO2/kg cement produced. For the quantity of cement produced, 0.234g of particulate matter, 1.5g of SO2, and 3g of NOx are generated as by-products or pollutants

blast furnace slag cement - an overview | sciencedirect topics

BFS cements have a lower permeability than Portland cements, which contributes to the lower diffusion rate of ions through the hardened cement and improved durability in the presence of salts such as chloride and sulphate.

Materials formed by acid-base reactions between calcium aluminate compounds and phosphate-containing solutions yield high-strength, low-permeability, CO2 resistant cements when cured in hydrothermal environments. The addition of hollow aluminosilicate microspheres to the uncured matrix constituents yields slurries with densities as low as approximately 1200 kg m3, which cure to produce materials with properties meeting the criteria for well cementing. These formulations also exhibit low rates of carbonation. The cementing formulations are pumpable at temperatures up to 150C.

Ceramic microspheres for cementing applications may replace blast furnace slag and Portland cement in any well cementing operation. They are a useful well cementing constituent that may be successfully implemented in differing temperature dependent processes, such as the steam injection technique employed for heavy crude oil extraction. They are manufactured by the following steps (Quercia et al., 2010):

As the pyrolized particles are propelled away from burner, they begin to rapidly cool in air and are spheroidized. The microspheres settle at a distance from the burner that is dependent on their diameters. The composition of typical slags is summarized in Table 10.10.

Lightweight cement slurries can be formulated using ceramic microspheres that are resistant to elevated temperatures and thermal cycling, such as are used in steam injection techniques (Quercia et al., 2010).

Fig. 7.3 shows the typical comparisons of CO2 emissions for secondary precast concrete products, such as bricks, curbings, and fish shelters according to the type of binder, namely, GGBFS cement (OPC 50%+GGBFS 50%) or GGBFS binder activated by 7.5% Ca(OH)2 and 3% Na2SO3. Table 7.4 gives also the typical mix details of secondary precast concrete products. The mix proportions of precast concrete products using GGBFS cement refer to the mixing tables practically applied in the plants, while those of AA GGBFS concrete products are determined from mock-up tests [13], considering the performance criteria specified in the Korean Industrial Standard (KS) [14] and economical efficiency. For the concrete bricks, CO2 footprint due to aggregate transportation accounts for 20% total CO2 emission, which matches that obtained from the binder. This is attributed to the fact that the amount of aggregates in the mix proportions is generally as much as 10 times that of the binder. As a result, the reduction of CO2 emissions in the AA GGBFS concrete bricks is minimal compared with the GGBFS cement concrete bricks, indicating a reduction rate of approximately 2.5%. The total aggregate-to-binder ratio by weight in the concrete curbings and concrete fish shelters usually ranges between 3.0 and 4.0, indicating that the portions of binder in the total mixing materials are higher than those in the concrete bricks. The CO2 emissions in the AA GGBFS concrete curbings are lower by 37kg per functional unit than those in the GGBFS cement concrete curbings, showing a reduction rate of 20%. Further, the CO2 emission of the AA GGBFS concrete fish shelters is reduced by approximately 19% compared with the case of using GGBFS cement as a binder. The CO2 reduction in the AA GGBFS concrete products is significantly dependent on the aggregate-to-binder ratio together with the type and dosage of the added alkali activators.

All the hydraulic hardening binding materials generated by grinding Portland cement clinker, pozzolana blended materials and appropriate amount of gypsum are called Portland pozzolana cement (simply called pozzolana cement), code-named PP. The mixing amount of pozzolana blended materials accounts for 20%~50% of the total mass. The technical requirements for pozzolana cement are the same to those of blast-furnace slag cement.

Pozzolana cement and blast-furnace slag cement have many common grounds in performance (see Table 4.6), such slow hydration, setting and hardening process, low early strength, high growth of the later strength, low heat of hydration, high corrosion, poor frost-resistance, and easy carbonization.

The water demand of pozzolana cement is large. The dry shrinkage is more obvious than blast-furnace slag cement in the process of hardening. And under dry and heat conditions, dry shrinkage happens and the cement cracks. Thus, the conservation should be strengthened in use and it should be kept in the moist state for a long time.

As discussed in Section 5.10 of Chapter 5, CLSMs, which typically consists of a mix of cement, sand and fly ash, are emerging as a promising alternative to soil as a backfill material. Advantages of these products include ease of placement and compaction, which can lead to reduced noise and vibration and overall less interference with the surrounding environment.

The effects of SSA on the leaching behaviour have been covered, in brief, for a CLSM mixture using SSA as filler, crushed stone powder as aggregate and Portland cement or blast furnace slag cement as binder. The testing focused on the release of hexavalent chromium [Cr(VI)], as this species is of particular importance because of its carcinogenicity and expected contents in SSA (Horiguchi etal., 2011; Fujita etal., 2011).

Leached concentrations of 0.13 and 0.02mg/L have been recorded for CLSMs with Portland cement and blast furnace slag cement, respectively, used as a binder alongside SSA. The mix with blast furnace slag cement satisfied the target 0.05mg/L Japanese environmental quality standards for soil and the improved performance with this binder compared to the Portland cement mix can be attributed to added pore-filling effects. Treatment of SSA to reduce the element solubility has also been suggested by researchers to minimize leaching (Horiguchi etal., 2011; Fujita etal., 2011), though the specific processing details involved are not outlined. It is also rather surprising that additional results on the leaching of Cr(VI) from SSA on its own were significantly lower than the aforementioned leached concentrations from the CLSM backfill mixes, though regrettably, details of the testing procedures were not specified.

The latent hydraulic properties of blast furnace slag and alkaline activation are also useful in soil stabilization. According to [6], a combination of lime and granulated blast furnace slag is commonly used in South Africa and Australia for soil stabilization. The sulphate in the soil is known to be able to cause serious swelling in stabilized clay. This sulphate-related swelling is associated with the formation of ettringite. Granulated blast furnace slag cement is known to be resistant to this kind of expansion and swelling.

As used in soil stabilization, a combination of ground granulated blast furnace slag and lime results in slower early-rate strength development, ensuring prolonged time for construction work (particularly compaction and levelling after slag addition), the ability to self-heal in the case of early-life overloading damage, long-term increased strength, and inhibited deleterious swelling caused by sulphates or sulphides in soil [6].

Deep soil stabilization for both road construction purposes and building foundations may also be accomplished with alkali-activated blast furnace slag. Results of laboratory investigation indicate that cement-activated, ground granulated blast furnace slag has the potential for clay stabilization in deep stabilization when the aim is to achieve great pillar strength [50]. These results indicated that compressive strength increased with water content for stabilized clay, and the water/binder ratio decreased. With smaller amounts of binder, it is possible to achieve strengths of 300600 kPa, and activated ground granulated blast furnace slag cement is a good binder when the aim of stabilization is high strength (in the range of 10003000 kPa).

The investigations described in this paper were carried out in order to investigate the influence of concrete technical parameters on the leaching behaviour of heavy metals from cement-based materials with and without the application of industrial by-products. The following concrete technological influences were investigated:

A normal portland cement (PC) and a blast-furnace slag cement (BFSC) in compliance with the requirements of German standard DIN 1164 were selected for the investigations. Four bituminous coal fly ashes (FA) with different heavy metal contents were chosen as concrete addition. For the investigations mortar mixtures with and without addition of FA were prepared according to European standard EN 196-1. The fly ash content (f) in the mixtures with FA was 20 mass.% in relation to the total binder content (c+0.5f). Mortars with different w/b were produced. Additionally, concrete mixtures with and without addition of FA were prepared. The grain size distribution of the concrete aggregates corresponded to grading curve A/B 16 in accordance with the German standard DIN 1045. The w/b was 0.5 for all concretes. Mortar specimen (40 by 40 by 160 mm) and concrete cubes (100 by 100 by 100 mm) were produced from the mixtures. After one day in the mould, the specimen were cured until the investigations in a climate chamber at 20 C and 95 % relative humidity.

The results from the tank leaching tests with the mortar and concrete specimen are summarised in table 1. Generally, the amounts of heavy metals leached are very low, often near the detection limits, due to the immobilisation of the heavy metals in the highly alkaline, dense cement matrix. The addition of fly ashes with relatively high contents of heavy metals does not result in higher emissions. Due to the pozzolanic reaction and the filler effect of the fly ashes the emissions from mortar with fly ashes are often lower than those for the mortars without fly ash addition (especially for zinc).

The leaching results from the mortars are represented for zinc in dependence on the total zinc content in Fig. 1. The total zinc content was each divided into the fraction from the aggregate, the cement and the fly ash that were calculated based on the composition and the contents of the ingredients.

Fig. 1. Tank leaching test results in relation to total zinc contents jar different mortars in dependence on cement and addition (PC: portland cement; FA: fly ash; BFSC: blast-furnace slag cement; w/(c+0.5f)=0.5 for all mortars)/1/

The amounts of zinc leached in the tank tests are, for mixtures with addition of fly ash, inspite of higher total contents, less than those of the corresponding mixtures without fly ash. This applies to the mortars as well as to the concretes. This effect can be attributed to the immobilisation of zinc due to the formation of insoluble complex salts. The diffusion of zinc in the leachant is, in addition to this, impaired due to the compacting effect of the fly ash. The amounts of leached zinc are especially low for blast-furnace slag cement mortars. The zinc is presumed to be fixed in the glassy matrix of the blast-furnace slag and thus cannot be mobilised.

Fig. 2 shows the tank leaching results from mortar specimen for chromium. The influence of the fly ash addition is not as distinct as for zinc. The chromium amounts leached for mortar with fly ash FA! lie above the value for the mortar without the addition of fly ash. Fly ash FAI has a high fraction of soluble chromium compared to the other fly ashes /2/. A decrease in the amount of leached chromium can be recognised in the other mortar mixtures with fly ash. This can also be attributed to the compacting effect of the fly ash.

Fig. 2. Tank leaching test results in relation to total chromium contents for different mortars in dependence on cement and addition (PC: portland cement; FA: fly ash; BFSC: blast-furnace slag cement; w/(c+0.5f)=0.5 for all mortars)

The amounts of leached chromium from the mixtures with blast-furnace slag cement are rather high compared to the total and available chromium content. The reason may be a reduction in the pH value of the pore solution due to the large content of blast furnace slag. This effect does not occur with zinc, since zinc has its minimum solubility at pH values around pH=10.

In Fig. 3, the leaching behaviour of chromium for mortars with and without addition of fly ash is shown in dependence on the age of the samples. As expected, the leached amounts decrease with increasing age of the samples. The influence of the hydration age occurred in the same manner for the other elements /2/.

The release of heavy metals from cement-based building materials is mainly controlled by diffusion. In order to be able to compare and assess the results from tank tests, the heavy metal amounts which were released due to diffusion (emission after 365 days) were calculated from the test results (details of the calculation see /2, 3/).

Smaller emissions due to diffusion were yielded for the PC mortars with addition of fly ash in relation to that without fly ash addition. This can be attributed to the filler effect of the fly ash and to a compaction in the pore system due to the pozzolanic reaction. This leads to a hindering in the diffusion process. All fly ashes had a diffusion hindering influence on the metals zinc, copper and lead /2, 6/. The influence was not so clear in the case of chromium.

The chromium emissions calculated for the mortars made of blast-furnace slag cement are larger than those for portland cement mortars, whereas only very small amounts are released for the heavy metals zinc, copper and lead. The addition of fly ash does not have a strong influence on the release rates. This can be explained by the fact that the use of blast-furnace slag cement strongly reduces the release rates. The additional effect of the fly ash is not noticeable.

The emissions calculated are generally larger for concretes than for mortars. This could be attributed to the higher porosity of the hardened cement paste in the contact zone areas between aggregate and hardened cement paste. This effect is more distinct for concrete, because of its higher fraction of contact zones, than it is for mortar. Another possible explanation is the smaller fraction of cement matrix resulting in lower pH values in the leachates.

In Fig. 5, the emission in dependence on water/binder-ratio are represented for mortar with and without addition of fly ash. Fig. 5 shows no distinct influence of the w/b-ratio on the emissions. The emission is partly reduced with higher w/b ratios (contrary to what was expected). This is attributed to higher dissolution of other ions (like calcium) which lead to a precipitation of the heavy metals. Generally the leached amounts are very low, therefore, the influence of the w/b-ratio could not be deduced. For main elements like sodium or potassium, the emission increase with increasing w/b-ratios /2/.

Highly contaminated sieve sand may not be utilised, neither if assessed on the basis of total content of Poly cyclic Aromatic Hydrocarbons (PAHs), nor if assessed on the basis of leaching of these PAHs.

This highly contaminated sieve sand (containing up to 1,000 mg/kg PAHs) can be stabilised well by adding 9% of blast furnace slag cement and 1% of an additive. Only 0.7 mg/m2 (being 0.002 % of the total concentration) is being leached during the 64 days lasting Dutch diffusion test.

However, if the stabilised sieve sand would have to be assessed on the basis of leaching, the material could be utilised in a category 2 application (because of the relatively high leaching of sulphate). The leaching of all other components (PAHs inclusive) is below the limit values of category 1 applications.

Both the ASTM C33 (2013) and the BS EN 12620:2002+A1 (2008) standards have a provision for manufactured fine aggregate to be used in concrete, although the definitions of manufactured aggregate therein are different. For the former, the definition is given in a separate standard, ASTM C125 (2013), in which manufactured sand is defined as fine aggregate obtained from crushing rock, gravel, iron BFS or hydraulic-cement concrete, whilst for the latter, manufactured aggregate is a material of mineral origin derived from an industrial process involving thermal and other modification (BS EN 12620:2002+A1:2008). Thus, it appears that CS can be considered as manufactured aggregate in the BS EN standard.

Among the compliance requirements, special attention must be paid to the grading of CS, which is governed by the type of CS. As shown in Section 3.4.2 of Chapter 3, quenched CS tends to be a coarse grading material, whilst spent CS is likely to be in the fine to medium zone in accordance with BS EN 12620:2002+A1:2008. On the other hand, air-cooled CS, which appears as rocklike material, needs to be crushed and sieved to a sand grading requirement, though its use as a fine aggregate is unlikely.

Alkali-activation technology has been used to produce inorganic cements for over a century, as a means of valorizing wastes or industrial by-products derived from different commercial activities. Granulated blast furnace slags (GBFS), derived from the iron-making industry, have been widely utilized for the production of alkali-activated cements. Significant advances in understanding the roles of different factors which govern the properties of alkali-activated GBFS cements have been made in recent decades, to the point that concretes based on alkali-activated GBFS are commercially deployed in several parts of the world. However, GBFS is not the only slag that can be used as a raw material for producing Portland clinker-free inorganic binders. Various other metallurgical slags, which currently have little or no commercial value, can also be utilized as raw materials for producing inorganic cements. The main difficulty to overcome in this area is the generally lower hydraulic reactivity of these slags compared with GBFS, and the high content of heavy metals which can limit the utilization of some such slags as building materials. Alkali-activation can thus, in some instances, also be seen as a means for the consolidation and safe disposal of such materials, which may otherwise pose an environmental hazard. This chapter provides an overview of inorganic cements produced via alkali-activation, particularly those which utilize nonblast furnace metallurgical slags including steel, ferronickel, titaniferous, stainless steel, lead, copper, zinc, nickel, manganese, silicomanganese, and phosphorus slags.

Test in distilled water (EN 12457-4, 2002): Zn (particularly because of its high concentration in the initial waste), Co, Ni, Cu and Sn are less leachable metals. In contrast, Mo, V and Cr, elements that typically form oxyanions, showed the worst results regardless of the matrix in which they are located. Materials prepared by activation with potassium and high calcium content (blast furnace slag and Portland cement) have obtained the best results. Ba and Pb are less effectively incorporated into the calcium matrix (Portland), and Sb in the geopolymers. This last element is the most problematic for inertizing in geopolymers. The above highlights once again the influence of the calcium present in the matrix on the immobilization of different metals.

Test in acetic acid (EPA TCLP (US EPA, 1986)): the results were compared with the limits set by the US Environmental Protection Agency (US EPA; www.epa.gov) for As, Cr, Cu, Zn, Pb, Ba, Cd, Hg, and Se. Concentrations of the metals were below the limits, but the best performances were obtained for metakaolin-based geopolymers activated with potassium.

Test in nitric acid (NEN 7345, 1993): Ba, Zn, Pb, Cr and Cd show concentrations below detectable limits. Comparing the different matrices, there is a lower release of Zn and Cr matrices with Portland cement and with blast furnace slag gepolymers, a fact probably related to the high alkalinity of the calcium-containing matrices. Conversely Ba shows worse behaviour in the calcic environment, according to the test in UNI 10802:2013, probably because of chemical interactions with the aluminosilicate lattice characteristic of geopolymers, which are able to immobilize them effectively.

GANC test (generalized acid neutralization capacity test) (Isenburg and Moore, 1992): this test is used to quantify the equivalents of acetic acid necessary to reduce the pH of an aqueous solution in contact with a solid. Thus it provides information on chemical neutralization, speciation and mobility during leaching of toxic metals from waste stabilized. The metals Zn, Pb and Cd showed amphoteric behaviour irrespective of the matrix. In particular, in geopolymers they show less leaching at neutral pH and acid.

In addition to the studies of the immobilization of heavy metals through the release test (UNI 10802:2013), the problem related to the presence of anions and their relative stability in the geopolymeric matrix is another important issue. Lancellotti et al. (2010) have conducted an investigation into the immobilization of soluble anions such as chlorides in metakaolin-based geopolymers containing a mixture of ashes from electrostatic and fabric filters following incineration of municipal waste. The activation was performed with sodium hydroxide and silicate and the leaching results show a significant decrease in the release of all metals with respect to as-received ashes. The leaching values were lower than the limits imposed by Italian law for the landfill for non-hazardous waste (DM 30/08/2005). An important effect was evidenced for Cd, Pb and Cu, which were strongly released from both ashes in the as-received state, while they are completely immobilized in the geopolymeric matrix. Cr behaved differently since it was partially released from the geopolymer containing ash from the fabric filter. Even though Cr values were maintained below the limits for a landfill for non-hazardous waste, its release was strongly influenced by the presence of chloride ions that facilitate its leakage despite being contained in these ashes in an amount lower than in the ash from the electrostatic precipitator. Assuming the chromium is in the form of insoluble hydroxide, it may be more leachable in solutions containing chlorides which are able to reduce the range of stability of the hydroxide of chromium in both acidic and alkaline conditions, promoting the dissolution in the form of Cr3+ and CrO33, respectively (Lancellotti et al., 2010).

The release of chlorides in geopolymers shows that the sample containing the ashes from the fabric filter presents higher values of leakage due both to the higher content of this anion in the ash itself and the co-presence of soluble cations. In the reference, which contains the ashes from the incinerator, the release of Na is very low and this indicates that both Na+and Cl come from the soluble salts in the ash and not just from the alkaline solution used as an activator. From these data the greater ability is apparent of the geopolymer containing ash from the electrostatic precipitator to retain only 30% the chlorides that are released, while this percentage increases to 81% for the material containing ash from the fabric filter.

The parameters of the production process of geopolymers can have a significant influence on the leaching of heavy metals. Izquierdo et al. (2010) studied the influence of open and closed conditions adopted for the aging stage in geopolymers obtained from blast furnace slag and coal fly ash enriched in Cu, Ni, P, Sn and Zn deriving from the co-combustion of sewage sludge. The activation was done with potassium silicate and the leaching test performed in open and closed solution vessels. In water (EN 12457-4, 2002) Mo, probably present in the form of soluble salts, is leached up to 75%. As, B, Se and V show a significant mobility (up to 10% of their initial content) linked to their presence in the form of soluble salts, which precipitate during the aging process rather than during the early stages of reticulation of the matrix. The mobility of all other metals such as Ba, Be, Bi, Cd, Co, Cu, Ni, Hf, Nb, and Pb, and also traces of rare earths, is very low and independent of the curing conditions. These results are reproducible for use in both the test EN 12457-4 and the test EA NEN 7375 (2004).

The ashes of urban incinerators could be subjected to pre-treatment before their geopolymerization. Given the high concentration of soluble ions, Zheng et al. (2011) conducted a study on the influence of a washing pre-treatment on ash from fabric filter. The results showed that the washing leads to higher values of mechanical resistance and to a reduction in the leaching fraction of Cr, Cu and Zn. This behaviour confirms the feasibility of a combined process of washing/immobilization, although an assessment is necessary of the appropriate management of the washing waters rich in K+, Na+, and Cl (Zheng et al., 2011).

slag cement - lehigh hanson, inc

Slag cement is used in concrete for virtually any construction application, either in conjunction with traditional portland cement, part of blended cement or as a separate component. It typically replaces part of the portland cement in concrete mixes.

According to the Slag Cement Association, an industry trade group, incorporating slag cement as a supplement in concrete offers higher strength, reduced permeability and improved resistance to chemical attack. And as a recycled material, its production requires far less energy and natural resources than the conventional cement it partially replaces.

Slag cement, originally known as granulated blast-furnace slag, begins with the production of iron. The heart of the process is the blast furnace that refines iron ore into iron. The ingredients are heated to nearly 1500 degrees Centigrade to form two components: iron and molten slag.

The iron is used to produce steel, and the molten slag is converted to a cement-like material by rapidly cooling it with water. This rapid cooling, called quenching, creates glassy granules, which are then ground into the fine powder known as slag cement.

Slag cement is as old as iron-making itself. In the 1700s, it was combined with lime to make mortar. One of the first major uses of slag-lime cements was construction of the Paris underground in the late 1800s. In the U.S., blends of slag and portland cements were introduced in 1896.

Slag cement is used in nearly all types of concrete construction: pavements, structures and foundations, mass concrete (i.e., dams and retaining walls) and precast concrete products such as pipe and block.

Advocates point to a number of ways in which slag cement makes concrete better and more consistent: Easier to place and finish Higher strength Lower permeability Better resistance to corrosive chemicals Lighter color than conventional concrete (better architectural and decorative finishes)

First and foremost, its a recycled product created from blast-furnace slag otherwise destined for disposal. The energy, emissions and raw materials required to produce slag cement is a fraction of that needed for traditional portland cement.

Slag cement requires nearly 90% less energy to produce than portland cement. According to the Slag Cement Association, substituting 50% slag cement for portland cement reduces greenhouse gas emissions by more than 40% and lowers the embodied energy of concrete by more than 30%.

Higher reflectance also mitigates the heat island effect, whereby highly developed urban areas tend to absorb heat and experience higher temperatures. Light-colored buildings and pavements reduce the energy needed for cooling and lowers ozone levels.

The Environmental Protection Agency recognizes the environmental benefits of using slag cement in concrete. It has classified slag cement as a recovered" product under the Resource Conservation Recovery Act and has issued a procurement guideline requiring its specification on most federally-funded projects.

mills for cement and granulated blast-furnace slag gebr. pfeiffer

Modernization projects which call for the integration of grinding plants into limited areas within existing cement works are one of the topics Pfeiffer engineers deal with in their everyday work but they also handle projects for the implantation of grinding plants in completely new cement works or separate grinding terminals. No matter what type of project is concerned, the basic idea is to meet individual requirements. This is what we do by optimally adapting our products and processes to the prevailing conditions. Cements consisting of the components cement clinker and gypsum and maybe also additional extenders can be ground in common, but the materials (like clinker, granulated blast-furnace slag, limestone, sulfate carriers) can also be ground separately for being mixed after having been finely ground.

What we also do is to discharge undesired material like metallic components and prevent extenders which already have the fineness required, e.g. fly ash, from being reground unnecessarily. In this way our plants remain as compact as possible, thus reducing capital expenditure significantly.

Cement clinker is produced on the basis of a raw material mixture which is sintered in a kiln system at more than 1400C. As a result, compounds form the so-called clinker phases which are specific calcium silicates and calcium aluminates. These ensure the characteristic properties of the cement during hydraulic hardening. The so produced cement clinker is ground together with gypsum and/or anhydrite, thus forming the final product cement. Depending on the availability of the raw materials and market-specific requirements, further extenders are added to the common grinding process such as granulated blast-furnace slag. Mainly consisting of lime, silica acid, aluminum oxide, and magnesium oxide, blast-furnace slag is normally processed in granulation plants nowadays. By quenching the liquid slag, a glassy, granular product is obtained, the so-called granulated blast-furnace slag which is particularly suited for use as an extender in the production of high-quality cements.

The cement components are ground, dried, and classified in the Pfeiffer vertical roller mill. Moist feed materials can be fed to the mill separately from warm and dry feed materials. Product quality and product fineness can be adjusted within a wide range (up to 6000 cm2/g Blaine). The ground and dried products are separated from the process gas by a dust collection filter which is followed by a fan. Downstream of the fan, the gas volume flow is divided: part of it flows back to the mill, the remainder is evacuated through an exhaust gas chimney. The mill is equipped with an external material recirculation allowing the reduction of pressure loss in the mill. It can also be used for emptying the mill in case of maintenance. When grinding granulated blast-furnace slag, the iron particles contained in the slag can be discharged.

cement plant and mainline | model railroad hobbyist magazine

I like building sprawling complexes and using structures that dwarf the trains and really seem to provide the railroad a reason to exist. I have my last eight feet on one level that needs to be filled. One of my concerns is taking up too much real estate with large footprint buildings that do nothing but "sit there".

As I was looking over my boxes of unbuilt Walthers kits, etc for a little inspiration, I noticed Valley Cement,Walthers Part # 933-3098. I bought this thing at a trainshow for about half its' current value. When I bought it, I had noidea how I would use it, I just knew that it was a deal and it could be worked into the plan.

Then it occurred to me that this structure would make an AWESOME backdrop industry with lots of switching, especially using A LOT of these buildings along the backdrop and as a portal to hide the holethrough the wall.

Here is how the story goes. Two railroads ran very close to each other and actually crossed each other near the old cement plant. At some point in history, a merger was allowed between the two railroads. The cement plant is still a healthy customer and the railroad reorganized the traffic flow and designated the primary main and the old secondary into primarily uni directional operations.

Nowadays, the junction is still used but trains now move off of the double track main using an upgraded interchange track near the old cement plant. Trains on the secondary main still cross the junction, but the secondary main ends not too far from this junction. The old line is now used to service a few industries and a small yard to hold cars for the cement plant. I do not know exactly what the plant will look like at this point, but something that straddles the tracks will cover the future opening through the wall.

In the bottom of the photo is the interchange switch that moves trains from the double track to the secondary main. In the background, it is possible to see the beginning of the wooden trestle that creates a speed restriction of 40mph over the bridge that spans the river. This area is seen in other blog posts.

I know of a few Portland cement plants located alongside mainlines. The plant at Howes Cave, New York on the D&H is one. The mill that is Clinchfield, Georgia (south of Macon on the former Southern) is another. Lehigh Cement's mill in Leeds, Alabama is close to the old Southern and CofG mains. Monolith, California (just east of Tehachapi summit) and Victorville (just east of Cajon Pass) also fit the bill.

Otherwise, most plants I can think of are on branchlines or long spurs off mainlines. In many cases the mill _is_ the town, and it's named something like "Portland" (as in Colorado) or "Limestone" or some such.

Most Portland cement plants are located very near limestone quarries, so their inbound supplies are usually limited to fuel (often coal), kaolin or bauxite, gypsum, and bags if the plant sells it that way. In the days before covered hoppers (late 1930s), most cement was shipped in barrels in boxcars so they would also get cooperage supplies. A plant that is not near a quarry, like the old Williams Brothers plant in northwest Atlanta got some of its stone in gondolas.

The Walthers "Valley Cement" is flawed in two important ways. Most noticably it doesn't have a preheater tower, that tall structure in the middle of your night shot. It also doesn't have clinker storage. Raw cement when it comes out of the kiln is called clinker and it's generally the size of pebbles or even bigger. It is mildly hydro-reactive so it is stored in large covered sheds. Then it is fed into the mill to be ground into a fine powder and mixed with gypsum to regulate the setting time. Only then is it stored in upright silos awaiting loading into covered hoppers.

So you need a track for inbound fuel (unless it's fed oil or natural gas by a pipeline), and other one or two for the other supplies. And you need a track for covered hopper loading and probably a storage track or two to store empties. Many mills have their own switchers because they need to move cars all day.

You asked about "how many switches off the mainline?" Be careful there. Mainlines busy enough to need directional running will be signaled and that means you try to minimize switches off them. A cement mill or other large industry will have a running track alongside the main and the switches will come off that. Just something to think about.

As far as aggregate coming in in gondolas, I guess this is primarily limestone from a Glacier gravel type of kitbash? If it doesn't come by rail, some large conveyors or something like that would look appropriate, I guess?

I think you have a point. I just wonder how many kits they could sell and at what price? For $250, I would probably just try and kitbash a decent representation. If they could offer something that would even be a backdrop offering, I would probably go for it, but I think part of the problem is that it is a tall structure, even selectively compressed. If it was a good kit, I could see it selling for between $50 and $75, especially with the detail items that would be necessary.

This still doesn't address the fact that Valley Cement is a BIG kit. It should be and I think Walthers did a pretty good job with it. I am just comparing walls and so forth and the only way to fit it into my enormous space is to have the mainlines run through the middle of the plant. The rotary kiln, which would happen to attach to the missing preheating tower, is about 18". I am modeling that as a full piece because I don't think it would look right being modeled as a backdrop structure.

The thought crossed my mind to use some Evergreen/Plastruct structural shapes or even the Walthers skyscraper under construction for the tower parts. I then thought I could use Lego rocket parts or something like that for the shapes for the big cyclones, but those pieces are ridiculously EXPENSIVE. I might as well just modify pieces of the blast furnace.

Ash Grove Cement is a major producer, and the Wikipedia article at https://en.wikipedia.org/wiki/Ash_Grove_Cement_Company has links to views of several of their plants that will help you with rail configurations and the siting of plant facilities.

The first thing you will notice about the site is that the railroad makes a return loop within the facility. at one point you can also look at the property and see that there was a spur of what was perhaps a second wider return loop along the south fence; in short, this is almost a perfect 4x8 Single Industry Layout candidate.

This plant's primary source of limestone is that long conveyer system that leaves the picture in the lower right, it is about four to five miles long in length. You'll also notice a string of gondolas along the southern fence, so it appears this site is also receiving or shipping raw material.

So looking at this site from the top, you have a coal pile that feeds via a flood loader (Flood loader kit) into the furnaces(Refinery kit) that is serviced by frontend loaders (Kibri kit). You have the raw material in the large reinforced building (Cement kit building #3) on the left and the prepared cement (Medusa Cement) on the right. Then you have a couple buildings that resemble a couple of the Kibri//Vollmer silos towards the bottom of the picture.

This brings us to the kilns. The plant at Rillito appears to have not one but three rotary kilns, each one lined up with one of three smokestacks towards the bottom of the picture. Finally, you have the blast furnace section, filling up much of the mid left of the picture, and a large long building to the right that ends in a large covered parking lot towards the coal yard.

I want to eventually model a cement plant, and I'll likely start with the Valley Cement kit for the rotary kiln and store house, a blast furnace for the tower, a couple Medusa Cements for the delivery side, a coal loader and a refinery kit for the furnace, and a couple other kits to round out the auxiliary silos/handling units around the perimeter of the plant. I may only build a one kiln plant, versus modeling this three kiln plant, but if I get to it and the fancy strikes me, I could easily see myself expanding out to capture all three kilns. As is, the Valley Cement kit would do well to get me close to a one kiln plant.