Rich or Pat Limes are those calcined from pure, or very nearly pure, carbonate of lime, not containing sufficient foreign constituents to have any appreciable effect upon either the slaking or setting actions.
The solubility and want of setting power of fat lime render it unsuitable for making mortar, except for the walls of outhouses and for other similar positions. It is nevertheless frequently used for the mortar in structures of a much more imposing character.
It is however better than hydraulic limes for sanitary purposes (being purer), and is very useful for plastering and for whitewashing. It is also extensively employed in the manufacture of artificial hydraulic limes and cements.
Fat lime requires to be mixed with a great deal of sand to prevent excessive shrinkage, but this addition does not materially injure it, as it attains no strength worth mentioning under any circumstances.
The only setting that takes place in it is the formation of a thin surface crust, bearing a small proportion to the whole bulk; mortar made from such lime may therefore be left and re-worked repeatedly without injury.
Some of the lime which finds its way into the London market, under the assumed names of Dorking, Hailing, and Mers-tham, is merely fat lime tinged with iron sufficiently to give it the buff colour characteristic of the hydraulic lime made out of the grey chalk from the localities above mentioned (see p. 155).
Of course, this stained lime makes mortar of the same inferior description as would be obtained from a common fat white lime, and has no hydraulic properties whatever.
Poor Limes are those containing from 6 0 to 90 per cent of carbonate of lime, together with useless inert impurities, such as sand, which have no chemical action whatever upon the lime, and therefore do not impart to it any degree of hydraulicity.
These limes slake sluggishly and imperfectly, the action only commences after an interval of from a few minutes to more than an hour after they are wetted, less water is required for the process, and it is attended with less hear and increase of volume than in the case of the fat limes.
If they contain a large proportion of impurities, or if they are over-burnt, they cannot be depended upon to slake perfectly unless first reduced to powder.
The resulting slaked lime is seldom completely pulverised - is only partially soluble in water, leaving a residue composed of the useless impurities, and without consistence.
The paste formed from the slaked lime is more incoherent, and shrinks less in drying, but behaves in other respects like that made from fat lime - in fact, it is like a fat lime mortar containing a certain proportion of sand.
Mortar made from poor lime is less economical than that from fat lime, owing to the former increasing less in slaking, bearing less sand (as the lime already contains some in the form of impurities), and requiring a more troublesome manipulation than the latter. It is in no way superior as regards setting, and should therefore only be used when no better can be had.
Hydraulic Limea are those containing, after calcination, enough quicklime to develope more or less the slaking action, together with sufficient of such foreign constituents as combine chemically with lime and water to confer an appreciable power of setting without drying or access of air.
Their powers of setting vary considerably. The best of the class set and attain their full strength when kept immersed in water.
Different substances have this effect, as already mentioned (see p. 147), but in the great majority of natural hydraulic limes commonly used for making mortar, the constituent which confers hydraulicity is clay.1
The phenomena connected with the slaking of limes varies greatly according to their composition. With none is it so violent as with the pure carbonate of lime (see p. 146), and the more clay the limes contain the less energy do they display, until we arrive at those containing as much as 30 per cent of clay, when hardly any effect at all is produced by wetting the calcined lime, unless it is first ground to powder.
1 In some varieties, as before mentioned, a portion of the carbonate of lime is replaced by carbonate of magnesia, which increases the rapidity of setting.
The setting properties of hydraulic lime also differ very considerably in proportion to the amount they contain of the clay or other constituent, which gives the lime its power of setting without drying or the access of air.
This led to their being subdivided by Vicat into three classes, as shown in the following Table: -