Slaking Tko Lime

SLAKING TKO LIME. Many persons seem to believe that the slaking of lime is such a simple process that any one can do it; but a little care and attention to the principles involved may materially increase the amount of paste obtained, and hence decrease the amount of lime required. Further, if the ':me is not completely slaked before being used, the swelling due to the subsequent hydra tion of the unslaked portion may damage and possibly destroy the structure in which the unslaked lime is used.

Hydrated lime (§ 107) is lime that has been slaked by the manu facturer. It is sold in the form of a dry powder, and is ready for mixing with the sand and water required to make the mortar.

There are three methods of slaking lime on the work, vii.: (1) drowning, (2) sprinkling, and (3) air slaking.

Slaking by Drowning.

The ordinary method consists in placing the lumps in a layer 6 or 8 inches deep in either a water-tight box or a basin formed in the sand to be used in mixing the mortar, and pouring upon the lumps a quantity of water 2i to 3 times the volume of the lime. If the quantity of water added is just right, the lime will be reduced to a thick paste; but if too much water is used, the lime will be reduced to a semiliquid condition and a considerable part of its binding quality will be destroyed. This method takes its name from the tendency to use an excessive amount of water, i.e., of drowning or killing the lime.

With a high-calcium or quick-slaking lime (I 105) the best results are obtained when all the water is added at once; but with a magnesian or slow-slaking lime (1 105) only a little water should be added at first, and then after the lime and water are hot, more water may be added gradually so as not to chill the mixture and retard the slaking. The slaking proceeds more rapidly and is more complete if the mass is hot. The lime absorbs the water, and the chemical action generates heat enough to change part of the absorbed water into steam which bursts the lumps of lime apart and thus exposes new surfaces to the action of the water; but if cold water is added after the slaking has begun, it chills the mass, prevents the formation of steam and the consequent bursting of the lumps, and hence the slaking is not complete, and the amount of paste formed is less than it should be. Further, when the slaking has been thus retarded, a thin paste forms on the outside of the fragments of the unslaked lime, which excludes the water from the interior or un slaked portion of the lump; and hence it is difficult, if not impossi ble, to thoroughly slake lime that has been chilled in the slaking. Partial air slaking is harmful in much the same way, since the slaked lime on the outside of a lump prevents the free access of the water to its interior.

Stirring the lime while slaking chills the mass and thereby retards the slaking; but, on the other hand, stirring breaks up the friabl e lumps and thereby aids the slaking. Therefore if the mass is stirred at all, the stirring should be done in such a manner as to cool the mass as little as possible. The swelling of the lime in the lower portion of the mass frequently lifts some of the lumps out of the water, the heat in the lump causes a column of steam to rise from it, and the lump is said to "burn." This "burning" is detrimental, since a film of slaked lime is formed on the surface of the unslaked portion which tends to prevent complete slaking. Therefore it is important that lumps which are "burning" should be pushed back into contact with the water. "Burning" can be prevented by covering the box with boards or a tarpaulin to retain the heat and the moisture.

There are two reasons why care should be taken to secure com plete hydration of the lime. 1. Imperfect slaking is uneconomical. With reasonable care the high-calcium limes will give a volume of paste equal to three or more times the volume of the unslaked lime; while unskilful slaking may reduce the paste to less than two vol umes. 2. The unslaked particles may do no harm if the lime is used in mortar for masonry; but if used for plastering, particularly for the white coat, they may slake after the mortar is on the wall and cause a portion of the surface to flake out.

Slaking by Sprinkling.

This method consists in forming the unslaked lime into a heap of convenient size, sprinkling it with water equal to one quarter to one third of its volume, covering with the sand to be used in the mortar, and allowing it to stand for at least a day or two. When the slaking is completed, the lime is in the form of a powder. This method was formerly very common in this country, but was abandoned because of the extra care and labor required. It is said to be in common use now in Europe.

Air Slaking.

Lime slakes spontaneously when exposed to the air by absorbing moisture from the atmosphere; but this is not a practicable method owing to the immense storage area, the long time, and the frequent stirring required. Lime that is thoroughly air slaked is as good, or even better, than that slaked in the usual way— a popular prejudice to the contrary notwithstanding. However, lime that is only partially air-slaked is undesirable, since it is more difficult to slake by the ordinary process than lime that is not partially air-slaked (see 4 223).