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.

Again, unusually strong mortar is required sometimes for the voussoirs of arches - to prevent sliding - for the lower joints of chimneys and walls, etc. 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.

In considering the proportion of sand to be mixed with different limes and cements it is necessary to bear in mind that the strength of the joint formed by the mortar will have an influence upon that of the wall. The proportion of the ingredients in mortar is generally specified thus: - 1 quicklime to 2 (or more) of sand, meaning that 1 measure of quicklime in lump is to be mixed with 2 measures (or more) of sand. The quantities of sand put at different times into a measure vary a little, according to the amount of moisture the material contains; but so little that practically it makes no difference, and this mode of measuring sand is very convenient and sufficiently accurate. With the lime, however, many conditions have to be fulfilled in order to make it certain that the same quantity always fills the same measure. The specific gravity of the calcined stone, the size of the lumps, the nature of the burning, the freshness of the lime, all cause the actual quantity contained in a given measure to differ considerably. In order to avoid this uncertainty, it has been proposed that the weight of lime for a given quantity of sand should be specified.

Practically, however, this has not been carried out to any great extent, and the bulk of lime to be used is generally specified as well as that of the sand. The following proportions are given by General Scott for mortar in brickwork built with ordinary London stock bricks.

Parts by Quicklime.

Measure. Sand.

Fat limes..........

1

3

Feebly hydraulic limes.........

1

2 1/2

Hydraulic limes (such as Lias)........

1

2

Roman cement..........

1

1 or l 1/2

Medina.............

1

2

Atkinson's...............

1

2

Portland...............

1

5

Scott's................

1

4

The proportions here recommended apply only to works above the surface of the ground, or free from the action of a body of water. For hydraulic purposes and foundations, 1 sand to 1 quicklime is as much as should be admitted. With cement mortar, 2 sand may be used with 1 cement, unless actually in contact with water, when 1 part of sand should be the limit allowed.

The quicklime and sand having been procured, and their proportions decided, the preparation of the ingredients commences. A convenient quantity of the quicklime is measured out on to a wooden or stone floor under cover, and water enough to slake it is sprinkled over it. The heap of lime is then covered over with the exact quantity of sand required to be mixed with the mortar; this keeps in the heat and moisture, and renders the slaking more rapid and thorough. In a short time - varying according to the nature of the lime - it will be thoroughly slaked to a dry powder. In nearly all limes, however, there will be found over-burnt refractory particles, and these should be carefully removed by screening - especially in the case of hydraulic limes; for if they get into the mortar and are used, they may slake at some future time, and by their expansion destroy the work. The fat limes may be slaked in any convenient quantity, whether required for immediate use or not. Plenty of water may be used in slaking without fear of injuring them, and they will be found ready for use in 2 or 3 hours.

Hydraulic limes should be left (after being wetted and covered up) for a period varying from 12 to 48 hours, according to the extent of the hydraulic properties they possess; the greater these are, the longer they will be in slaking. Care should be taken not to use too much water, as it absorbs the heat and checks the slaking process. Only so much should be slaked at once as can be worked off within the next 8 or 10 days. With strong hydraulic limes, or with others that are known to contain over-burnt particles, it is advisable to slake the lime separately, and to screen out all dangerous lumps, etc, before adding the sand; or the safest plan is to have the lime ground before using it. When lime is purchased ready ground, there is sometimes danger of its having become "air-slaked," by which, wear and tear of machinery in grinding is saved at the expense of loss of energy on the part of the lime. At the same time, if unadulterated and fresh, ground lime is likely to be of good quality. The quantity of water required for slaking varies with the pureness and freshness of the lime, and is generally between 1/3 and 1/2 of its bulk. A pure lime requires more water than one with hydraulic properties, as it evolves more heat and expands more in slaking.

A recently-burnt lime requires more water than one that has been allowed to get stale.

The great object in mixing is to thoroughly incorporate the ingredients, so that no 2 grains of dry sand should lie together without an intervening layer or film of lime or cement. On extensive works, a mortar-mill is universally adopted for mixing the ingredients, and, indeed, is absolutely necessary for the intimate incorporation of large quantities. The heap of slaked lime covered with sand is roughly turned over and shovelled into the revolving pan of the mortar-mill, enough water being added to bring the mixture to the consistency of thick honey. When the ingredients are thoroughly mixed and ground together, the mortar is shovelled out of the pan on to a" banker " or platform to keep it from the dirty ground, whence it is taken away by the labourers in their hods. A good deal has been said regarding the number of revolutions that should be given to the pan. Nothing seems to have been settled upon this point except that the mortar should be thoroughly mixed, yet not kept so long in the mill as to bo ground to pap. On very small works the mixing is effected by hand or in a pug-mill. It is evident, however, that such a mixture must be very incomplete unless a great deal of time is devoted to it.

Before hydraulic lime is mixed in this manner it is absolutely necessary that it should first be ground to a fine powder, and with any description of lime the smallest refractory unslaked particles should be carefully screened out. Mortar, especially when made with cement, is sometimes mixed dry, the ingredients being carefully turned over together 2 or 3 times before the water is added. By this process a very thorough incorporation of the materials can be effected, but in many cases it would involve a separate grinding of the lime, and would be too expensive. If a hydraulic mortar is allowed to commence to set and is then disturbed, it is greatly injured. Care should be taken, therefore, to mix it only so long as is required for thorough reduction and incorporation of the ingredients, and only to prepare so much as can be used within a few hours. With fat limes it matters little whether large or small quantities of mortar are made at once, because they set very slowly. Very quick-setting cements must be used immediately they are mixed. The bulk of mortar produced in proportion to that of the ingredients differs greatly according to the nature of the lime or cement and the quantity and description of the sand added to it.

The more hydraulic limes produce a smaller quantity of mortar because they expand less in slaking.