Bourne Archive:
Muspratt: Cement: Mortar
http://boar.org.uk/aaiwxw3Muspratt3Mortar.htm Latest edit
17 May 2011
The Bourne Archive
Muspratt’s Chemistry, Theoretical,
Practical & Analytical (ca. 1859)
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.