If the quicklime is treated (either by sprinkling or dipping) with as much water as it will easily absorb, it almost immediately cracks, swells, and falls into a bulky powder with a hissing crackling sound, slight explosions, and considerable evolution of heat and steam; - this is the process of "slaking." By it pure lime is increased in volume from 2 to 31/2 times its original bulk, - the variation depending both on the density of the original carbonate and on the manner of conducting the process.
Air-slaking. - If the pure quicklime be exposed to the air, it will gradually absorb moisture, and fall into a powder with increase of volume, but without perceptible heating; it is then said to be "air-slaked." Some carbonic acid is also absorbed in "air-slaking."
If a small pat be made of paste from the slaked lime and placed under water, it will slowly dissolve, until (if the quantity of water be sufficient, or is changed often enough) it entirely disappears.
1 Calcium carbonate. 2 Ferric oxide.
3 Called also "anhydrous" or "caustic" lime.
In air the surface of the pat will absorb carbonic acid, which reconverts it into a carbonate of lime. This action continually decreases, and practically ceases after forming a surface crust less than half an inch thick - the interior remaining pulpy or friable, according as the situation is damp or dry, and undergoing no further change of any kind.
Sand, of an ordinary description (such as that from flint or grains of quartz) occurring as an impurity in the limestone, has by itself no chemical action with the quicklime, when forming part of a limestone calcined at the temperature ordinarily reached in a kiln, but constitutes with it a mere mechanical mixture; forming what is called a "Poor Lime," and having the effects described at page 152.
The substances above noticed give the lime no hydraulic properties whatever.
It is most important to understand distinctly what constituents are necessary in a limestone to confer upon it the characteristic of hydraulicity.
These will now be shortly referred to.
The effects produced by the presence of clay in a limestone are as follows : a. It greatly modifies the slaking action. When a large proportion of clay is present, such action does not take place at all.
b. It confers the power of setting, and remaining insoluble under water, or in other positions where the air has no access.
In order that the clay may properly fulfil its functions, it is necessary -
1. That the amount of clay should be properly proportioned to that of the remaining constituents.
(The effects above mentioned are more marked as the proportion of clay is greater, up to a certain limit when the excess of clay becomes injurious.)
2. That the stone should be calcined at the proper temperature.
(This is a very important and very intricate portion of the subject. The same stone will give very different results according to the degree of calcination to which it is subjected.)
The nature of the changes undergone by the clay, and the evils caused by over-burning or under-burning the stone, are explained at page 230.
These changes are of a somewhat complicated nature, and it will be sufficient at present to note the fact that after proper calcination of a limestone containing clay, the result is a substance containing a proportion of free quicklime together with compounds (formed by the clay and lime) which have the property of becoming hard when formed into a paste, even if secluded from the air or placed under water.
There are several forms of silica, such as sand, flint, etc., which, as already noticed, are useless in lime, for they are only in a state of mechanical mixture with it. The silica must be in combination with other substances and in a peculiar soluble state, or it will not combine with the lime; in such a state it is found in clay.
Unfortunately, in most analyses of limestones the soluble or usefully active form of silica is not distinguished from the sand, or silica in an inert state; this leads to some confusion, and renders the analyses less useful than they would otherwise be.
Carbonate of Magnesia 1 combined with lime reduces the energy of the slaking, and increases that of the setting processes; when other substances are present, its behaviour and combination with them are similar to those of lime.
When carbonate of magnesia is present in sufficient quantity (about 30 per cent), it renders lime hydraulic independently of and in the absence of clay.
These if exposed to a great heat become fused and quite inert; but when subjected only to lower temperatures sometimes tend to produce soluble silicates, and thus to cause hydraulicity.
Sulphates in small quantities tend to suppress the slaking action, and to increase the rapidity of setting.
The introduction of these is the basis of a class of cements which will be considered presently. (See p. 179.)