Pure lime, in itself, has neither power of setting nor ultimate strength, and it is only when combined with clay and burnt that it is of value as a building material. The presence of clay, up to a certain percentage, regulates the value of its strength and setting power, a pure, rich, or fat lime (as explained and used for plastering) containing little or no clay, which allows it to slake at once with a loud crackling noise and considerable steam. It is only fit for plastering and other sanitary purposes, and can be made out of most pure limestones, including the Bath and Portland varieties, marble, and chalk.
Poor lime is the next degree, containing from 80 to 90 per cent, of carbonate of lime, with a residue of sand, and other qualities which give it no hydraulic properties, as its power of setting under damp conditions and in water is called. It may be termed a useless variety as compared with the others, considering that it slakes sluggishly and imperfectly, which makes it valueless for plastering; while for mortar it is worse than "fat" lime, because it will neither increase so much by slaking, nor take such a quantity of sand, being neither one thing nor the other. Rich and poor limes never set bard, though sometimes they may harden a little on the exterior only.
The first best, as a good, quick-setting, and strong lime, is called a feebly hydraulic lime, containing 80 to 90 per cent, of carbonate of lime and 8 or 10 per cent, of clay, which are the required ingredients. The small amount in which it is present in this kind is not as much as is wanted, though its slaking may be considerably reduced. The clay is required to such an extent that its silica and alumina will take up in combination most of the lime, forming silicates and aluminates of lime, the former being formed at an early stage of the burning, and the latter after subjection to a considerable degree of heat; and it is this fact which makes it necessary that the stronger limes and cements should be the more burnt.
This combination reaches its state of perfection (as about to be explained), and then it declines in efficiency and value, owing to the presence of too much clay.
Hydraulic limes, the fourth class, contain from 15 to 20 per cent, of clay, which takes some time before it begins to slake, and then only in a small degree. It sets in about a week, and is fairly strong. It is the product of grey chalk, found in Sussex and Surrey.
An eminently hydraulic lime contains from 20 to 30 per cent, of clay, is of a greenish-yellow colour, found in the lias lime formations at Barrow-on-Soar, in Leicestershire; Rugby and Stockton, in Warwickshire; Whitby, in Yorkshire; Lyme Regis, in Dorset; and Holywell, in Wales. It sets hard and quickly, and is very difficult to slake.
When the clay is mixed with the lime in a natural state, or otherwise artificially in the proportion of 1 and 2, the resulting combination of chemicals obtained at a great heat is called a cement, the Portland variety being the best, of which there are two different kinds, slow and quick setting - the former being heavy and the latter light in weight.