[Sidenote: Materials]
Mortar, as the bonding material between brick, must be given
careful attention as the strength of the wall depends on its
quality. Mortar is composed of sand and either lime or cement, or
both. In all cases the materials should be the best. The cement
should pass the test of the American Society for Testing Materials.
The lime should be either fresh and well-burned lump lime, slaked
so as to produce a smooth, puttylike mass without lumps, or the
prepared hydrated kind found on the market. The sand should be
sharp, clean, and free from foreign matter, and screened through a
mesh of the size to secure the proper fineness. For wide joints,
the mortar needs, instead of sand, fine gravel of the size required
to give it sufficient body.
[Sidenote: Kinds of Mortar]
In large cities the building ordinances require certain mixtures of
mortar. For various mixtures see page 104. But where there are no
legal requirements, the contractor's own judgment and experience
should guide him in the selection of the proper mix, as each one
has certain qualities which recommend it. Portland cement mortar is
stiffer and harder to work than lime mortar, but should always be
used where permanence and strength are required, as in exposed or
heavy bearing situations, or in cold weather when the mortar must
set before it freezes. Lime mortar may do for the usual work, above
the grade in dry warm climates. It should not, however, be used
for basement walls, except possibly where the soil is extremely
dry, nor for piers or points in the wall that carry heavy loads.
It is smooth and works easily, so that more brick can be laid per
day than with cement mortar, but its use should be limited to the
proper situations. Cement-lime mortar has the combined qualities
of both cement mortar and lime mortar. It has both strength and
good working qualities, and costs less than cement mortar. It is
preferred by many to cement mortar; we recommend its use in proper
situations, throughout this Manual.
[Sidenote: Mixing Cement Mortar]
Cement mortar must be mixed and used immediately, as cement sets
very quickly. On this account, it should not be made in large
quantities and, once it has obtained a partial set, should never
be re-tempered. The cement and sand for cement mortar should be
thoroughly mixed while dry, by turning the material over three or
four times with a shovel, when sufficient water to make a plastic
mass should be added, turning again several times. It is always
desirable to add a small quantity of lime, when tempering the
cement mortar, in order to make it easier to work.
[Sidenote: Mixing Lime Mortar]
Lump lime should be slaked as soon as possible after delivery on
the job. If it is allowed to stand for any length of time, it
should be stored in an air-tight box; otherwise, it may become air
slaked, reducing mostly to powder and soft, crumbly lumps. The
fresh lime is always in hard lumps and should be free from cinders.
Lime is slaked in a mortar box by adding water to the lump lime.
On adding water the lime becomes very hot, gives off vapor, and
finally burst into a powder which gradually reduces to a paste
known as lime putty. The exact amount of water necessary varies
with different limes, but in all cases, the right proportions used
in slaking must be watched carefully. An excess of water will stop
the slaking process. If not enough water is used the lime will
"burn" and lose its strength. If left overnight, the mortar box
should be covered with planks.
When slaking is completed, the lime is reduced to what is called
"putty", which should be of uniform consistency throughout,
containing no unslaked lumps. The sand should now be added, the
exact amount depending upon the quality of mortar desired, and
thoroughly mixed with the putty, and then shoveled out on wooden
planks where it should remain for at least a week before using.
[Sidenote: Hydrated lime]
Hydrated lime is in powdered form and needs no other preparation
than the addition of water. Its quality is uniform and produces
satisfactory results, even when experienced labor cannot be
obtained, and may be used at once on the job. Its uniformity and
the facility it gives in handling the mortar make it preferable to
lump lime although it costs somewhat more.
The mixing of cement-lime mortar is very simple, as all that is
needed is to add the proper amount of cement to the lime mortar,
at the time it is tempered for use, and very thoroughly mix into a
consistent mass.
[Sidenote: Tempering Mortar]
Working the mortar and adding water to bring it to proper
consistency for actual use on the job is called tempering, a
process that should be continued until the mortar slides easily off
the trowel. All white spots, which are really small lumps of lime,
should be made to disappear; otherwise, these lumps will swell and
"pop" after the mortar is laid in the wall.