XXXI

IMPROVED PORTLAND CEMENT,

By JOHN W. DICKINSON, Asso. Mem. W. S. E.

Read March 16, 1898.

The world does move, and while it would sound like stating an axiom to say that progress is making in all branches of industry, it is true that there are those who regard any improvement in Portland Cement as impossible, and even appear to deprecate the great advance which has been made by the American manufacturer. A clear understanding of "What is Portland Cement?" may assist us to note the wonderful progress made in the science in the last few years.

Portland cement is a product consisting of an intimate mixture of lime and clay-bearing materials burned to incinderation and ground to the fineness of flour. A perfect Portland Cement is a product consisting of an intimate mixture of lime and clay-bearing materials, with the elements in exact and perfect proportions and free from any deleterious or even adulterating substances ALL burned to a perfect clinker and the clinker ground to an absolute flour. Should the raw materials, even if of perfect composition, not be in a finely divided state or not intimately mixed, the chemical union of the different elements which should be accomplished by heat, is impossible, and the resulting product is simply calcined clay or calcined lime and not unground Portland Cement as it should be, and it would be immaterial how finely such so-called clinker was ground, it could never be Portland Cement.

Marked improvement has been made in the last few years in the preparation of the raw materials. Instead of going to the kilns in a coarse, unmixed condition and with the different elements varying from 3 to 5 per cent as was the former practice, and unfortunately is today in many mills, such precision is used in the best mills, that the raw materials are brought to a condition approaching impalpability, then thoroughly mixed and by the use of improved methods and competent supervision, the proportion of the chemical elements, viz: lime, silica and alumina, are so controlled that the variation never exceeds of I per cent. Great advance has also been made in burning. Bear in mind that the lime and silica in a Portland Cement must all be chemically combined by heat, and to effect this combination, a great heat is necessary when the fluxing elements, alumina and iron are kept at their proper minimum,and no product of the kilns that has not been thoroughly cindered and consequently chemically combined should be used with the clinker making the finished cement, and

it is safe to state that owing to the improved process of burning adopted in the past few years, the proportion of underburnt material coming from the kilns has been reduced from 25 per cent to 30 per cent, to less than 5 per cent, and in some cases the product of the kilns is invariably perfect clinker. The higher the percentage of combined lime the stronger the cement, and owing to the improved methods employed both in preparation of raw material and in burning, the lime has been increased from about 55 per cent of the total to an average of 60 per cent, and in a few cases the entire process is so perfect that a percentage of 65 per cent of lime is successfully incorporated.

But it is probably in the matter of grinding that the greatest advance has been made. Before any great reform is possible it is necessary to realize first the NEED; second, the CURE, and as in the social and political world, so is it true in the physical, that the creating of a realizing sense of the NEED, is the greater part of the problem. Mr. Thomas Bevan, senior member of the firm of Knight, Bevan & Sturge, no doubt the most widely known name in the cement world, told me not long since that the coarse particles of clinker, or what would be the residue on a No. 50 sieve, were of equal value to the flour, and certainly the custom of the English and many of the continental factories, in shipping with their cement such a large proportion of coarse particles, confirms the belief that Mr. Bevan is on the side of the large majority. But, fortunately, this country while occupying in many ways the position of scholar, and we do not fail to appreciate our teacher, and to acknowledge the debt of the Portland Cement industry of this country owes to the English and German manufacturers, has by investigations made known the NEED, by determining that the residue on any known sieve is clinker and not

cement.

Perfect clinker is sufficiently hard to scratch glass, and it is evident to anyone that will take two pieces of clinker, say the size of large diamonds, and wet the surfaces, that on putting them together no bond will result. Now, if we break up one of these pieces and attempt to cement together even the smaller particles, we find that these are no more cement than the larger pieces. By taking the residue on the 50-100 and 200 sieve, it will be found that the so-called cement represented by such residue is equally inert. Some investigators have reported slight strengths from the residue obtained by taking the intermediate of the 150 and 200 sieve, but I am positive that the strength thus obtained was wholly owing to the cement flour that adhered as dust to the coarse particles.

I have made many tests with the residue on the No. 200 sieve and in no case, where proper care was used to remove the impalpable powder, have I been able to get the least strength, so with the knowledge that no portion of a barrel of cement is active or possesses any cementing properties save that which has been

ground to an absolute flour, it was comparatively a short step to devising machinery and methods which produces 85 to 90 per cent of flour instead of only about 60 per cent, as was the product ten years ago. The progressive mills in this country are not satisfied with even this great advance, but are constantly striving to improve their quality, and effort will not be slackened until the contents of every barrel of cement is not only of the highest quality, but is so ground that there is the least possible amount of "cement sand." As adobe was superceded by lime, lime by natural cement, natural cement by Portland, so have the coarse-ground, bicalcic-silicate Portlands of ten years ago been practically supplanted by the fine-ground tricalcic-silicate cements manufactured to-day, and there are forces and ideas at work that in my opinion, will in the future, produce a Portland Cement superior to the best now manufactured.

While opinions may differ as to the probable soundness and permanency of different cements as indicated by their varying characteristics, it is certainly true that other qualities being equal, the quicker a cement attains its maximum strength, the better it is for all purposes.

Cement work suffers more from careless and mischievous attacks during the first month than in any future twelve, so the quicker a cement sidewalk, floor, or foundation is hard and out of danger of attack from any source, the better, and it has been the aim of the American manufacturer to produce a cement that will get its maximum strength in the shortest possible time. How fully they have succeeded in this particular is seen by the following tables.

Cement is used only to bond other materials, so in the comparisons given I shall confine the figures to sand tests, as these alone show the relative values of different cements in the work which they are called upon to perform.

Not feeling at liberty in this paper to give the name of the American Cement tested in the accompanying comparisons, it will be called American X, while the foreign cements are indicated by brand.

R. L. HUMPHREY, INSPECTOR OF CEMENTS, PHILADELPHIA, PA.

In the great lines of improvement indicated, viz.:

Ist-Better preparation and more accurate mixing of the raw materials.

2nd-Uniform and complete incinderation.

3rd-By fine grinding.

This country has taken the foremost position, equalling at least in the first particular that of any other country, and in the 2nd and 3rd, easily leading the world.

American kilns are now being adopted by Dyckerhoff & Sons, of Germany, a name long the fetich of those worshiping at the shrine of prejudice, and in the method of grinding universally used ten years ago, the old-fashioned burr-stone has been so completely deposed by American grinding machinery or European adaptations, that it is no exaggeration to state that in the entire making of Portland Cement, this country is copying the manufacturers of the old world far less than they are following in the footsteps of the progressive manufacturers of America.

DISCUSSION.

A member: What are the chief qualities in the cement that give to it the quick setting properties spoken of?

Mr. Hennessy: It has been demonstrated by practical experience that the improved Portland cements of America get their maximum strength quicker than the Portlands made twenty to thirty years ago. There are several reasons for this quicker action, but it is no doubt principally owing to the finer grinding. Mr. Adler: You have given us a number of test figures-none, however, extending beyond twenty-eight days. Have you any figures for a longer period?

Mr. Hennessy: I have none with me. Mr. Dickinson, no doubt, considered it unnecessary, as all the cements used in this list of tests are perfectly sound, and all continue to increase in strength. Climatic and other conditions cause all materials to fluctuate in ultimate strengths. The variation in strength of the natural stones, which of course must be considered to have obtained the maxim strength, vary materially in strength at different times, and, as would be expected, the same variation is shown in cement tests. This variation would be apparent in fifty years, or even a hundred.

Mr. Adler: Really, the two or three-year tests would be very interesting, because certainly cements which develop a great deal of strength in the first few months, and afterward sometimes, not until after a year or two begin to deteriorate, but deteriorate very rapidly. I believe that is the case where there is an excess of sulphur, or an excess of magnesia, and it would be very interesting to have long-time tests.

Mr. Hennessy: As I have said, I haven't them with me, but the relative strength obtains all the way through. Scientific knowledge has advanced sufficiently to determine whether cements are sound and stable, and whether there is any danger of any deterioration. The boiling test is the most accurate in this particular. Any cement that will remain intact in boiling water for eight or ten hours is permanent, and there is not the slightest possibility of any cement standing this test ever showing any but the fluctuations in test referred to previously.

Mr. Stephens: I would like to ask whether the specimen which shows maximum strength in a short time, that is would harden very quickly, whether such cement would in practice not be as good a cement as that which takes longer to harden?

Mr. Hennessy: I did not understand that Mr. Dickinson meant to say a cement that would set quickly was the best cement. What he said, was, "that would harden quickly." There is a difference between quick setting and quick hardening. The cement he had in mind does not get its initial set under two hours, and final set in over eight hours. That is slow enough for any work, giving ample time for all manipulation in the mixing and placing of the concrete, or mortar.