Christopher Hall De~artmentof Bulldlnq University of Manchester Institute of Science and Technology Manchester. England M60 1QD
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On the Historv. oi Portland 150 Years
Cementitious building materials have a long history, and the date 1824, traditionally taken as the origin of portland cement, is in fact only the date of a patent ( I ) granted to Joseoh Asodin (1778-1855) of Leeds. Eneland. who mav not a t that'time have made portland cemeit a t all. ~ o d a y ; cement is the successor to a line of materials with a historv reaching hack to the ancient world; it has a chemistry o? perhaps unsurpassed complexity amongst major inorganic industrial materials and is still the subject of intense study. At some ~ o i n in t the second quarter of the 19th century, ago, the cement as we know it now was fir& about 150 produced. The very simplest of the mineral cements are those based on gypsum, hydrated calcium sulfate. Gypsum loses its water of crystallization easily on gentle heating to yield various olasters. These dehydrated suhstances when mixed with water reform a crystalline mass of synthetic gypsum, which however is mechanically weak and is slowly dissolved hv water. There is evidence that the ancientEevotians ".. mixed burnt gypsum with sand to make masonry mortars 12). Such mortars were unsuited to less arid climates. and the early bu~ldrrsOII [he nurthtm shores of the Xlediterranean dt.wlc>oed the ; k ~ l l ; of lime-hurninr ,conversion of calcium carbonate to the oxide). The quickkme produced is slaked with water to yield a lime putty, to which can he added sand to produce a mortar or coarse mineral to produce a concrete. Such pure lime mortars and concretes harden slowly and often incompletely by reaction with the carbon dioxide of the air to give calcium carbonate once more. The Roman builders could achieve superb quality in pure lime mortars of this kind-many examples remain today-hut these materials did not set from within the mass of the paste, nor did they have the quality of hydraulicily: the capacity to set in contact with water, so valuahle in many engineering situations. Hydraulic limes were produced by hoth Greeks and Romans by adding to the lime putty fine volcanic ash or crushed volcanic minerals, the Romans using the deposits a t Pozzuoli, so that now all such materials are known as pozzolanas. The Romans also discovered that crushed tiles could act as artificial pozzolanas. Both natural and artificial pozzolanas are composed chiefly of silica with some alumina in forms which on mixing with water will react readily with calcium hydroxide from the lime to give hydrated calcium silicates and aluminates. These important suhstances are closely similar to the cementing compounds in portland cements themselves. Because hoth the pozzolana and the lime are present in the mortar, the setting proceeds from within the mass; the extreme insolubility and useful strength of these hydrates leads to the property of hydraulicity. Lime-pozzolana cements were the only cements available for work in contact with water from the Roman period until as late as the 19th century ( 3 ) . In the 18th and 19th centuries, we see the beginnings of an understandine of whv it is that lime-oozzolana mixtures Iln\,e t h r w rcmarknhle hydraulic pr,,ptrries, and !he ;tart of .a I,?horwus ernoiricnl i t r u n ~ l p 1 0 make Iwtter and herter cements. But t h e understanding was a long time coming, and the early pioneers had very little help from the chemists of the day. In 1757 the great engineer John Smeaton 222 1 Journal of Chemical Education
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began to build the third Eddystone lighthouse off the southwest coast of Britain ( 4 ) ,apparently a labor of Hercules in the absence of a good hydraulic cement. Smeaton, appreciating the need to obtain the best component materials fur the lime-pozzolana cement he intended to use, examined a number of mixtures, and found that the quality depended on the limestone (5). He made the important discovery that the best mortars were made from limes containing clay impurities. We recognize now that the aluminosilicate clay minerals are closely related to those in the pozzolana, with this essential difference: that they are burned intimately with the lime in the calcining process. I t soon emerged that certain heavily clay-bearing limestones were hydraulic in the absence of the pozzolana (6).Such materials became known as Roman cements. Various natural cements of this kind (whose properties depended in an erratic way on the composition of the mineral taken from the ground) achieved prominence in various places in the earlv vears of the 19th centurv. The essential roles of lime andsikca in producing hydra;lic cements were established some time before 1818 hv the Frenchman Vicat in his scientific studies of innumerable artificial mixtures of limestones and clays (7). Thus about the year 1824, a good many people in several European countries were interested in trying to make their fortunes by perfecting these cements. Some were spurred on by the belief that the Romans had had a secret for making fine cement which had been lost, and many unlikely substances were added in the hope of stumbling upon a missing ingredient. Lacking any clear notion of the nature of the cementing reactions and using the most variable of raw materials.. thev labored lareelv in the dark. Scientifically speaking the nature of portland cement did not emerge until well into the second half of the 19th centurv. Todav's cement is made by heating an intimate mixture of finely eround limestone and clav. .(in ~rouortions carefullv . . " adiusted by reference to analyses of raw materials) to a temperature sufficientlv . high . for oartial meltine" to occur (8).The cooled granular product, a complex multiphase clinker, is ground to a powder of large surface area. This is portland cement. It consists primarily of a number of calcium silicate and aluminate comoounds. When mixed with water it forms a hydraulic paste which sets and hardens to a strong solid, larpelv c o m ~ o s e dof hydrates of calcium silicates but with a remarkahfy intricate physical microstructure. The critical variables closely controlled in manufacture are the composition of the feed (chalk:clay, 7525) and clinkering temperature (over 1400°C). Neither Aspdin nor anyone else for a long time after 1824 fully understood these basic facts. What then is the basis of Aspdin's claim to be the inventor of portland cement? On November 6, 1824, the gossip column of the newspaper the Leeds Mercury was preoccupied with its usual business of warning the honest citizens about the dishonest ("Last Saturdav. - . three females. handsomelv dressed. and having the appearance of respectable ladies, paid a five ~ o u n d. . . hnnknote.. . to a haberdasher in Leeds, which is not worth a farthing") and gave only a single passing sentence to Aspdin's news. "We hear that Joseph Aspdin, bricklayer of this town, has obtained a patent for a superior
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cement representing Portland stone." His patent B P 5022: 1824, "An Improvement in the Modes of Producing an Artificial Stone" consisted of two hrief pages and the roughness of his manufacturine methods is apparent. His Drocess calls for the use of a "specific quantityGf limestoneand of clav hut sadlv . thev . are not specified. Information about the burning temperature is of course also absent, as are all details of the kiln which might have helped to establish this. However, Aspdin did introduce the name portland cement, a shrewd idea to link the unproven qualities of his new cement with those of a fine building stone. Unfortunately little is known of Aspdin's manufacturing activities at this time. The scanty records have been examined with great Aspdin was carrying on business as care by Halstead (6,9). patent portland cement manufacturer a t times during the next ten years both in Leeds and Wakefield, and certainly later in Wakefield, hut few details have come to light. His son William entered the business, and spent the rest of his rather short life in the cement industry, moving from one venture to the next without ever apparently really prospering (10). The question of whether what Aspdin made in the early davs of his business. sav from 1824 to 1830. should be rega;ded as a true portland cement is a debated but still open auestion. No specimens appear to he available, though it to establish his connection with some may yet he existing building of that period. His product may simply have been yet another artificial hydraulic lime (excess lime) or Roman cement (excess silica). No record has been found of his early kiln, hut the claim made by a commentator much later in the century that he used a glass-making kiln rather than the cooler lime-hurnine kiln a t least holds out the possihility that he attained the temperatures necessarv for clinkerine - .(that is. for more complete reaction of lime and clay), today regarded as essential to cement formation. Other evidence which bears on whether Aspdin's early cement should be considered a portland is inferential. Professor Skempton's detailed studies (11) of the cements of the middle of the 19th century (including those produced by William Aspdin) provide the basis for making a case that Joseph Aspdin had stumbled upon the essential facts of manufacture. Briefly the argument is this. In 1841 William Aspdin entered into an agreement with a firm of Roman cement manufacturers, Maude, Son & Co., of Rotherhithe, in East London to make the Aspdin cement. There is no suggestion that the son made any innovations to his father's process. At this time work was under way on the building of the new Houses of Parliament following the fire of 1834 and in 1843 the contractors carried out tests to compare Maude's new cement with the available Roman cement. The results of these tests seem definitely to show that the new cement was almost twice as strong- in compression as the finest Roman cements available, compelling evidence that this was indeed a true portland cement. If William Aspdin's Rotherhithe cement i n 1843 was a portland, there can be little doubt that Joseph Aspdin's product at that time was also. However, whether this was the same cement as that which he was producing as earlv as 1824 we cannot at present say. Once the comparative tests of the 1840's showed publicly the superiority of portland cements over the other calcareous cements production began in a number of places in Europe and the gradual empirical improvement of the process and the material gathered speed. The development of sound test methods, especially for strength, played an important part in this, as did the refinement of methods of chemical analysis of raw materials and changes in kiln de-
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sign - .(12). . The understandine of the nature of the cement reactions-both the composition and phase relationships in the clinker and the details of the hvdration reactionshave been more recent, and the materials science of cement and concrete remains an active and important area of study. Advances in knowledae have been brought about by a number of modern experimental methods,-such as X-ray diffraction techniques for compound identification, optical and electron micr©, diffeiential thermal analysis, adsorption isotherm techniques, and so on (13). There is no doubt that for the first few decades the production of portland cement was a primitive manufacturing enterprise, even by the standards of the day. The material took a considerable time to penetrate into constructional and civil eneineerine nractice. and indeed not until the de". velopment of the techniques and structural theory of reinforced concrete construction were its full potentialities realized. These facts help explain why portland cement was not produced in the US until 1871 when David 0. Saylor was eranted his US patent (14). The earlv American portland cement manufac&ers had to face strong dual competition from the natural cement industry and from the portland cements imported from Europe. ~ a t u r a hydrauliE l cement had been discovered as earlv as 1818 in New York State, and later in a number of other Eastern seaboard states, and Kentucky and Illinois. The natural cement rocks were found much more abundantly in these areas than in Europe and many of the products established good reputations in the middle of the 19th century. They were used in huge quantities in the great canal construction projects of those years-starting with the Erie Canal in the 1820's. The importation of European portland cements started in the later 1860's. The trade was viable only because the cement could travel free of charge westwards across the Atlantic as ballast in tramp careo ships seekine erain or cotton cargoes. The ~ u r o p e a nindustry at thisdate was well established. The product was reliable and auicklv earned a good name. By thk 1890's however the ~ m e r i c a nmarket for cements was expanding so rapidly that the indigenous portland cement industry was able to establish dominance. w
Literature Cited i l l Aspdin. J.,"An lmprovemenlin the Modes of Produeinpan Artif~eialStone." B r i t ~ irh Patent 5022:1824. (21 Lea. F. M., "The Chemistry of Cement and Concrete: 3rd Ed.. Edward Arnold Ltd.. London. 1970. ~ N., S& ~ ~ Enainaar. ~ : 52,193 (197"). (31 D of the construction of (41 Smeaton. J.. ',A narrative of the buildins and a description . the Edystone 1i~hthou~e:'C.NICOI.~ ~ n d 1791. m . (51 For a life of Smeaton. see Smiles, Samuel, '"Livesof the Engineers: vol. 2, pl, 6. reprint of the 1862 ed., David and Charles. Newton Abbot. England, 1968; Turner. T.. Endenuour, 33.29 (1974. (61 Halstead. P.. T r o w Necueomen Sor , 3 4 3 7 (1861.2). (71 Vicat. L. J., "Treatise on Calcareous Mortars and Cements," John Weale. London. 1837. I81 For a de~criptionof the modern manufacturing process, see Pollitt. H. W. W., in Tsylor. H. F. W.. "The Chemistrv of Cements.'' Academic Presi. Landon and ~ & Y o r k . LSbd, p. 27. (91 Goading, P.. and Halstead, P. E.. P i o r Thirdlnli. Symposium on the Chemistry of Cement. London. 1962. Cement and Concrete Annociation, London. 1954. (10) Much interesting information an the history of the Aspdin iamily, especialiy the suns .lamer and William. has been eathered toeether in a recent ertic1e bv Bari0ot.R. .I., Conemla. R, 18i1974). (11) Skempton. A. W.. Trons. N ~ w c o m s nSoc., 35,117 11962). 112) A v i e r of the s a t e of cement msnufaeture at the heginning of this century is provided by Butlor, D., "Portland Cement: its Manufacture, Testing and Uses." Soon. Ltd..Londonand New York. 1905.
Volume 53,Number 4, April 1976 / 223
