When a stone yielding hydraulic lime is subjected to too high a temperature, the effect will be to partly fuse the particles, which prevents them from absorbing water and slaking at once. They thus form either a totally inert substance, or one which slakes after a lapse of some considerable time.
Hydraulic limestones should therefore be burnt at a moderate temperature.
With a stone containing the smallest quantity of clay required to form a cement, a slight calcination will not carry the combination far enough to form a strong cement, and the result will probably be either a hydraulic lime which slakes on the addition of water and sets afterwards, or a mixture of quicklime and quick-setting cement, the latter of which sets first, and is then broken up by the slaking of the lime.
A high degree of calcination produces a cement of great strength; the best Portland cement is therefore produced by burning at a high temperature.
If, however, the calcination be carried to far, the extreme heat will vitrify the cement, and make it almost entirely inert.
Stones containing much clay give, on the other hand, the best result with a slight calcination, many indeed at a point short of the expulsion of the carbonic acid. A higher degree of heat, sufficient to make the whole of the lime caustic, sometimes gives a mixture of lime and cement like that produced by under-burning a slow-setting cement, or it may give a slow-setting cement.
The point of vitrification is reached much sooner in such a stone, especially if the clay contain much soda, potash, or iron.
Roman cement and others of the same class are produced from stones containing a large proportion of clay and of iron, and are therefore burnt at a low temperature.
The foregoing is only a general sketch of the results of the burning process, to which there are many exceptions caused by peculiarities of composition.
Some stones yield - 1, a cement; 2, an intermediate lime; 3, a cement; 4, an inert substance; 5, a cement; and 6, an inert substance again, in the order given, and at progressive increasing degrees of calcination.
This action also is influenced by the proportion of clay contained in the lime.
1 General Scott; R.E. Corps Papers, vol. xi.
If the burnt stone contains so small a proportion of clay that the silicates and aluminates cannot combine with all the lime, a certain proportion of quicklime is left in an uncombined or free condition. This causes a modified slaking action, more or less marked in proportion to the amount of free lime that is present.
In proportion as the amount of clay increases, and therefore in proportion as the free quicklime diminishes, e.g. in hydraulic and eminently hydraulic limes, this slaking action is more and more suppressed.
Finally, in the case of cements, where the quantity of uncombined lime is reduced to a minimum, the slaking action entirely disappears, and the setting process begins immediately upon the addition of water.
We see, then, that after the processes of burning and slaking hydraulic limes containing clay, there is left within them a mixture of pure lime and silicates, or of pure lime, silicates, and aluminates, ready to combine, if a proper communication is provided to bring them into contact.
When the lime is placed under water, this is effected, for the water at once commences to disseminate and in some degree to dissolve the particles of pure lime; these mingle with the silicates (many of which are also partially soluble), and combine with them, one by one, by slow degrees, to form a new set of hydrated silicates.
It is evident, then, that the water acts by enabling the particles of alumina and silica to get at the particles of lime, and thus to attack them; whereas if the particles remained in a dry state, they would lie within a short distance of one another, without ever combining.
A certain proportion of the water also plays another part, by itself combining with the silicates and aluminates to form hydrated compounds, which set by crystallising, and pass into the solid state.
This explains why mortars of hydraulic limes should not be allowed to dry too quickly. The dissemination and dissolution of the particles is thereby stopped, and the setting process impeded.
A properly burnt lime, containing sufficient clay, when saturated with moisture after calcination, or when quite immersed, is therefore in a favourable condition for forming the hard and insoluble compound above mentioned; in fact its composition adapts it for setting under water.
Even in the case of fat limes the presence of moisture for a certain time is useful, for it enables them more readily to absorb carbonic acid from the air. In hot countries it is necessary that work in which they are used should be kept moist for some time, otherwise the mortar will be in a granular crumbly state, it will not readily absorb carbonic acid, and the lime will not enter the crystalline condition which is essential for proper setting.
To give a perfect result there must be sufficient clay to combine with all the lime in the mixture; otherwise some of the lime, having nothing to combine with, remains pure and soluble, and reduces the average setting property of the whole.
This occurs in those hydraulic limes which contain from 8 to 15 per cent of silica. When all this silica has entered into combination, there is still some quicklime left (more or less in inverse proportion to the amount of clay). This remaining lime develops the slaking action as before explained, and impedes the action of setting.