Bourne Archive: Muspratt: Cement: Mortar
http://boar.org.uk/aaiwxw3Muspratt3Mortar.htm Latest edit 17 May 2011
The Bourne Archive
Extracts Concerning Building Cements, 3.
This is part of an article on chemical bonding materials under the heading ‘Cement’. It is presented here in three web pages, respectively dealing with fairly simple lime burning kilns; with a large lime kiln in Brandenburg; and the present one, dealing with mortar.
Vol. 1. pp. 455-6
MORTAR is a mixture of slaked lime and sand in proper proportions. When this is spread in thin layers between stones or bricks, as in building, it gradually hardens and acquires a great degree of tenacity, combining at the same time with the substance of the brick or stone, so that the whole becomes one perfect solid. The time required to effect this change is long; for although the mortar becomes sufficiently hard, in a few days or weeks, for the wall to bear considerable pressure, still it does not acquire the maximum degree of hardness till after the lapse of many years, and even centuries. One of the causes of the durability of old buildings might be attributed to the long period during which the mortar has been exposed to the hardening influence exercised upon it by age; but even this, as is well known, will be ineffectual in bringing the mortar to its greatest power of endurance, unless it be made with the proper admixture of lime, and silicious or other matters.
Lime, alone, is capable of forming a hard cement, but only when interposed between building materials in a very thin stratum. If it be used in masses of considerable thickness, the degree of cohesion which it acquires is very slight, and therefore, it is in this state almost useless as a cement. The reason why it is not adapted for the purpose here specified when used alone is, that only the portion in contact with the stone or brick adheres with the required firmness, the body of the mortar remaining friable. Were the surfaces of contact sufficiently extensive and regular, then the effect of the thin layer of hydrate of lime would be sufficient to unite the parts, but it is impossible to supply this condition. As, however, it is found that the lime reacts with such bodies as are different from it in composition, especially if they be of a silicious nature, the fact has been taken advantage of, and sand employed from time immemorial to multiply the surface of contact, and bring the whole of the hydrate of lime into active combination, as well with this as with the surfaces of the stones or bricks, and thus the union is more thoroughly accomplished. Another requirement for insuring the solidification of the mortar is the presence of sufficient moisture. If water be not properly supplied, no matter how true the admixture of the other ingredients may be, the mortar will not become firm; if kept in large quantities of this liquid, it will not solidify; and if it be dried as in a stove, it will not form a cement, but remain friable. Exposure to air is likewise necessary to develop all the properties of the mortar.
Bearing these facts in mind, it will be easy for the builder to prepare his mortar or cement, so that it may possess all the requisite qualities. The best material is quartz sand, not very fine. When the sand is very minutely divided, then the matter becomes too dense to admit the air which is necessary for its proper solidification; on the contrary, if it be very coarse, the interstices are too large to be filled up with the lime. A good result is obtained when irregularly-shaped stones are employed, by taking a mixture of coarse and fine sand, for it has been observed that the more irregular the foreign body mixed with the lime, provided the surface of contact is sufficiently extensive, the more binding is it. When the mortar is intended to form a thin coating exteriorly, fine sand is more appropriate.
After ascertaining the materials best suited for the preparation of the mortar, the next step is to proportion them in such a way as to give the best result. This is a work of considerable importance, and any error committed in it cannot be subsequently remedied. As to the quantity of sand which should be taken, much depends upon the quality or the lime; If it be a fat lime, that is, if it be devoid of much foreign matters, and fall into a very fine powder in the slaking, it will require about six times the weight of sand; or three or four cubic feet of sand must be added to one of the semifluid milk or lime. Should the lime contain much insoluble matter, or be what is usually known as poor, then the volume of sand must be lessened to two, or two and a half cubic feet. As a general rule, the sand should be mixed with as much of the cream as it will take up without increasing its bulk, and the lime should be so finely divided as to form the thinnest possible stratum between the grains of sand, which it should envelop and bind together.
The cause of the setting of mortar was a problem unsolved for a long time; it was thought that the hardening was due to mechanical agency exclusively; now, however, it has been found that this is not the case, and that chemical combination contributes materially to render it firm and durable. Such chemical action is not wholly confined to the formation of carbonate of lime, as was once supposed. Indeed, it has been experimentally proved, that the mortar in the interior parts of the edifice contains, after three hundred years and longer, no carbonate of lime; and that what has remained without otherwise entering into combination with the material of the stone or brick, is still caustic; and, at most, only half an equivalent of carbonic acid is absorbed, producing a basic or subcarbonate of lime, the formula of which is Ca O, CO2, Ca O, H O.
Analyses have shown that a silicate of lime is formed, partly from the sand of the mortar, and partly from that of the stone or brick in the building; and the constitution of this body has a considerable influence in blending the material and mortar together so intimately. Besides the generation of carbonate and silicate of lime, there are other changes which play an important part in the solidification of the mortar; such as the crystallization of the hydrate of lime, as the excess of water in it spontaneously evaporates, or is absorbed by the bricks. This hydrate being dissolved in the mixture, and therefore permeating the whole of the mortar, forms very minute crystals, which, though they are brittle and break very readily when they are bent, yet offer great resistance when the force tends to tear them asunder. Hence, if a layer of such crystals be deposited upon surfaces in close proximity, a power tending to separate them will act upon the ends of very short levers, and tend to break the crystal in that direction in which it is calculated to offer the greatest resistance.
Whenever the moisture in the mortar comes in contact with the air on the outside, carbonic acid is absorbed from the atmosphere by the hydrate of lime, giving rise to carbonate of lime, which deposits in a crystalline state, and this offers the same opposition to fracturing as the hydrate in the foregoing instance.
The formation of the carbonate continues as long as the mortar retains moisture to dissolve the base and the air has access. Whatever may be the advantages gained from these combinations, they are not sufficient to impart firmness to the material; it is only from the combined influence of the adhesiveness of the mortar to the stone, rendered more intimate by chemical combination, that the full effects are obtained.
Much injury arises to buildings in consequence of impurities being in the materials of the mortar, more especially if they consist of humus, nitrogenous matters, or alkaline chlorides. When these are present, they produce, by their decomposition, deliquescent salts, which attract moisture and occasion obnoxious damps, besides disintegrating the mortar, and consequently, destroying its cohesive power. Particular observation should therefore be made, as to whether the sands and water employed in the making of mortar are free from the substances mentioned. When alkaline chlorides are contained in them they undergo decomposition, giving rise to an alkaline carbonate and chloride of calcium, which is a most deliquescent salt. So ready is this interchange, that it was once foolishly proposed as a means of preparing carbonate of soda on a large scale. The humus and nitrogenous matters, when decomposed in the presence of hydrate of lime, produce nitric acid, which, combining with the latter, yields nitrate of lime – also a deliquescent salt – which is observed in the form of an efflorescence.
Incrustations, however, appear on the walls, which are not occasioned by any of the forementioned causes, and, consequently, do not affect the quality of the mortar like those enumerated. They are composed, according to the researches of KUHLMANN, VOGEL, and others , who examined efflorescences of this kind, of sulphate of soda, carbonate of soda, and chloride of sodium, together with carbonate of potassa, and chloride of potassium; they take their rise from the limestone employed containing these bases. Probably the ash from the fuel used in burning the lime contributes more or less to their formation.