USES OF LIME MORTAR. Mortar composed of common lime and sand is not fit for thick walls, because it depends upon the slow action of the atmosphere for hardening it; and, being excluded from the air by the surrounding masonry, the mortar in the interior of the mass hardens only after the lapse of years, or perhaps never.* The mortar of cement, if of good quality, sets immediately; and con tinues to harden without contact with the air. Owing to its not setting when excluded from the "air, common lime mortar should never be used for masonry construction under water, or in soil that is constantly wet; and, owing to its weakness, it is unsuitable for great strength, or that are subject to shock.
For data on the strength of lime mortar see Table 10, page 51, and Fig. 7, page 122.
The freezing of lime mortar retards the evaporation of the water, and consequently delays the combination of the lime with the carbonic gas of the atmosphere. The expansive action of the freezing water is not very serious upon lime mortar, since it hardens so slowly. Consequently lime mortar is not seriously injured by freezing, provided it remains frozen until it has set. Alter nate freezing and thawing somewhat damages its adhesive and cohe sive strength. However, even if the strength of the mortar were not materially affected by .freezing and thawing, it is not always per
missible to lay masonry during freezing weather; for example, if the mortar in a thin wall freezes before setting and afterwards thaws on one side only, the wall may settle injuriously.
When masonry is to be laid in lime mortar during freezing weather, frequently the mortar is mixed with a minimum of water and then thinned to the proper consisterlcy by adding hot water just before using. This is undesirable practice (see * 223). When the very best results are sought, the brick or stone should be warmed—enough to thaw off any ice upon the surface is sufficient—before being laid. They may be warmed either by laying them on a furnace, or by suspending them over a slow fire, or by wetting with hot water, or by blowing steam through a hose against them.
The following data are useful in making estimates: Lime weighs about 200 pounds per barrel. One barrel of lime will make about 2i barrels (0.3 cu. yd.) of stiff lime paste. One barrel of lime paste and three barrels of sand will make about three barrels (0.4 cu. yd.) of good lime mortar. One barrel of unslaked lime will make about 6.75 barrels (0.95 cu. yd.) of 1 : 3 mortar.