This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
In mixing lime mortar, a bed of sand is first made in a mortar box, and the lime is distributed as evenly as possible over it, both lime and sand being previously measured. The lime should be slaked by pouring on from 1 1/2 to 2 times as much water, and covered with a layer of sand, or preferably, a tarpaulin, to retain the vapor given off. Sufficient water should be used at the start; if more must be added later, it chills the hot lime and makes it lumpy. Too much water makes the paste thin, weak, and slow in drying. Additional sand is then added, if necessary, until the mortar contains the proper proportion, which is usually 3 of sand to 1 of lime, although 2-to-l is much better. The bulk of the mortar will be about 1/8 greater than that of the dry sand alone, so that 20 cu. ft. or 16 bu. of sand, and 4 cu. ft. or 3.2 bu. of quicklime, will make about 22 1/2 cu. ft. of mortar.
For cement-and-lime mortar, the materials should be well mixed together before water is added, and the mortar should be used before the cement sets.
A good method of mixing cement mortar is as follows: One-half the quantity of sand is first spread over the bottom of the mortar box; next, the cement is spread evenly over the sand; and the remainder of the sand is then put on. The dry materials are thoroughly mixed together, either by hoe or by shovel; water is then added to so much of the mass as is required for immediate use, and this portion is mixed until it has the uniform consistency of a stiff paste. The quantity of water required depends upon the cement used, but it is better to have an excess than a deficiency. Owing to the rapidity of setting, only small lots should be mixed at a time.
In winter, a small proportion of lime is sometimes mixed with the cement, the heat generated in slaking being supposed to prevent freezing until the cement has set. Salt is often added for the same purpose, the quantity being about 1 lb. of salt to 18 gal. of water, an additional ounce being added for each degree of temperature below 32° F. Salt is objectionable, however, as it causes efflorescence.