Cement Kilns

I C Johnson's experiments on Portland Cement

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Isaac Charles Johnson (28 Jan 1811 - 29 Nov 1911) was the son of a labourer at Francis & White's Nine Elms (Battersea) Roman Cement plant. He wrote much autobiographic material, and his life is well summarised in A. J. Francis' book. His formal education was minimal and his literacy and numeracy were largely the result of his innate intelligence and application. In his employment when young, he gained experience in many aspects of the building industry, including periods working at the cement plant. His father obtained for him an introduction to John Bazley White, who was sufficiently impressed to take him on as plant manager at the Swanscombe plant. There, he was charged with emulating the Portland cement that William Aspdin had introduced at Rotherhithe in 1841. An attempt was made to buy the technology from Aspdin, but this was rebuffed, and Johnson rashly claimed that he could "work it out for himself". Johnson took three years to do this, but eventually founded a production facility for the product that became Britain's largest, from which grew Blue Circle. The chatty openness with which Johnson described this early work - in distinct contrast with William Aspdin's paranoid secrecy - allowed him to make an unchallenged claim that he was the "inventor" of "true Portland cement". Although this claim is now known to be specious, his account is of interest as a description of the discovery of the long list of pitfalls that await the new entrant to cement manufacture. The following is taken from an article that he had published in The Building News in 1880.

Mr Aspdin began work at Rotherhithe in connection with Messrs Maude & Son on a small scale, and did sometimes make a strong cement, but, owing to want of scientific method, the quality as respects strength and durability was not to be depended upon.

I was at this time (about 1845) manager of the works of Messrs White, at Swanscombe, making only the Roman cement, Keene's plaster, and "Frost's cement", the latter composed of 2 chalk to 1 of Medway clay, calcined lightly, and weighing 70 to 80 lbs per bushel.

My employers, attracted by the flourish of trumpets that was then being made about the new cement, desired to be makers of it, and some steps were taken to join Aspdin in the enterprise, but no agreement could be come to, especially as I advised my employers to leave the matter to me, fully believing that I could work it out.

As I before said, there were no sources of information to assist me, for although Aspdin had works, there was no possibility of finding out what he was doing, because the place was closely built in, with walls some 20 feet high, and with no way into the works, excepting through the office.

I am free to confess that if I could have got a clue in that direction I should have taken advantage of such an opportunity, but as I have since learned, and that from one of his later partners, that the process was so mystified that anyone might get on the wrong scent - for even the workmen knew nothing, considering that the virtue consisted in something Aspdin did with own hands.

Thus he had a kind of tray with several compartments, and in these he had powdered sulphate of copper, powdered limestone, and some other matters. When a layer of washed and dried slurry and the coke had been put into the kiln, he would go in and scatter some handfuls of these powders from time to time as the loading proceeded, so the whole thing was surrounded by mystery.

What then did I do? I obtained some of the cement that was in common use, and, although I had paid some attention to chemistry, I could not trust myself to analyse it, but I took it to the most celebrated analyst of that day in London, and spent some two days with him. What do you think was the principal element according to him? Sixty per cent of phosphate of lime! "All right," thought I, "I have it now." I laid all the neighbouring butchers under contribution for bones, calcined them in the open air, creating a terrible nuisance by the smell, and made no ending mixtures with clay and other matters contained in the analysis, in different proportions and burnt to different degrees, and all without any good result.

The question was, what was the next thing to be done? I had an idea that the elements were those contained in Roman cement, and I had read somewhere that the older chemists had taught that the value of Roman cements was due to the iron and manganese in them. I knew that these matters gave rise to the peculiar colour of Roman cement, but they were absent in Portland.

I had a laboratory and appliances on the premises, so I worked day and night to find out the component parts of the stones from Harwich and Sheppey. Having found these, and having tried many experiments, spreading over some months, in putting different matters together, I began to think that lime and alumina were the chief ingredients necessary. I, therefore, tried quicklime powdered and mixed with clay and calcined, by which means I got something nearer. It was a cement very much like Frost's. After this I used chalk and clay as used in Frost's cement, but with more chalk in proportion. The resulting compound being highly burned, swelled, and cracked.

By mere accident, however, some of the burned stuff was clinkered, and, as I thought, useless, for I had heard Colonel Pasley say that he considered an artificial cement should feel quite warm after gauging, on putting your hand on it, and that in his experiments at Chatham he threw away all clinkers formed in the burning.

However, I pulverised some of the clinker and gauged it. It did not seem as though it would harden at all, and no warmth was produced. I then made mixtures of the powdered clinker, and powdered lightly-burned stuff, this did set and soon became hard. On examining some days later the clinker only, I found it much harder than the mixture, moreover the colour was of a nice grey.

Supposing that I had nearly got hold of the right clue, I proceeded to operate on a larger scale, making my mixture of 5 of white chalk to 1 of Medway clay. This was well burned in considerable quantities and was ground finely, but it was of course a failure from excess of lime, although I did not then know the reason of it. The whole of this material was tossed away as useless into a kind of tunnel near at hand, and laid there for some months, after which I had the curiosity to take a sample of it and gauged it as before, when, to my astonishment, it gauged smoothly and pleasantly, and did not crack and blow as before, but became solid and increased in hardness with time.

Cogitating as to the cause of this difference, it occurred to me that there had been an excess of lime, and that this exposure in a rather damp place had caused the lime to slake.

This was another step in advance, giving me as it did the idea of there being too much chalk, so I went on making different mixtures until I came to 5 of chalk to 2 of Medway clay, and this gave a result so satisfactory that hundreds of tons of cement so mixed were soon afterwards made. Some of this cement was sent to the French Government Works at Cherbourg, and was, as I believe, set up as a standard of quality to which all subsequent purveyors had to conform.

Johnson had had 35 years to rationalise all this, and it is unlikely that the process of discovery was really quite as linear as described. He had certainly forgotten the chemical analysis, which actually showed 45% calcium phosphate.

His final clay content of 2 parts to 5 of chalk was a very safe option, producing a clinker with only around 20% alite, but this was fairly typical of Aspdin's early product. 2.25 parts of clay would produce no alite at all, while 1.63 was the minimum that would make a sound cement. The great sensitivity of the product to clay content was just emerging.

Johnson, with his self-publicity, always claimed that his cement represented a step-change improvement over that of William Aspdin, and succeeded at persuading people of this, to the extent that, even today, he is often falsely represented as the "inventor" of "true" Portland cement. However, almost all comparative tests performed at the time showed Aspdin's cement to be at least as good as that of Johnson. While Aspdin's work was undoubtedly unscientific, Johnson's plants were, even at the start of the twentieth century, still making cement rawmix in the most primitive manner.

Original content © Dylan Moore 2011: last edit 27/08/14.

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