§ 24. Classification of plastic cements with respect to limes and to puzzolanas. As we said in the beginning of this chapter, the limestones that af ford plastic-cements contain a large proportion of clay: larger than hydraulic limestones, and less than calcareous puzzolanas. Establishing then the continued series of combinations which may be formed of pure lime and pure clay, we shall be led, as in the following classification, to three distinct products. The distinctive characters given in the table, show, as the proportions vary, the effects of a predominance of lime or clay. But the plastic cements have two maxima of energy, one at the point of incomplete calcination, and the other at the point of supercalcination: do they enjoy this singular property, exclusively? It is important to ascertain this. 1st. Pure lime stone incompletely calcined, slakes with difficulty, and even requires to be pulverised like plastic cement. According to Mr. Minard, if it be tempered with water and immersed, it comports in the beginning like a very energetic substance; and after four days, according to Mr. Vicat, it is impossible to make an impression with the finger. But beyond this, the solidification does not advance. On the other hand Hassenfratz (at page 203 of his Traité des mortiers) says he has noticed that the fat lime of Moustier, supercalcined, gives, after being pulverised and tempered, a mortar which sets strongly in water. Mr. Vicat, also, announced in 1818 that common fat lime supercalcined, in contact with a mixture of charcoal and seacoal, became incapable of slaking, and gave, on pulverizing and moistening it, a paste which hardened under water. It is hardly necessary to say that at the term of complete calcination, fat limes are entirely incapable of acquiring, alone, under water, any consistency. However feeble, therefore, may be the hydraulic property of fat lime at the points of incomplete and supercalcination, it is not the less true that it does exist, as in plastic cements. 2nd. Before Mr. Lacordaire had engaged in the Pouilly enterprise described $12, he had ascertained that a limestone but little hydraulic, from which all the carbonic acid had not been expelled by calcination, afforded a true cement; and he applied this observation to profit, in the works of the canal de Bourgogne, which he had in charge. With this view he used the ordinary kilns of the country, reducing to three days, the burning which was commonly extended to six or eight days: he afterwards slaked the lime by immersion, separating the subcarbonated portions, which he incorporated in the mortar, after having reduced them mechanically to pow der. We were of opinion that the common limestone of Pouilly, alternating with the variegated marles, contained, like the marles, the claystone (septaria) of which Parker's cement is made, and that if Mr. Lacordaire preferred extracting these materials by subterranean galleries, rather than by open quarries, it was, no doubt, because he found, below, richer deposits and a better choice of materials. But one of our friends, Mr. Avril, an Engineer, to whom we had communicated our researches, profited by his being at Pouilly to examine this point with much care. He has announced that, in fact, the common limestone of Pouilly properly treated, and retaining about fifteen per cent. of carbonic acid, sets under water in five minutes, and does not yield in any thing to plastic cements; that therefore, the preference accorded to septaria, could not be explained except on the supposition that it was easier, with it, to secure the degree of calcination necessary, and because the limits of greatest energy were more extended. He also discovered that the hydraulic limestone of Pouilly, suitably supercalcined, enjoyed the same properties as when in the state of subcarbonate. Some direct experiments on a few fragments of a hydraulic lime stone from Pompean, (Ille-et-Vilaine,) and from Doué (Maine-et-Loire,) lead us to believe that these properties are common to all limestones of analogous composition. 3rd. The good puzzolanas produced, on the one hand, by a torrefaction of some minutes on plates of iron, and on the other, by ochreous and refractory bricks, well burned, seem at first to establish two maxima of energy for clays also; but the experiments of the 17th chapter, (§6 and §7) demonstrated that these two degrees of heat are in fact but one and the same, and that these substances become so much the more inert as the term of vitrification is approached. As to clays containing at least a tenth of lime, they require, according to M. Treussart, that the degree of calcination should correspond with that of a slightly burned brick; beyond which they become of a quality more and more deteriorated. Is it still the same, at the transition from puzzolanas to plastic cements, when the proportion of lime is from twenty to thirty per cent.? We are unable to say. But it is possible that the influence of lime is then prominent enough to give rise to two maxima of energy. § 25. Geometrical representation of the influence of Heat on the several compounds of Lime and Clay. In order the better to exhibit the influence of heat on the several compounds of clay and lime, we will attempt to represent it geometrically. To do so, let us conceive that from a fixed point two lines are drawn at right angles to each other, of which the horizontal one shall be taken as the axis of the abscissas, and the vertical one, the axis of the ordinates. Then suppose that on the axis of the abscissas we take, from the point of intersec tion, lengths proportionate to some of the principal degrees of torrefaction; that, for example, we choose for the first the degree of moderate burning of bricks; for the second, the degree of thorough burning of bricks; for the third, the degree of complete calcination of fat lime; and lastly, for the fourth, the degree of super-calcination of the same lime. If, then, the ordinates be raised on the points of division, and we consider the compounds of lime and clay to be submitted to the corresponding degrees of heat, we might lay off on these ordinates, lengths proportionate to the hydraulic energy of each particular product; and afterward, through the extremities of these lengths, pass a continuous curve, which will be the curve of energy of the compound, whether hydraulic lime, plastic cement, puzzolana, or even fat lime. In this manner we have constructed the figures 4, 5, 6, 7 and 8, of plate II. Let us go into some particulars respecting them. 1st. Fig. 4 represents the curve of energy of fat lime. This curve has one ordinate null at the abscissa No. 3, which is the term of complete calcination; but at the abscissas 2 and 4, the ordinates have some magnitude, for they correspond to a hydraulic power, feeble it is true, but real. 2d. Fig. 5 represents the curve of energy of hydraulic lime. At the abscissa No. 3, the ordinate will vary much according to the portion of clay contained; but is greater than zero and less than the ordinates Nos. 2 and 4, because on one hand the lime is hydraulic by supposition, and on the other hand we obtain an instantaneous induration, and an improvement in the hardness of mortars, as in plastic_cements. 3d. Fig. 6 represents the curve of energy of plastic cements. In some the ordinate No. 3 is nearly null: is it the same in all? we think not. We know in fact that the best hydraulic limes are not those which contain the most clay; that, for example, in the manufacture of artificial hydraulic lime, the proportions of twenty to twenty-five per cent. of clay are recommended as being most suitable. Accordingly, the ordinate No. 3, which is null for fat limes, will augment progressively up to lime containing twentyfive per cent. of clay, then descending in a continuous manner for the limes with thirty per cent. of clay, for the plastic cements with forty, fifty and sixty per cent., finishing by becoming null. We give this explanation as a simple theoretical induction only. 4th. Fig. 7 represents the curve of energy of calcareous clays employed as puzzolanas. The maximum of this curve is at the abscissa No. 1-the term of bricks but little burned. The clays which, from the proportion of lime, approach most nearly to plastic cement, enjoy, perhaps, a second maximum of energy: but we have traced it only by a dotted line. 5th. Fig. 8 represents the curve of energy of clays not calcareous. This curve has only one maximum, which is at No. 2-the term of bricks well burned. To resume we see that each of the compounds of lime and clay is characterised by certain properties which vary, more or less, according to the predominance of one or the other of these constituents; that clays have only one maximum of energy, but that limes have two, and that, therefore, plastic cements only offer a particular case of a general phenomenon. CHAPTER XIX. On the change in Hydraulic Limes, and on the solidification of Mortars in general. § 26. Influence of spontaneous slaking on Lime in general. As lime is not always used immediately on leaving the kiln, it is im portant to know what modifications time will bring about in its properties; and what precautions are necessary for its preservation, or in its use, after it may have become deteriorated. When quick lime is abandoned to the contact of air, it absorbs moisture and carbonic acid from the atmosphere, and a certain quantity of oxygen also, according to Gen. Treussart. At the same time it splits and falls to powder; this being what is called spontaneous slaking, or air slaking. For a long time it was thought that lime thus slaked was good for nothing; but this was improperly generalizing a consequence which is true only with one particular kind of lime. In fact, M. Vicat has ascertained, 1st, that fat limes do not deteriorate, and that they even give superior results to those obtained by slaking in the ordinary mode of immersion: 2d, that for hydraulic limes, on the contrary, spontaneous slaking is the more disadvantageous as the energy of the lime is originally the greater. We owe to Gen. Treussart the proof of this last observation; obtained while he was in search of the relation between the degrees of alteration in the air; and time of exposure to the air. The following are some of his results, Table No. LIV. Made After 3 months. Obernai hydraulic lime with two parts of sand lbs. lbs. lbs. lbs. lbs. [121] 77 44 3322 These specimens were all kept under water. We see, therefore, that all delay in the use of hydraulic lime tends to convert it into common lime, and that in large works a circumstance of this kind might be of serious consequence. § 27. On the manner of preserving Hydraulic Limes. In the deterioration of hydraulic limes it is necessary to consider the influence of the water applied in slaking, and the influence of the contact of air. While the quantity of water absorbed does not exceed a quarter of the weight of lime, this last will remain in dry powder; it cannot solidify; and experience demonstrates that it may then be preserved without change, provided it be carefully covered from the contact of the air, in very tight barrels for example, like plastic cement. But if this powder, which is easily obtained by immersing the lime for twenty or thirty seconds in water, is exposed to the air, it comports nearly as in the case of air slaking; it becomes common lime. At works where there must be large supplies, it is hardly possible, however, to put the lime in casks, because they would be expensive from their number, and embarrassing from their bulk. A middle course remains, which consists in storing the quick lime in very close sheds, and enveloping it, on all sides, with a layer of lime already reduced to powder, in any way: an obstacle will thus be opposed to the circulation of air, and the interior parts of the mass will be preserved from its influence. If this process is not perfect, it is at least economical: and although all the layers of powder which serve as an envelope will be unfit to be used as hydraulic lime, it still may be used in a way that will soon be pointed out. § 28. Or the use of Puzzolanas in correcting the deterioration of Hydraulic Lime. In order to cause fat lime to harden under water, it is mixed with puzzolana; but deteriorated hydraulic limes, as we have said, act like fat lime: if this assimilation be exact, then the same process ought to be applied to both. This is, in fact, what happens; and it is observed that the amount of alteration has no sensible influence on the hardness acquired after about one year. Gen. Treussart was the first who insisted important fact; the following are some of his experiments. Table No. LV. on this Obernai hydraulic lime slaked spontaneously and mixed with one of sand and one of trass lbs. lbs. lbs. lbs. lbs. 209 389 352 308 495 Table No. LVI. Made Made 1 month. Obernai hydraulic lime slaked by immersion and mixed with one of sand and one of trass lbs. lbs. lbs. lbs. lbs. lbs. 493 330 297 286|304|399| Hydraulic limes employed while fresh, will, according to M. Vicat, support puzzolanas the more advantageously, as the limes are the less energetic; and on the other hand, they deteriorate so much the more rapidly as they are the more energetic: we may thence conclude that if a hydraulic immediately. After |