This section is from the book "Building Construction And Superintendence", by F. E. Kidder. Also available from Amazon: Building Construction And Superintendence.
The sand used in making mortar should be angular in form, of various sizes, and absolutely free from all dust, loam, clay or earthy matter, and also from large stones. It is generally necessary to pass the sand through a screen to insure the proper degree of fineness. For rough stonework a combination of coarse and fine sand makes the strongest mortar. For pressed brickwork it is necessary to use very fine sand. The architect or superintendent should carefully inspect the sand furnished for the mortar, and if he has any doubts of its cleanliness, a handful put in a tumbler will at once settle the question, as the dirt will separate and rise to the top. Another simple method of testing sand is to squeeze some of the moist sand in the hand, and, if upon opening the hand the sand is found to retain its shape, it must contain loam or clay, but if it falls down loosely it may be considered as clean. Sand containing loam or clay should be at once rejected and ordered from the premises. As a rule, it is better that the sand should be too coarse rather than too fine, as the coarse sand takes more lime and makes the strongest mortar. Some unscrupulous masons may attempt to use fine sandy loam in their mortar, as it takes the place of lime in making the mortar work easily; but, of course, it correspondingly weakens the mortar, and its use should never be permitted.
White and colored mortars to be used in laying face brick should be made from lime putty and finely screened sand. After the slaked lime has stood for several days the water evaporates and the lime thickens into a heavy paste, much like putty, and from which it takes its name of lime putty. By the time the putty is formed the lime is sure to be well slaked and will not then swell or "pop." Colored mortar is made by the addition of mineral colors to the white mortars. Colored mortar should never be made with freshly slaked lime, but only with lime putty at least three days old. For Mortar Colors see Section 148.
Common lime when slaked and evaporated to a paste may be kept for an indefinite time in that condition without deterioration, if protected from contact with the air so that it will not dry up. It is customary to keep the lime paste in casks or in the boxes in which it was slaked, covered over with sand, to be subsequently mixed with it in making the mortar. Clear lime putty may be kept for a long time in casks, for use in making colored mortar, only a little mortar being made up at a time.
Lime paste or mortar does not set like cement, but gradually absorbs carbonic acid from the air and becomes in time very hard; the process, however, requires from six months to several years, according to the thickness of the mortar and its exposure to the atmosphere. If permitted to dry too quickly it never attains its proper strength. If frozen, the process of setting is delayed and the mortar is much injured thereby. Alternate freezing and thawing will entirely destroy the strength of the mortar. Lime mortar will not harden under water, nor in continuously damp places, nor when excluded from contact with the air.