PROPORTIONING THZ MORTAR. Lime mortar consists of a mixture of lime paste and sand. The requisites for good sand for mortar making have been considered in Art. 1, Chapter V.
There are four reasons for adding sand to the lime paste: (1) to divide the paste into thin films and make the mortar more porous, thus allowing the penetration of the air and facilitating the absorption of the carbonic acid which causes the setting of the mortar; (2) to prevent excessive cracking of the mortar owing to shrinkage due to the evaporation of the water in the lime paste; (3) to give greater strength to the mortar against crushing (practically the only stress that comes upon mortar), since sand has a greater resistance to compression than lime paste either before or after it has set; and (4) to reduce the amount of lime necessary to make a given bulk of mortar, thus decreasing the cost.
Since the paste sets or hardens very slowly, even in the open air, unless it be subdivided into small particles or thin films, it is im portant that the volume of the paste should be but slightly in excess of what is sufficient to coat all the grains of the sand and to fill the voids between them. If either more or less sand than this is used,
the mortar will be injured. An excess of lime paste will prevent the mortar from setting properly, and will cause it to shrink unduly; while a deficiency will make the mortar porous and weak. An excess of lime paste also decreases the compressive resistance of the mortar. With most sands the proper proportion will be from 2.5 to 3 volumes of sand to 1 volume of lime paste.
In proportioning cement mortar it is necessary to measure the sand and the cement to get the proper relation; but with lime mortar the proper proportion of sand and paste can be determined from the way the mortar behaves during the mixing, as will be explained in the next section.