Fine sand requires more water than coarse to give the same con sistency. Dry sand will take more water than that which is moist, and sand composed of porous material more than that which is hard. As the proportion of sand to cement is increased the proportion of water to cement should also increase, but in a much less ratio.
The amount of water to be used is such that the mortar when thoroughly mixed shall have a plastic consistency suitable for the purpose for which it is to be used.
The addition of water, little by little, or from a hose, should not be allowed.
Cement flortar. In mixing cement mortar the cement and sand are first thoroughly mixed dry, the water then added, and the whole worked to a uniformly plastic condition.
The quality of the mortar depends largely upon the thoroughness of the mixing, the great object of which is to so thoroughly incorporate the ingredients that no two grains of sand shall lie together without an intervening layer or film of cement. To accomplish this the cement must be uniformly distributed through the sand during the dry mixing.
The mixers usually fail to thoroughly intermix the dry cement and sand, and to lighten the labor of the wet mixing they will give an overdose of water.
Packed cement when measured loose increases in volume to such an extent that a nominal 1 to 3 mortar is easily changed to an actual 1 to 4. The specifications should prescribe the manner in which the materials are to be measured, i.e., packed or loose.
The quantity of sand will also vary according to whether it is measured in a wet or dry condition, packed or loose. On work of sufficient importance to justify some sacrifice of convenience the sand and cement should be proportioned by weight instead of by volume.
In mixing by hand a platform or box should be used; the sand and cement should be spread in layers with a layer of sand at the bottom, then turned and mixed with shovels until a thorough incor poration is effected. The dry mixture should then be spread out, a bowl-like depression formed in the center and all the water required poured into it. The dry material from the outside of the basin should be thrown in until the water is taken up and then worked into a plastic condition, or the dry mixture may be shovelled to one end of the box and the water poured into the other end. The mixture of sand and cement is then drawn down with a hoe, small quantities at a time, and mixed with the water until enough has been added to make a good stiff mortar.
In order to secure proper manipulation of the materials on the part of the workmen it is usual to require that the whole mass shall be turned over a certain number of times with the shovels, both dry and wet.
The mixing wet with the shovels must be performed quickly and energetically. The paste thus made should be vigorously worked with a hoe for several minutes to insure an even mixture. The mortar should then leave the hoe clean when drawn out of it, and very little should stick to the steel.
A large quantity of cement and sand should not be mixed dry and left to stand a considerable time before using, as the moisture in the sand will to some extent act upon the cement, causing a partial setting.
Upon large works mechanical mixers are frequently employed with the advantage of at once lessening the labor of manipulating the material and insuring good work.
Retempering flortar. Masons very frequently mix mortar in considerable quantities, and if the mass becomes stiffened before being used, by the setting of the cement, add water and work it again to a soft or plastic condition. After this second tempering the cement is much less active than at first, and will remain for a longer time in a workable condition.
This practice is condemned by engineers, and is not usually allowed in good engineering construction. Only sufficient quantity of mortar should be mixed at once as may be used before the cement takes the initial set. Reject all mortar that has set before being placed in the work.
Freezing of flortar. It does not appear that common lime mortar is seriously injured by freezing, provided it remains frozen until it has fully set. The freezing retards, but does not entirely suspend the setting. Alternate freezing and thawing materially damages the strength and adhesion of lime mortar.
Although the strength of the mortar is not decreased by freezing, it is not always permissible to lay masonry during freezing weather; for example, if, in a thin wall, the mortar freezes before setting and afterwards thaws on one side only, the wall may settle injuriously.
Mortar composed of one part Portland cement and three parts sand is entirely uninjured by freezing and thawing.
Mortar made of cements of the Rosendale type, in any propor tion, is entirely ruined by freezing and thawing.
Mortar made of overclayed cement (which condition is indicated by its quicker setting), of either the Portland or Rosendale type, will not withstand the action of frost as well as one containing less clay, for since the clay absorbs an excess of water, it gives an increased effect to the action of frost.
In making cement mortar during freezing weather it is cus tomary to add salt or brine to the water with which it is mixed. The ordinary rule is: Dissolve 1 pound of salt in 18 gallons of water when the temperature is at 32° F., and add 1 ounce of salt for each degree of lower temperature.
The use of salt, and more especially of sea-water, in mortar is objectionable in exposed walls, since the accompanying salts usually produce efflorescence.
The practice of adding hot water to lime mortar during freezing weather is undesirable. 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.