By FREDERICK RANSOMS, A.I.C.E.
So much has been said and written on and in relation to Portland cement that further communications upon the subject may appear to many of the present company to be superfluous. But is this really so? The author thinks not, and he hopes by the following communication, to place before this meeting and the community at large some facts which have up to the present time, or until within a very recent date, been practically disregarded or overlooked in the production of this very important and valuable material, so essential in carrying out the great and important works of the present day, whether of docks and harbors, our coast defenses, or our more numerous operations on land, including the construction of our railways, tunnels, and bridges, aqueducts, viaducts, foundations, etc. The author does not propose to occupy the time of this meeting by referring to the origin or the circumstances attendant upon the early history of this material, the manufacture of which has now assumed such gigantic proportions - these matters have already been fully dealt with by other more competent authorities; but rather to direct the attention of those interested therein to certain modifications, which he considers improvements, by means of which a large proportion of capital unnecessarily involved in its manufacture may be set free in the future, the method of manufacture simplified, the cost of manipulation reduced, and stronger and more uniformly reliable cement be placed within the reach of those upon whom devolves the duty and responsibility of constructing works of a substantial and permanent character; but in order to do this it will be necessary to allude to certain palpable errors and defects which, in the author's opinion, are perpetuated, and are in general practice at the present day.
Portland cement is, as is well known, composed of a mixture of chalk, or other carbonate of lime, and clay - such as is obtained on the banks of the Thames or the Medway - intimately mixed and then subjected to heat in a kiln, producing incipient fusion, and thereby forming a chemical combination of lime with silica and alumina, or practically of lime with dehydrated clay. In order to effect this, the usual method is to place the mechanically mixed chalk and clay (technically called slurry), in lumps varying in size, say, from 4 to 10 lb., in kilns with alternate layers of coke, and raise the mass to a glowing heat sufficient to effect the required combination, in the form of very hard clinker. These kilns differ in capacity, but perhaps a fair average size would be capable of producing about 30 tons of clinker, requiring for the operation, say, from 60 to 70 tons of dried slurry, with from 12 to 15 tons of coke or other fuel. The kiln, after being thus loaded, is lighted by means of wood and shavings at the base, and, as a matter of course, the lumps of slurry at the lower part of the kiln are burned first, but the moisture and sulphurous gases liberated by the heat are condensed by the cooler layers above, and remain until the heat from combustion, gradually ascending, raises the temperature to a sufficient degree to drive them further upward, until at length they escape at the top of the kiln.
The time occupied in loading, burning, and drawing a kiln of 30 tons of clinker averages about seven days. It will be readily understood that the outside of the clinker so produced must have been subjected to a much greater amount of heat then was necessary, before the center of such clinker could have received sufficient to have produced the incipient fusion necessary to effect the chemical combination of its ingredients; and the result is not only a considerable waste of heat, but, as always occurs, the clinker is not uniformly burnt, a portion of the outer part has to be discarded as overburnt and useless, while the inner part is not sufficiently burnt, and has to be reburned afterward. Moreover, the clinker, which is of excessively hard character, has to be reduced by means of a crusher to particles sufficiently small to be admitted by the millstones, where it is ground into a fine powder, and becomes the Portland cement of commerce.
This process of manufacture is almost identical in principle and in practice with that described and patented by Mr. Joseph Aspden in the year 1824; and though various methods have been patented for utilizing the waste heat of the kilns in drying the slurry previous to calcination, still the main feature of burning the material in mass in large and expensive kilns remained the same, and is continued in practice to the present day. The attention of the author was directed to this subject some time since in consequence of the failure of a structure in which Portland cement formed an essential element, and he had not proceeded far in his investigation of the cause of the failure when he was struck with what appeared to him to be the unscientific method adopted in its manufacture, and the uncertain results that must necessarily accrue therefrom. Admitting, in the first place, that the materials employed were considered the best and most economical for the purpose readily accessible, viz., chalk and an alluvial deposit found in abundance on the banks of the Thames and the Medway, and being intimately mixed together in suitable proportions, was it necessary, in order to effect the chemical combination of the ingredients at an intense heat, to employ such massive and expensive structures of masonry, occupying such an enormous space of valuable ground, with tall chimney stacks for the purpose of discharging the objectionable gases, etc., at such a height, in order to reduce the nuisance to the surrounding neighborhood? Again, was it possible to effect the perfect calcination of the interior of the lumps alluded to without bestowing upon the outer portions a greater heat than was necessary for the purpose, causing a wasteful expenditure of both time and fuel? And further, as cement is required to be used in the state of powder, could not the mixture of the raw materials be calcined in powder, thereby avoiding the production of such a hard clinker, which has afterward to be broken up and reduced to a fine powder by grinding in an ordinary mill?