Before attempting to explain this action it will clear the ground to make a few remarks regarding the nature and composition of clay. Some of the information now about to be given has, however, been anticipated in the chapter on Bricks.
Clay is a compound of silica and alumina with water, chemically known as "hydrated silicate of alumina."
Silica and alumina alone, or in the presence of each other, are infusible, except at extremely high temperatures.
The same effect is produced to a still greater degree by potash, soda, and chlorides of potassium and sodium.
Many clays naturally contain iron and also the alkalies above mentioned.
Lime is also an infusible substance. When burnt with clay the lime is attacked by the alumina as well as by the silica of the clay, and both silicate of lime and aluminate of lime are formed.
When a limestone containing clay is burnt, the carbonic acid from the carbonate of lime and the water from the clay are partially or wholly driven off, and the ingredients are re-arranged in a new set of compounds, the exact nature of which varies both with the original composition of the stone, and with the degree to which it is burnt In general terms it may be said that these compounds consist of quicklime mixed with silicate of lime and aluminate of lime.
The silicate of lime is formed at a comparatively early stage in the burning, but it is only at the higher temperatures that the alumina and lime enter into combination to form aluminate of lime.
When the burning has been carried to the proper point, these substances are left in a condition in which they will combine with one another and with a proportion of water (when made into a paste with the latter). During this combination they form a new set of compounds, and eventually yield a hard substance insoluble in water.
"When Portland cement is thoroughly well made, hardly any causticity can be detected by the tests owing to the silicates which have formed round the particles of quicklime." 1
The limit of temperature to which the burning should be carried varies for different stones, and can only be found out experimentally.
The proportion of clay contained by the limestone, also the composition of the clay, both affect the question of the degree of burning, and it will be well to consider these points separately.
Effect in Stones burnt at a moderate temperature. - If the stone contain a large proportion of clay and is burnt at a moderate temperature, the silicic acid in the clay attacks the lime, forming calcium silicate. The alumina in the clay does not combine with the lime as long as the temperature is moderate.
If there be sufficient clay present, the whole of the lime is so converted into silicate of lime, and the result is a quick-setting cement like those of the Roman class.
1 Scott and Redgrave; M.I.C.E., vol. lxii. p. 78.
If there is only a small amount of clay (that is, not sufficient to provide the necessary amount of silica for the conversion of all the lime into silicate), some of the lime will be left uncombined - i.e. in a quick state. This uncombined lime will slake upon the addition of water.
The slaking action will, however, be sluggish as regards the mass of the stone, for it is impeded by the presence of the clay.
Such a state of things exists in hydraulic limes; in these the greater part of the compound body has been converted into a silicate of lime; but there is sufficient uncombined quicklime remaining to develop a slaking action. This action, however, is in most cases feeble, and sometimes almost suppressed, in consequence of the bulk of quicklime being so small in comparison with that of the mass.
Effect in Cements burnt at a high temperature. - When, however, the calcination is carried to a further stage, and the stone is burnt at a very high temperature, not only is the carbonic acid driven off and some of the lime converted into silicate of lime, but a further combination takes place - the alumina of the clay combines with the lime, forming aluminate of lime, and at the same time a further silicate of lime is formed. In addition to these combinations, there are others of an intricate character which result in the formation of double silicates of lime and alumina.
The aluminate of lime was found by M. Fremy to set readily, when powdered and wetted, without access of air, and also to be capable of cementing together inert particles such as those of sand.
The double silicates of lime and alumina also have the property of setting when hydrated - i.e. when mixed with as much water as they will take up.
When Portland cement is raised to the very high temperature necessary for the proper burning of that material, the whole of the lime in the mixture is converted into either silicate or aluminate - the entire mass is composed of either one or the other of these compounds, and the result is great strength.
In an underburnt Portland cement, however, the aluminate is not formed, some of the lime is left free; the resulting cement is quick-setting, but weak and apt to "blow," the uncombined particles of lime slaking either when they are wetted, or after a considerable lapse of time.
If the clay contains a large proportion of iron and alumina (especially of iron) as compared with the silica, the calcination must be at a comparatively low temperature, or the particles will be fused.
In the Roman, Medina, and Atkinson's cements the quantity of iron and alumina together nearly equals the silica. These are therefore burnt at a low temperature.
When, however, the iron and alumina are in comparatively small proportion compared with the silica, the mixture can be burnt at a very high temperature without danger of fusion. This is the case with Portland cement.
The presence of potash or soda in conjunction with the alumina produces the same effect as the presence of iron, but to a greater degree. If material containing them be exposed to a high degree of calcination, it will fuse into glass or slag.
The same relation holds good between the composition of the clay in hydraulic limestones and the temperature at which they are burnt.
"The larger the amount of the iron and alumina present, the more readily will the lime and the clay, when the limestone is raised to a red heat, pass successively from that condition in which the lime retains all its own proper energy for water, to that in which the lime and clay prefer, in partnership as it were, to enter into combination with it in a gradual and quiet manner, and to that in which the formation of the silicates is completed without the intervention of water, and the resulting vitrified compounds show themselves quite indifferent to it, or are only affected by it after having been submitted to its action for some time." 1