Any of the substances mentioned at page 195 may be used as substitutes for sand in mortar, some of them with advantage, as there pointed out.
Salt Water is objectionable in some situations, as it causes damp and efflorescence.
Dirty water, and water containing organic matter, are of course objectionable for the same reasons as dirty sand.
Mr. Dyce Cay gives a table of experiments made with. 8 oz. fresh water to 36 oz. neat Portland cement, and 7 oz. sea water to 36 oz. Portland cement, which seems to show, as far as experiments with neat cement could show it, that "roughly speaking the salt water briquettes are as strong in a week as the fresh water ones are in a fortnight, and as strong in a fortnight as the fresh water ones are in a month." 1
Lime is much more expensive than sand. It is, therefore, a source of economy to add as much sand as is possible without unduly deteriorating the strength of the mortar.
So long as the joints of masonry or brickwork are weaker than the stones or bricks, the strength of the wall will increase in proportion as the strength of the mortar increases, until they are nearly equal in power of resistance.
The mortar need not be quite equal in strength to the bricks, because in a bonded wall the fracture is constrained to follow a longer path than when the work is put together without breaking joint.
The object, then, is to produce such an equality of resistance as will compel the fracture to follow a straight line, i.e. to break the material of the wall straight across rather than to follow the joints.
This cannot always be done, with a due regard to economy, where the wall is built with very hard stone, but it can be done with the generality of bricks.
In some cases a stronger mortar, no doubt, adds to the strength of the wall. For example, when the bricks are very bad, they will sometimes weather out on the face, leaving a honeycomb of mortar joints.
As a rule, however, it can hardly be economical to make the strength of the mortar joints greater than that of the bricks or stones they unite.