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Slag Cement at Coltness

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The following is a transcript of an anonymous article that appeared in The Engineer, 109, 21/1/1910, pp 60-61, which is believed to be out of copyright. Although the article is anonymous, it was certainly written by Bertram Blount (1867-1921). Blount was retained as a consultant by the Collos Portland Cement Company, which was set up in 1907 by Edward John Vavasour Earle (1851-1923). He used his company, Martin, Earle and Co. Ltd, as a source of working capital. The purpose of the company was to acquire the 1905 patent of Heinrich Colloseus for an activated slag cement, then to sell world-wide licences to produce the cement, and to set up demonstration manufacturing plants. The underlying philosophy was a belief that, given the relatively large production (at the time) of slag as a zero-value waste material, the slag cement, produced at much lower cost, could completely displace Portland cement as a building material. The psychology of the promoters is demonstrated by the fact that, whereas the production plant described here cost £35,000, the value placed on the patent was £1 million. In addition to the grandiosity of the promoters' expectations, the article also demonstrates a persistent tendency of promoters of such products, to re-engineer the English language to make the product sound slightly less second-rate.


The great similarity in chemical composition existing between some types of blast furnace slag and Portland cement has long been recognised (Note 1), and has given rise to several attempts to utilise this waste product of the smelting works, so as to convert it from an encumbrance into a profitable source of trade (Note 2). The advocates of research in this direction maintain that the intense heat of the blast furnace brings about a complete fusion between the component substances of the slag, whereas the lesser heat employed in the ordinary kiln producing Portland cement is insufficient to do so. They hold that this incipient fusion of the latter process is a point of weakness in the system, the incomplete chemical combination resulting therefrom being regarded as the cause of numerous defects in the cement or mortar when used under certain conditions (Note 3). Thus the disintegrating effect of some alkaline liquids, of sea water, and of great heat is attributed by them to this cause. Again, the uneven coloration seen in some cements and the consequent stains produced on certain stones, such as marble, is due, they contend, to the non-union of the metallic salts in the cement with the other constituents.

Until lately, most attempts to transform the slag from blast furnaces into a commercially useful cement have resulted in failure. The "Puzzolan" cements of America (Note 4) are derived from this source, but their use is restricted in practice to cases where the cement or concrete after setting is kept moist, as in sea-water constructions, the thoroughly dry material being, we understand, found defective in strength.

Within recent years a new treatment for blast furnace slag has been designed by Dr. Heinrich Colloseus, of Berlin (Note 5). The results obtained, both commercial and technical, have been such as to induce several iron companies to adopt this method as a further aid to the economical working of their furnaces. In addition to this, the cement produced is said to compare so favourably to that obtained from the ordinary processes of manufacture that in some cases blast furnaces of a specially designed type have been erected for the purpose of producing the cement directly instead of its being produced as a by-product of iron smelting. The composition of blast furnace slag necessarily varies with the ores used, and it is only in those cases where the chief constituents are in a certain ratio that the process is possible. Thus a slag containing above 42 per cent. of lime and not more than about 37 per cent. of silica is suitable for the Colloseus method of cement production (Note 6).

In this country the Coltness Iron Company, Limited, Newmains, Lanarkshire, has acquired for the present the exclusive rights to work the process in Scotland (Note 7). In other countries several ironworks are employing it most successfully. At the Newmains works, which we recently visited to inspect the process as carried out there, considerable time has been given to perfecting the method and adapting its details to local necessities. The composition of the slag obtained from the hematite ores used at these works is as follows:-

per cent.
Total sulfur2.60
Manganese oxide0.50

The company has laid down an extensive plant for the economical production and handling of the slag cement. In addition to the actual producers and transporting arrangements a highly modern grinding mill has been erected and completely furnished testing houses laid down. In the accompanying engravings the extent of the plant will be seen.

General view with the cement store and packing plant nearest, attached to the mill house behind, and the slag store beyond.

The routine of the process is simple, although the chemical reactions taking place are complicated and in some cases unknown. During the smelting of iron it is customary to run off at regular intervals the slag which collects on the top of the molten iron in the furnace. At the Coltness works this slag is caught in a ladle, holding about eight tons, as it leaves the tapping hole. During the running of the iron any slag remaining in the furnace is led off from the "sow" and added to that already in the ladle. An overhead crane running on gantries past the furnace is then employed to carry the slag ladle to the "granulators" - see Figs. 4 and 5. The ladle is then deposited on an iron cradle mounted on journals about which it can be rotated by an electric motor directly geared to one of the trunnions. As the slag flows from the ladle it is caught by an open spout, which guides the stream on to a revolving drum running within a housing provided with a flue for carrying off the gases generated in the process. The drum is formed with open ends, and is perforated with numerous slot openings. Through each end of the drum a pipe is led into its interior, and as the slag falls on to the horizontal surface of the rotating drum, a jet of water containing five per cent. of magnesium sulphate in solution is sprayed into the drum, where it finds its way through the holes, and becomes intimately mixed with the slag. A third pipe at the same time delivers a jet of the same solution directly into the slag as it falls from the spout on to the drum. Partially as a result of the aqueous spray, and partly due to the impact with the revolving drum, the slag falls away from the granulator in a disintegrated condition. It is, however, still hot, and in a fairly coarse state (Note 8). The rate of tipping of the slag ladles and the speed of the drum can be varied as these are found to have at marked influence on the process of granulation.

Fig. 4: Front of the granulator platform, with the end of the overhead crane gantry. Two liquid slag ladles can be seen, the one on the left in process of tipping. The end blastfurnace is to the left. The tall domed structures behind are two of the seven hot blast stoves. Fig. 5: Side view, showing the crane carrying ladles to the left, the two granulators centre, and the elevator raising the product buckets for transportation to the grinding plant. The row of six blastfurnaces is behind the granulators, and the solution tank is visible through the ropeway staging.

The chemical changes taking place during this stage in the process are complex, and form an essential feature of the Colloseus method. What these changes are would appear to be for the present incapable of complete statement, but it is evident that they are chiefly concerned with the sulphur contained in the slag. Quantities of sulphur dioxide and hydrogen sulphide are given off, and the greater part of the calcium sulphide originally present is oxidised to the sulphate (Note 9).

In the event of any slight variation in the composition of the slag the strength of the spraying solution is altered, this apparently having compensatory effects. Although magnesium sulphate is employed in the spray, we understand that any salt of the earthy oxides which is soluble, in water can be used. These can again be replaced by the sulphate of calcium, aluminium, sodium or potassium, or mixtures of these with magnesium sulphate, while solutions of salts derived from the iron group - chromium, nickel, manganese, &c. are also available. The sulphate or chloride of iron is said to be especially favourable to the production of a cement which would resist the action of sea water effectually (Note 10).

Plan based on the 1912 map showing the layout of the furnaces, granulators, ropeway and grinding plant.

As the clinker (Note 11) falls from the drums it is guided through shoots into buckets suspended from an overhead runway. The buckets when filled with clinker are elevated and attached to an aerial ropeway (Note 12) which conveys them to the clinker house, where they are automatically emptied, and the slag is stored until required for grinding. An end view of the "granulators" and the elevating gear of the ropeway is shown in Fig. 5. The circular brickwork tower to the right of the engraving contains a boiler for use in preparing the spray solution, which is stored in a steel tank on the top of the tower. Fig. 6 gives a view of the "granulator" from the rear, and in Fig. 7 a portion of the "Bleichert" aerial ropeway as it leads into the clinker storehouse is seen.

Fig. 6: Rear of the granulator platform. The ropeway buckets are queued up to receive slag from the chutes under the granulators. Fig. 7: The entry of the ropeway into the slag store.

The succeeding processes in the manufacture of Colloseus cement are identical with those employed in the ordinary course. From the clinker house the granulated slag is transported by the buckets of an overhead runway to the first of the grinding mills. In succession the clinker is passed through as screw crusher, ball mills, and tube mills, the ordinary degree of fineness being such as to leave a 10 per cent. residue on a sieve having 32,400 meshes per square inch. After grinding is completed the cement is carried by a spiral conveyor and chain elevator to the hopper at the top of the building, being automatically weighed on its journey. The arrangements connected with the discharge of the hoppers are such as to allow a mixing in any proportions of the contents of two or more of them (Note 13). In Fig. 5 will be seen an exterior view of the mill-house, with the clinker store in the background. The small building between these is employed as a testing house, and here the cement is subjected to rigorous comparisons with ordinary material. Figs. 1 and 2 show views of the ball and tube mills, the photographs having been taken during the construction of the buildings.

Fig. 1: Ball mills under construction. Fig. 2: Tube mills under construction.

The entire plant, granulators, conveyors, and mills, is electrically-driven, power being obtained from the blast furnace gases used in conjunction with a Cockerill gas engine, and from a large Rateau turbine running on the exhaust steam from several small Parson turbines, which are employed to drive the exhausters of the by-products recovery plant. Thus the entire industry, both as regards raw material and the power to work it, is based on the utilisation of "waste" products. We may add that this is a side of its business to which the Coltness Iron Company devotes considerable attention in several directions. As witnessing the extent to which it has adopted this process, it may be noted that at present there are nine furnaces connected with the granulators, each furnace producing 120 to 140 tons of cement per week, thus giving a total which will compare quite favourably with that obtained at most works devoted exclusively to the manufacture of cement (Note 14).

With regard to the strength of the material thus produced, we have been supplied with the following figures:

lb per sq in
7 days468
28 days571
12 months775
18 months778
3 to 1 sand
7 days326
28 days400
12 months482
18 months556
compressive, 3 to 1
7 days2300

A typical analysis of the cement is given below (Note 15):-

per cent.
Iron oxide1.23
Calcium sulphide2.18
Sulphuric anhydride1.11

We are indebted to the Coltness Iron Company for permission to inspect the process at the Newmains works and for the photographs from which the engravings accompanying this article were produced. We also desire to thank the Collos Portland Cement Company (Note 16), Limited, 139, Cannon-street, London, E.C., for information supplied on behalf of Dr. H. Colloseus.

- o - O - o -


Note 1. The similarity in chemistry mentioned here and elsewhere is a similarity in the bulk chemical analysis of the materials. Portland clinker relies for its performance on the particular minerals present in it. The amount and character of these in the clinker entirely define the way the clinker reacts in the finished cement. The minerals present in Portland clinker are entirely absent in blastfurnace slag, so a trivial comparison of the chemical analyses of these materials is no guide whatever to the usefulness of the slag. However, in 1910, an understanding of the mineralogy of clinker was still rare among cement manufacturers, and the writer of the article (who most certainly did understand this mineralogical difference) takes advantage of the general ignorance in the rest of the article. John Hudson Earle quotes Vavasour Earle (5/9/1907) as saying "slag cement is the same analysis as the old Roman cement, which has stood the test of time", which pretty well sums up the technical competence of those promoting this project.

Note 2. During the subsequent century, this problem has been solved. Proper quenching of fresh blastfurnace slag, so that it solidifies as a glass rather than a crystalline mass, produces a material that, when ground finely, is used as a concrete component that has many beneficial properties.

Note 3. The sole proponent of this philosophy (although he refers to himself here in the plural) was Bertram Blount. The various potential defects of cement that he lists are not the result of "incipient fusion" at all. A cement rawmix can indeed be completely melted at a very high temperature (around 2100°C), and provided that it is crystallised with rapid cooling, the properties of the product are indistinguishable from that prepared by sintering at a normal (1400°C ) temperature. So complete melting produces no quality benefit, and is much more costly by virtue of the higher processing temperature needed. But in 1910, the "cheap fuel" mindset made the latter consideration less important.

Note 4. Granulated blastfurnace slag is not a pozzolan, but in the USA it was termed thus for a long time (and is still occasionally so-termed by the ignorant). The "puzzolan cements" achieved a small consistent market for a while in the USA, mainly as masonry cements, and were made by intergrinding granulated slag with about 10% lime.

Note 5 The first Colloseus patent was 1905. Vavasour Earle launched a company to acquire the patent, to set up manufacturing facilities, and to sell licenses. He retained Bertram Blount to be his technical front-man.

Note 6. A high level of lime/silica ratio is required for the slag to be reactive. On the other hand, with defective quenching, a high lime slag crystallises belite, which undergoes β-γ inversion, causing the whole mass of slag to "fall" to an inactive powder. None of this was understood at the time, and the knife-edge control of the slag composition was way beyond the capabilities of iron works at the time.

Note 7. However, about this time, it was also being made by GISCo at Wishaw. In the north of England, both Casebourne and Trechmann tried it, and an independent got involved. Coltness had been experimenting with slag and slag/lime cements for a decade before they set up their very substantial plant for the Colloseus process in March 1909 (commissioned end of August 1909). The particular concentration of slag use in Scotland was the result of the almost total lack of Portland cement production in Scotland. The Scottish market was largely supplied from the Thames area, at high price.

Note 8. Experience with air-cooling of slag indicates that the larger (i.e. greater than sand-sized) particles are crystalline and therefore unreactive. The amount of water used is unspecified, but it was a magnesium sulfate solution, and very little magnesium sulfate appeared in the product (as was necessary for economic production). A minimum of 0.7 t of water per tonne of slag is needed for water quenching, but the amount used must have been a tiny fraction of that.

Note 9. Most of the sulfur in the blastfurnace feed ends up as calcium sulfide in the slag. Water can destroy this at high temperature, forming calcium oxide and hydrogen sulfide. Oxidation to sulfate only occurs if a lot of new surface is created while the slag is still liquid.

Note 10. It might occur to the reader that, if the success of the process is independent of the chemistry of the "magic ingredient" added, then the process is not a chemical one. One might also wonder how they make a 5% calcium sulfate solution. The list is typical of the more desperate types of patent specification.

Note 11. Suddenly, it's clinker! Of course, it isn't clinker - it's granulated slag. The suggestion that the addition of a homeopathic dose of magnesium sulfate to slag converts it into "clinker" is a deliberate misrepresentation.

Note 12. The ropeway was still visible in the 1931 aerial photographs. The conveyor had to be elevated because it crossed the plant's main rail tracks, and there was no room for the grinding plant closer to the furnaces.

Note 13. This apparent "blending" operation was probably to hold the product until it could be passed as acceptable by the lab. Product found unacceptable was probably dumped.

Note 14. The attached plan shows six furnaces feeding the granulator shown. However, there are three others south of the rail tracks, and the aerial photographs show a junction gantry on the ropeway that might have communicated with these, in which case there must have been another granulator - perhaps this was the first installed. The claimed capacity is significant: an output of 1170 tons a week would indeed make it a first-division cement plant of the time, but this depends on a 100% yield of cement. An intimidating public image was part and parcel of such promoter projects. The John Hudson Earle diaries document the rise and fall of the "Collos" project from the sidelines, and he quotes Vavasour Earle as saying: "Bertram Blount told a merchant who went to see him about Collos cement, that he might consider Portland Cement as done with, as this slag cement would take its place". But the yield of good product from 1170 tons a week of raw slag was low on average, and extremely erratic, with long periods yielding no product at all.

Note 15. Note the dramatic difference between this and the raw slag analysis given before. This is probably inadvertant, and reflects the appalling variability of the slag.

Note 16. The name of the company, and the title of this article, calls the product "Portland Cement", and the unground product is referred to as "clinker". The claim that this product is Portland cement is, of course, fraudulent, but it was an intrinsic part of the promotion of the product. Robert Lesley, in his book, describes how slag cement producers made a similar claim in the USA. The claim was successfully challenged in court. He also describes the arrival in the USA of Vavasour Earle. Earle's UK company - the Collos Portland Cement Company - had been undersubscribed and he was on the verge of bankruptcy. He was therefore making a desperate bid for US backing. Lesley describes it thus:-

The Colloseus patents were brought to this country by Vavasour Earle, of England, and Dr. Susskind, of Germany, who endeavored to enlist capital in the process. That they attached high value to their patents was shown during a dramatic meeting they had sought with the Board of Directors of the North American Portland Cement Company. Panic was thrown into the souls of the American manufacturers when Susskind, being asked what the price of the invention was, said: "A million!" One of the directors present asked: "A million dollars?" "No," Susskind replied, "a million pounds". Thereupon the Americans, in a fainting condition, retired for deliberation. The result was that a commission consisting of Dr. Clifford Richardson, the well-known chemist and scientist, and Robert W. Lesley, cement manufacturer, was appointed to visit Europe and investigate the process. This they did, finally reporting against its practicability. Subsequently William R. Warren and associates bought a small works in Buffalo and started manufacturing Colloseus cement, but without achieving very satisfactory results.

Lesley also described the effect of the failure of slag cements in the US market. The main producer - US Steel at Gary, Indiana - promptly decided to redesign their plant to burn a slag/limestone rawmix and produce a true Portland clinker. Within a very short time, the plant, with forty kilns, became the world's largest, and was extremely successful commercially. This, as it turns out, was also the path followed by the Scottish slag plants, but somewhat later, and considerably less successfully. Collos cement, having been distributed at below-cost price to a reluctant market, soon drowned in compensation claims following many expensive failures, and it ceased production within a few years. Coltness installed a kiln and started true Portland cement production just before the outbreak of the Great War.

Note 17.

Text and notes for this article are in preparation.
Original content © Dylan Moore 2015: commenced 06/07/2015: last edit 16/02/2017.

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