Page:The Building News and Engineering Journal, Volume 22, 1872.djvu/518

 496 ———<——S———————————eeeeeere_eoqQnteaeeeeeeeeeeeee ees treatment at Dunton which he would recommend was to mix lime and clay with the sewage shortly before it reached the tauks. That would produce a complete deposit in the tanks, and would leodorise the sewage, which would flow on to the land without any smell arising from it, and the deposit also would be absolutely inodorous.” ‘After such important testimony concerning the economy and efficiency of the process of precipita- tion by lime and clay, I may venture to take you a step further, and remind you of the fact that these are the substances which are the essential compo- nents of all hydraulic limes and cements. The pre- cipitation which ensues on their addition to sewage water is chiefly due to the combination of the lime with the carbonic and fatty acids which it finds there, but partly also to its affinity for the clay which is added with it. These substances in sub- siding carry down with them, first, some of the car- bonate of lime which was held in solution in the sewage water, and small quantities of other soluble salts; second, the clayey mineral matters in suspen- sion in it; third, the whole of the organic matter in suspension, together with some portion of that which is in solution. Now, it is manifest that we have in the deposit thus produced, both as respects the mineral matters removed from the sewage water and in the materials added to produce precipitation, substances analogous tothe components of limestones, such as are employed in the manufacture of hydraulic cements and limes. After calcination, the products are perfectly similar, for all the organic matter has been destroyed, the carbonic acid and water has been expelled, and nothing remains but lime and clay, together with some other substances, such as the alkalies, phos- phates, &c., which are found in small quantities in all hydraulic limes and cements. Though it will probably appear unnecessary to most of my hearers to insist upon the inferiority of the pure or fat limes for the purposes of the archi- tect, to those which have hydraulic properties, yet, as it willconduce very much to the appreciation of my process for improving limes that this inferiority should be strongly present to your minds, I will ask you to pardon my quoting also from some dis- ‘tinguished authorities on this point. I wish, as far as possible, to tell my story by the words of those whose opinions must carry weight with you. “Tf, then,” says Vicat, “in what follows, we treat of ordinary mortars, or the mixtures of sand and rich limes, it is because we are compelled to do so to com- plete the history of the phenomena we have to describe ; for it is our most decided opinion that their use ought for ever to be prohibited, at least in works of any importance.” ‘We are in the habit of composing our mortars,” says M. Treussart, “ of fat lime and sand. Our mortars, consequently, have little durability. We shall not obtain durable masonry in the air until we make use therein of hydraulic mortars. In countries where good hydraulic lime is to be had, no other kinds should be used for any purpose whatever.” Sir C. Pasley’s testimony is equally strong. “I had ascertained,” he writes, more than twelve years ago, “that the pure limes, such as chalk lime, Carrara marble, &e., were utterly unfit for the purposes of hydraulic architecture, as they dissolved away on the outside, and never set at all in the inside of walls exposed to the action of water. But I was of opinion at that time that the mortar of these limes was good for dry situations and for inside work, provided that the external joints were protected against the effects of beating rains, by pointing them either with cement or some superior sort of lime.” He then goes on to say that further experience showed him that such pure lime mortar rather dried than set in inside work and dry walls, and he con- cludes the subject with the pithy remark, “ Thus chalk lime mortar, when wet, is a pulpor paste, and when dry it is little better than dust.” We may fairly assume, then, that in all districts in which pure limes are the only limes now available at a moderate price, any scheme which would supply the architect with a good description of hydraulic lime or hydraulic cement without any considerable increase of the cost of his works, would be an im- portant boon. Now, that which constitutes, essen- tially, the difference between a pure lime, a feebly hydraulic lime, and an eminently hydraulic lime, or a cement, is, as I have stated, and as, indeed, you must all be perfectly aware, the amount of clay present. The results may be modified by the greater or less proportion of iron and alun.ina or alkalies in the clay, or by the temperature of calcination, but if any one point of difference be selected for pur- poses of classification, it is the quantity of clay which must be our guide. On this principle limes have been classified by Vicat, as—first, fat limes, when the quantity of clay present in the limestone THE BUILDING NEWS. is less than six per cent.; second, as hydraulic limes of medium quality, when the quantity of clay is from eight to twelve per cent.; thirdly, ordinary hydraulic limes, when it reaches fifteen or eighteen per cent.; and fourthly, eminently hydraulic limes, when it amounts to twenty or twenty-five per cent., the last-named quantity being that which is present also in Portland cement mixture previous to calci- The resistances which Vicat assigns to these four qualities of limes, commencing with rich nation. or fat Jimes, are as— 23h. The resistances assigned by Colonel Raucourt de Charleville to the mortars of similar descriptions of lime as the above and quartzose sand are scarcely above ll ...... 2t04 AMO Gi nccsos) OO: and the results obtained by Treussart were very similar to those given by Vicat. If we endeavour to assign a place to the limes of this country which are to be met within the market, we shall find that none of them are such as to merit being classed with the “eminently hydraulic limes ” of the French writers. It is true that there are certain beds of lias and gray chalk which wouid yield limes that would come up to this standard, but they are of very irregular composition, and have never found favour either with builders or architects. Nevertheless, it would be quite possible, by artificial mixtures of lime and clay, to prepare such eminently hydraulic limes of equable composition, just as it is possible, by triturating chalk and clay with water in proper proportions to produce a mixture which will burn to Portland cement, the strongest cement known. Whether the mixture is made by precipita- tion from sewage-water, or subsidence from ordinary water, would be of little moment, either in the pre- paration of lime or cement. We have merely to add the ingredients to the sewage-water at some distance from the outfall into the depositing tanks, and we obtain as perfect an admixture of the ingredients’ as could be made by the operations of nature in the production of limestones, and probably more con- stancy in the proportions of the ingredients than nature ever attains. With the object, indeed, of sanitary improvement in our towns, I introduce the lime and clay not only at a distance from the outfall, but at different points of the system of sewerage through the streets. At Ealing the lime and clay are introduced into the main sewer at a point in the village about one mile and a quarter from the pre- cipitating tanks. The lime, previously slaked, and the clay, in the proportion of five measures of slaked lime to one of clay, are mixed together in a tub by means of revolving arms, and the impalpable mixture thus obtained flows into the drains, and passes on with the sewage towards the outfall. Thre joint pre- cipitating action and clarifying effect of the lime and clay, or other chemicals, is thus promoted to the fullest extent by the natural agitating process to which the mixture is subjected; the sewage under- goes defecation in the drains themselves, and no longer emits the noxious gases which now find their way into our streets and houses ; and the precipitate formed exercises a scouring action upon the bottom and sides of the drain, quite obviating the tendency of the sewage to coat them with filthy slimy decom- posing matter. The daily flushings which house drains ordinarily receive ought to keep such drains clean, and therefore, under this system, if these be laid with sufficient fall, no lodgment of noxious matters, which will create disease by their decom- position, can take place, unless the street drains are themselves defective. Professor Abel states, in reference to this system, that “the various cement- producing ingredients could scarcely be more thoroughly mixed by the most efficient mechanical appliances than they are found to be in the deposit described, which, therefore, after separation of the water, is at once in a proper condition for conversion into cement by simple calcination.” Mr. Hawksley, the President of the Civil Engineers, also advocates this system of introducing the precipitants, ‘in order allow the chemical matter to mix thoroughly.” The deposit obtained by the lime and clay process is then dried, and subsequently calcined, as is done with the deposit in the manufacture of Portland cemeat, or the artificial hydraulic limes made at Meudon, near Paris; but itis to be observed that, in the case of the sewage deposit, there is a considerable amount of organic matter which acts as fuel. Indeed, the deposit, when dried, is so combustible as to enable the material to burn of itself, when in quantity and fairly lighted. Accordingly Mr. Bramwell, speaking of the Birmingham sewage, says, ‘The sludge de- posited at Saltley contains about fifteen per cent. of solid matter, and of that forty per cent. is com- bustible, and could be used as fuel;” and Mr. Hawksley states, ‘‘It was practicable to burn the sludge, which contains a great deal of combustible ordinary mechanical appliances; thirdly, the sewage or nearly so, to effect the calcination of the deposit. June 21, 1872. matter, and the experiment of burning the sludge had been very successfully carried out by General Scott.” The destruction of the sludge by fire is indeed a very important point, independently of the manufacture of cement, as it gets rid of every possible germ of disease. It perhaps may, however, occur to you that the process of drying and burning deposit mixed with so much organic matter may in itself be a nuisance; but here again I can produce unexcep- tional testimony as to the innoxious character of the process. Mr. Bramwell asserts: ‘ No injury would be likely to arise, in the way of nuisance, from dealing with the sludge in the manner proposed; ” and Dr. Voélcker says he ‘‘ wished to bear testimony to the fact that the process of drying and manu- facturing the dried material into cement could be carried on in the immediate neighbourhood of the town where the sewage was obtained, without creating the slightest nuisance.” The kiln at Ealing burns with a down draught, so that all the organic products, which are evolved at a low temperature, and which might create a nuisance, are subsequently raised toa high temperature, and are thus resolved into inor- ganic matter. The waste products of combustior are carried through flues under a drying-floor or which the sludge is prepared for the kiln. The steam, &e., from the drying process may also be carried into the kiln chimney shaft, if necessary. Experience has taught me that another question suggests itself to most minds in respect of my pro- cess, and that is, ‘‘ Would it not be better to make hydraulic limes and cements in a more direct way than this ; and what gain is there, or what advantages have you, in making them through the intervention of sewage?” Theadvantages in this case, which do not exist in the ordinary processes, are these: First, as I have already stated, the materials used undergo a considerable increase from the lime and clay removed by the precipitants from the sewage water ; second, the agitation in a long length of sewer pro- duces better admixture than can be effected by gives up a large amount of fuel, which is sufficient, But, as Mr. Bramwell very well put it in replying to an objection of this sort, ‘‘ Supposing the sewage- water contained no ingredient whatever to make Portland cement, but supposing that the materials to cause it to become inoffensive were such materials as made Portland cement, then you put those in, not because you wanted to make Portland cement, but because you wanted to make the sludge harmless and inoffensive; and if you could afterwards make Portland cement, why should you not do so? That he understood to be General Scott’s proposition.” I must also meet a third doubt which will be sure to arise in your mind: ‘Is the cement worth any- thing when it is made?” Treply again in the words of others. Professor Abel, chemist to the War Department, who has probably examined, in the course of his official duties, more limes and cements than any man alive, stated at the Society of Arts that he had examined the cement made by the pro- cess, and ‘‘found that it was equal to excellent Portland cement, or in another case to an excellent hydraulic cement of a different character.” Dr. Frankland’s opinion I have already quoted, and 1 will add to this testimony that of two distinguished engineers practically well acquainted with the subject. Mr. Hawksley stated before the Select Committee of the House of Commons, already many times referred to— By adopting General Scott’s process, it (the deposit from sewage) could be converted into a very useful and valuable cement.” And Mr. Bramwell states, in the same inquiry, ‘General Scott's plan of converting the sludge into cement is very satis- factory, and from what I have seen of the cement, I have the highest opinion of it.” By omitting the clay an excellent lime for agri- cultural purposes (because comparatively rich in phosphoric acid, sulphuric acid and alkalies) may be prepared, bat this application has only an interest for the architect as affording an outlet for a portion of the products of the process in cases in which there is a small demand for building materials. As, however, the sewage of a population of 100,000 will only yield from 40 to 50 tons of lime or cement per week, there are few localities in which the demand would not exceed the supply. I have shown, I hope, in the course of what I have stated, that both hydraulic limes and cements can be produced by the above process. I have also brought evidence that Portland cement of the best quality can be prepared by it; but the manufacture of such cement necessitates powerful crushing and grinding machinery, and the ingredients must be proportioned with a degree of nicety only to be looked for in large establishments. The so-called selenitic cement, however, is of a “tender” nature very readily ground, and succeeds perfectly well, though