ON HYDRAULIC AND COMMON MORTARS. Sect. 1.-ON MORTARS PLACED UNDER WATER. On Lime. Actual state of our knowledge of this substance. Lime has been employed from time immemorial. Mixed with sand, or certain other substances, it forms what is called mortar. Although the solidity and durability of masonry depends on the goodness of mortar; still, few experiments have been made with lime; and the manner of making mortar has almost always been given up to workmen. It is only within about fifty years that a few scientific men have attended to this important subject. Comparing the mortars of the ancients, and especially of the Romans, with those of modern times, it was perceived that the old mortars were much better than ours; and the means have, consequently, been sought of imitating them. Several constructors have thought they had discovered the secret of making Roman mortars: others, on the contrary, have thought that the Romans had no particular process, but that, of all their constructions, those only which were made of good lime had survived to our day. We shall see that my experiments tend to confirm this latter opinion. Lime used in building, is obtained by the calcination of calcareous stones, which occur abundantly on the surface of the globe. Marbles, certain building stones, chalk, calcareous alabaster, and shells, are employed in making lime. The effect of calcination is to drive off the water and the carbonic acid which are combined with the lime. The water and the first portions of carbonic acid pass off easily; but it requires an intense, and long continued heat to dispel the remainder of the acid. Lime, as used in constructions, contains, almost always, a considerable quantity of carbonic acid. When the stone submitted to calcination is white marble, pure lime is obtained, provided the calcination be carried far enough. According to an analysis which I made of white marble, this substance contained, in 100 parts, as follows: lime 64; carbonic acid 33; water 3. Lime obtained by calcination possesses the following properties. It has a great avidity for water, imbibes it from the air, and has its bulk enlarged thereby. If a certain quantity of water be thrown on lime recently calcined, it heats highly, breaks in pieces with noise, and a part of the water is evaporated by the heat produced. The disengaged vapour carries off some particles of lime. Water dissolves about one four-hundredth of its weight of lime, forming what is called lime-water.* Lime is caustic and turns the syrup of violets, green: its specific gravity, according to Kirwan is 2.3, it attracts carbonic acid from the air, and finally returns to the state of carbonate of lime. preserve it, it is necessary to keep it in very tight vessels. Το Lime was formerly ranked among the alkalis, and it is only lately that the true nature of the substance became known. Davy, the English chemist, succeeded in 1807, in decomposing, by means of Volta's pile, the sulphate, and the carbonate, of lime, or more properly lime derived from these compounds; obtaining a brilliant substance, having so strong an attraction for oxygen that it absorbs it rapidly from the air, and from water, which it decomposes. The brilliant substance obtained from lime is regarded as a metal and has received the name of Calcium. Accordingly, lime is only a metallic oxide. It is rare that lime derived from white marble is used in the arts; that which is commonly employed, and which is derived from ordinary lime stone, almost always contains oxide of iron, and sometimes a certain quantity of sand, alumine, magnesia, oxide of manganese, &c. Some of these substances combine with the lime by calcination: and the lime thus acquires properties which it had not before, and of which I shall speak in the sequel. If we take lime derived from white marble, or from common lime stone, and reduce it as it comes from the kiln, to a paste with water, and if we place this paste in water, or in humid earth, it will remain soft forever. The same result will be obtained, if lime be mixed with common sand and the resulting mortar be placed in similar situations. It is a common practice to deluge lime, fresh from the kiln, with a large quantity of water, and run it into large basins, where it is allowed to remain in the condition of soft paste. Alberti says (book II., chap. XI.) he has "seen lime, in an old ditch, that had been abandoned about 500 years, as "was conjectured from several manifest indications; which was still so moist, "well tempered, and ripe, that not honey or the marrow of animals could be "more so. There is another kind: eflime which possesses a singular property: if it be slaked as it comes from the kiln, as above, and be then placed in the state of paste, in water, or, in moist, earth, it will harden more or less promptly, according to the substances it contains. The same result is obtained if the lime, being mixed with sand, is made into mortar and placed in similar situations. If this:lime be slaked and run into vats, as is done with common lime, it will become hard after a little time, and it will then be impossible to make use of it. On slaking lime, fresh from the kiln, with enough water to reduce it to paste, it is found to augment considerably in bulk; this augmentation is such that one volume of quick lime will sometimes yield more than three volumes, measured in the condition of thick paste. When lime which has the property of hardening in water is slaked in the same manner, it affords a much smaller volume than common lime. Sometimes one volume of this lime, measured before slaking, will give, when slaked to thick paste, scarcely an equal bulk. For a long time, those limes which had the property of hardening in water were called meagre limes, and those which had not this property were called fat limes. These denominations were affixed because * One four hundredth, Davy-one seven hundred and fifty-eighth, Thompson—one seven hundred and seventy-eighth, at 60° Fahr., Dalton.-Tr. the first kind increased but little in bulk when made into paste, while the other give a considerable augmentation of volume; and because fat limes formed, with the same quantity of sand, a mortar much fatter or more unctuous than meagre lime. But the designation "meagre lime" is altogether improper to indicate limes which enjoy the property of hardening in water; because there are limes which augment their volume very little, on being made into paste, and at the same time possess no hydraulic property. Belidor gave the name of béton to lime which had the quality of hardening in water; but many engineers continued to call it meagre lime. The denomination of béton is not suitable; and, in this sense, is not now in use. The following are the terms now employed. In England, the name of aquatic lime has been given to lime which indurates in water; in Germany it is called lime for the water; Mr. Vicat, Engineer of roads and bridges, has proposed the name of hydraulic lime, and this denomination, which is a very good one, has been generally adopted. I shall therefore call that lime which swells considerably in slaking, fat lime, that which swells but little and does not harden in water, meagre lime, and that which possesses the property of hardening in water, hydraulic lime. Fat lime is often called common lime, also. The term quick lime is applied to all unslaked limes whether fat lime, meagre lime, or hydraulic lime. Although meagre lime and hydraulic lime may have been calcined exactly to the proper degree, still they are slower to slake, and give out less heat than fut lime. When fat lime has been too much burned, it, also, becomes slow to slake; while, if properly burned, it begins to slake the instant water is thrown on. Experiments, to be given in the sequel, will show that iron, in the state of red oxide, causes fat lime to slake sluggishly. Some of the ablest chemists have, at different times, sought to detect the substances which impart to lime the property of indurating under water. Bergman, a Swedish chemist, was, I think, the first who gave an analysis of a hydraulic lime-stone. That from Léna in Sweden, he found to contain, in 100 parts, the following substances; lime, 90; oxide of manganese, 6; clay, 4. Bergman seems to have attributed the peculiar property of hydraulic lime, to the oxide of manganese; and this opinion prevailed for a long time. On the other hand we find in the Bibliothèque Britannique of 1776, vol. III, page 202, that Smeaton, the English Engineer, who built the Edystone Light-house, in 1757, attributed this property to clay: for he says that it is a curious question, which he leaves to chemists and philosophers to decide, why the presence of clay in the tissue of a calcareous stone should give it the property of hardening in water, while clay added to common lime produces no such effects. Guyton de Morveau, announced in a memoir published in the year 9, that he had detected the presence of oxide of manganese in all the lime stones which afforded hydraulic limes; he announced, further, that in calcining together 90 parts of common lime stone pulverized, 4 parts of clay, and 6 parts of black oxide of manganese, an excellent artificial meagre lime would be obtained. It was stated above, that at that time, the name meagre lime was given to lime that would set under water; the French chemist was the first therefore to make artificial hydraulic lime; but he, as well as Bergman, was mistaken in supposing that the presence of the oxide of manganese was necessary to the result. He would have obtained his result by burning the pulverized lime stone with clay alone. Mr. Saussure, in his Voyage des Alpes, says that the property possessed by certain limes of hardening in water is due solely to silex and alumine (that is to say, to clay) combined in certain proportions. Mr. Vitalis, chemist of Rouen, made, in 1807, the analysis of the limestones of Senonches and St-Catherines, near Rouen; the analysis is contained in the memoir on the schists of Cherbourg (page 58) published in 1807, by Mr. Gratien, Sen., engineer of roads and bridges. This lime stone contains, according to Mr. Vitalis, in 100 parts, the following substances, water, 12; carbonate of lime, 68; alumine, 12; sand, 6; oxide of iron, 2. In addressing these results to Mr. Gratien, senior, Mr. Vitalis expresses himself thus: "It follows from the analysis that the lime-stones of Senonches and St-Catherines, are two calcareous marles, in which the chalk predominates it is true, but wherein the clay performs an important part. It is this portion of clay which, in my opinion, makes the lime of these two lime-stones, meagre; whence it follows that the presence of oxide of manganese is not indispensable to the constitution of such limes, since the analysis proves that the lime-stone in question contains no oxide of manganese, as it would, if present, have coloured the glass violet." I noticed above that these hydraulic limes were then called meagre limes. We see that the analysis of these stones confirms the opinion of Mr. Saussure, who had attributed to the clay alone, the property of hardening in water. Thompson, an English chemist, was of the same opinion. Mr. Descotils, engineer of mines, also made an analysis of the lime-stone of Senonches; which analysis may be found in the Journal des Mines of 1813, page 308. According to this trial, the Senonches lime-stone contains a quarter part of silex, disseminated in very fine particles, and only so small a quantity of iron and alumine, that these substances can have no influence on the lime; whence this engineer concludes that the hydraulic property of this lime-stone is owing to the silex. We have, however, seen above, that, according to Mr. Vitalis, it contains twice as much alumine as silex. Mr. Berthier also inserted in the Journal des Mines an analysis of the Senonches lime-stone, which will be given further on, and according to which the stone contains very litte alumine. This contradiction has not yet been explained. Perhaps the quarries at that place afford stones of different kinds. If so, it would be important to ascertain what is the composition of the best. The analysis of the Senonches lime-stone afforded Mr. Descotils occasion to make an important remark on the silex contained in lime-stone: namely, that the silex found in these stones does not dissolve in acids before calcination, but does dissolve after calcination. This fact proves that the properties of silex are changed by calcination with lime, and that it combines in the dry way with this substance. Mr. Vicat, engineer of roads and bridges, published in 1818 a very important memoir on hydraulic mortars. This engineer set out with the opinion generally admitted at that time, that it was the clay which gave to lime the singular property of hardening in water. He, in consequence, took fat lime, which he mixed with various proportions of clay, according to the following process, extracted from page 7. "The operation we are about to describe (says Mr. Vicat) is a true synthesis, reuniting in an intimate manner, by the action of fire, the essential principles which are separated from hydraulic lime, by analysis. It consists in allowing the lime, which is to be improved, to fall spontaneously to powder in a dry and covered place; afterwards to mix it, by the help of a little water, with a certain quantity of gray or brown clay, or simply with brick earth, and to make balls of this paste, which, after drying, are to be burned to the proper degree. 66 Being master of the proportions, we may concieve that the factitious lime may receive any degree of energy desired, equal to, or surpassing at pleasure, the best natural lime. "Very fat common lime will bear 0.20 of clay to 1.00 of lime; moderately fat lime will have enough clay with 0.15; and 0.10, or even 0.06, of clay will suffice for these limes which are already somewhat hydraulic. When the proportion is forced to 0.33 or 0.40, the lime does not slake, but it pulverizes easily, and gives, when tempered, a paste which hardens under water very promptly." Such is the process indicated by Mr. Vicat. But this engineer did not content himself with experiments on a small scale: a manufactory was established near Paris by his means, where artificial hydraulic lime is made in large quantities; he moreover exerted himself to extend the use of hydraulic mortar every where, and he succeeded. He has, therefore, rendered an important service to the art of construction, and I have done him the justice to make this acknowledgment, in the notices I have heretofore published. In 1818, Dr. John, of Berlin, presented to the Society of Sciences in Holland, a memoir, which was published in 1819. This memoir, crowned in 1818, by the Society, answered the following question proposed by the Society: "What is the chemical cause, in virtue whereof stone lime makes generally more solid and durable masonry than shell lime, and what are the means of improving shell lime in this respect?" Dr. John has remarked that shells require to be more highly calcined than common lime-stone: he thinks this owing to the shells being purer carbonate of lime than common limestone, which contains earthy substances facilitating the disengagement of the carbonic acid. In making the analysis of sundry lime-stones, he found that those which afforded hydraulic lime contained clay, oxide of iron, &c. He called the foreign matters which gave the property of hardening in water, cements; and says that it is possible, by introducing cement in the dry way to ameliorate lime which contains none. On these considerations he made the following experiments. He mixed the powder of oyster shells, 1st, with of silicious sand-2d, with several proportions of clay, varying from to 3d, with of oxide of manganese. He tempered these mixtures with water, formed them into balls, let them dry in the air, and then burned them in a lime-kiln for 96 hours. The following results were obtained: the first mixture was agglutinated but friable, and was not a good result; the second mixture gave good results; and the third possessed no peculiar property. The author concludes that clay is the ingredient which gives to common lime the property of hardening in water; and he says that nothing can be easier than to procure good hydraulic lime, either from shells or from pure lime-stone, following the process indicated: he adds that it is for constructors to determine the best mixture to be made in each case. The memoir of Dr. John contains the analysis of several ancient mortars; and offers several important observations of which I shall have occasion to speak. In the third number of the Annales des Mines of 1822, there is a very interesting memoir by Mr. Berthier, Ingénieur en Chef des mines; it contains the analysis of different lime-stones, and several new views which will contribute to form a more perfect theory of mortars. I shall have more than one occasion to cite his experiments, and his opinion on several important acts. Mr. Raucourt, engineer of roads and bridges, published at St. Petersburgh |