LIME. In building operations lime, the dchydrised oxide of calcium, in its various modifications is used for the purpose of forming the material by means of which the small materials used in masonry or brickwork, are bound or cemented together; and the precise word lime is used in contradistinction to plaster (or the sulphates of lime), or of cement (or the highly silicated carbonates of lime and alumina), to designate the materials obtained from the calcination of carbonates of lime of comparatively pure and simple composition. The various descriptions of limes, technically considered, are those which are known by the names of the rich, the poor, the moderately hydraulio, the hydraulic, and the eminontly hydraulic limes ; and the application of these names depends exclusively upon the manner in which the re spective limes pass through the stages of slaking, or of setting. Very little attention is paid in practical building operations to the mechan ical structure of the stones from which the various limes are obtained, and the qualities of the caustic limes themselves constitute the only recognised elements of the classification.
The materials obtained from the calcination of limestones contain ing less than 6 percent. of the silicate of lime, and otherwise composed of the pure carbonate, are those called the rich limes, on account of the remarkable increase of volume they assume when water is added to them (or when they are slaked), for the purpose of being reduced to the state of the plastic material, to be subsequently mixed with sand, for the for mation of mortar. They increase, in fact, as much as 3i or 41 of their original volume when measured in the lump, and about of their original weight ; they absorb in so doing about times their bulk of water. Naturally contractors prefer limes of this description ; for, to use one of their own phrases, "they carry more sand ;" but such pure rich limes never set (or harden by crystallisation), when in large bodies, or large masses of work, and they are very soluble in frequently renewed water. It follows, therefore, that pure rich limes should never be used for external walling, or for works to be exposed to the action of running or of tidal water. In sea water another source of danger arises from the tendency of the lime to pass into the state of the sulphate of that base. Some curious illustrations of the length of time during which large bodies of slaked rich lime will remain in a plastic state are to be found recorded in Vicat's 'Recherches stir les Chaux." When the limestones operated upon contain from 1 to 30 per cent. of impurities, such as the oxides of iron, manganese, magnesia, and sand in the form of insoluble silica, they yield limes which do not increase in bulk when slaked (in any notable manner); and are there , fore known as the poor limes. The hydraulic limes, to be noticed
hereafter, are also poor, in so far as their expansion is concerned ; but they differ from the class usually known by that name in this respect, namely, that the poor limes, containing impurities such as are above-named, do not set under water within any reasonable time, if at all ; whereas, the hydraulic limes possess that faculty in variable degrees.
The hydraulic limes are those which are obtained from the calcina tion of limestones containing proportions of the silicate of alumina, in combination with the pure oxide of calcium, varying within the limits of from 6 to 30 per cent. of the silicate of alumina in the whole mass ; and it appears that it is upon the greater or less rapidity of formation, and of the stability of the hydrated silicate of lime and alumina thus formed by the addition of water to the caustic lime, that this class of materials owes the power of setting under water, and of resisting the solvent action of the latter, from which circumstance its name is derived. When the proportion of the silicate of alumina ranges between the limits of from 6 to 8 or 12 per cent., the lime becomes simply moderately hydraulic ; when the proportion ranges between 12 and 20 per cent., the lime is hydraulic ; and when it ranges between 20 and 30 per cent., the lime becomes eminently hydraulic ; and at last closely approaches the character of the cements. According also to the proportion of the silicate of alumina present in the limestone is the lime obtained from it more or less rich, for the moderately hydraulic limes swell nearly as much as the rich limes in slaking, and with nearly equal rapidity and evolution of heat ; whereas the hydraulic limes only increase from 1 to 2i of their original volume, they slake slowly, and give out comparatively little heat. In contradistinction to the rich limes, however, they set rapidly in the open air, and permanently under water ; for which reasons they alone should be used for exposed or submerged walls. Artificial hydraulic limes are made by the addition of clay; containing soluble silicate of alumina, to the pure carbonate of lime, and by calcining those mate rials together ; or it is made by the addition of pounded and rather under-burnt brick or tile-dust to the mortar of rich limes. The arti ficial hydraulic limes obtained by the simultaneous calcination of the limo and clay are, it may be observed, much more fitted to resist the solvent action of running waters, than those in which no pyrogenous combination takes place between those elements.