When cement is substituted for the lime, the mixture is called Cement Mortar.
The use of mortar in brickwork or masonry is to bind together the bricks or stones, to afford them a soft resting-place, which prevents their inequalities from bearing upon one another, and thus to cause an equal distribution of pressure over the beds. It also fills up the spaces between the bricks or stones and renders the wall weather-tight.
It is also used in concrete (see page 210) as a matrix for broken stones or other bodies to be amalgamated into one solid mass; for plastering, and other purposes.
Fat Limes should only be allowed for inferior or temporary work.
On account of their being cheap and easy to manipulate, they are often used in positions for which they are entirely unfit.
Mortar made from fat lime is not suitable for damp situations or for thick walls. In either case it remains constantly moist; when placed in positions where it is able to dry it becomes friable, and in any case is miserably weak.
Even the economy of fat lime mortar is in many cases doubtful; for walls built with it are injured by frost, require constant repointing, and perhaps before many years rebuilding.
M. Vicat says of fat limes : - "Their use ought for ever to be prohibited, at least in works of any importance."
Sir Charles Pasley adds with regard to fat lime mortar that "when wet it is a pulp or paste, and when dry it is a little better than dust."
If a pure or feebly hydraulic lime mortar is used in massive brickwork or masonry, it is only the outer edges of the joints that are affected by the carbonic acid in the air. A small portion of the exterior of the joints sets, but the mortar in the inside of the wall remains soft. The result of this is that a heavy pressure is thrown upon the outer edges of the bricks or stones, and they become "flushed," that is, chipped off. In some cases, from the same cause, the headers of brickwork are broken, so that the face of the wall becomes detached, and liable to fall away.
Again, these weak mortars retain or imbibe moisture, which, when it freezes, throws off the outer crust. Pointing is then resorted to. If this is done with the same sort of mortar, the same result ensues, and in an aggravated degree, for as the operation is repeated, the joint becomes wider. In the end it will often be found that more has been expended in patching up work done with bad mortar than would have sufficed to provide good mortar at the first.
Hydraulic Lime or Cement should, therefore, always be used in mortar for work of any importance. In subaqueous constructions it is, of course, absolutely necessary.
If there is any choice, the class of hydraulic lime used will depend upon the situation and nature of the work to be done.
For ordinary buildings, not very much exposed, slightly • hydraulic limes will suffice to form a moderately strong joint, and to withstand the weather.
For damp situations, such as foundations in moist earth, a more powerful hydraulic lime should be prepared.
For masonry under water an eminently hydraulic lime or cement mortar will be necessary. If the work be required to set very quickly, Roman cement, or a cement of that class, would be used; whereas, if quick setting be not necessary, but great ultimate strength is required, a heavy Portland cement should be adopted.
Cement is also generally used for copings, plinths, arches, and other important parts in ordinary house-building.
Sand is used in mortar to save expense and to prevent excessive shrinkage.
Ordinary sands are not in any way chemically acted upon by the lime, but are simply in a state of mechanical mixture with it; with hydraulic limes and cements the effect of sand is to weaken the mortar.
When fat lime is used, however, the porous structure, caused by the sand, enables the carbonic acid of the air to penetrate farther, and to act upon a larger portion of the joint.
It is of the utmost importance that the sand used for mortar should be perfectly clean, free from clay or other impurities which will prevent the lime from adhering to it.
Sand for this purpose should have a sharp angular grit, the grains not being rounded, their surfaces should not be polished, but rough, so that the lime may adhere to them.
It has been found that, speaking generally, the size of the grains of sand does not influence the strength of the mortar.
Mr. Mann's experiments tend, however, to show that in samples four weeks old Portland cement mortar made with fine sand was weaker than that made with coarse sand.
Very fine sand is objectionable for mortar, as it prevents the air from penetrating, which is necessary in order that the mortar may set.
Although coarse irregular-grained sand may make the best mortar, when very thin joints are used finer sand is sometimes necessary.
Calcareous sands, on the whole, give stronger mortars than siliceous ones.
Sea sand contains salts, which are apt, by attracting moisture, to cause permanent damp and efflorescence.
This moisture will effectually prevent a fat lime from setting, or rather drying, but would tend to increase the strength of a hydraulic lime or cement (see page 235).
Great care must be taken to exclude all organic animal matter from the sand, or substitutes for sand, that may be used in mortar for building or plastering the walls of dwellings, otherwise they will putrefy, and render the walls and ceilings sources of unwholesome emanations.