Fresh burned lime will readily absorb moisture from a damp atmosphere, and will in time become slaked thereby losing all of its valuable qualities for making mortar. It is therefore important that great care should be taken to secure freshly burned lime and to protect it from dampness until it can be used. If the lime is purchased in casks it should be kept in a dry shed or protected by canvas, and if it is bought in bulk it should be kept in a water-tight box built for the purpose.
On no account should the superintendent permit of the use of air-slaked lime, as it is impossible to make good mortar of it.
Good lime mortar, when protected from moisture, has sufficient strength for all ordinary brickwork, except when heavily loaded, as in piers, and continues to grow harder and stronger every year. The writer has often seen instances in old walls where the lime mortar was as strong as the bricks, and where the adhesion of the mortar to the bricks was greater than the cohesion of the particles of the bricks.
A specimen of mortar, supposed to be the most ancient in existence, obtained from a buried temple on the island of Cyprus, was found to be hard and firm, and upon analysis appeared to be made of a mixture of burnt lime, sharp sand and gravel, some of the fragments being about ½ inch in diameter. The lime was almost completely carbonized.*
Lime mortar, however, attains its strength slowly, and where high buildings are built rapidly the mortar in the lower story does not have time to get sufficiently hard to sustain the weight of the upper stories, and for such work natural cement should be added to the lime mortar.