Type N One part portland cement, one part
hydrated lime or lime putty, and six parts sand;
or, one part type II masonry cement and three
parts sand. Type N mortar is suitable for
general use in above-grade exposed masonry
where high compressive or lateral strength is
Type O One part portland cement, two parts
hydrated lime or lime putty, and nine parts
sand; or, one part type I or type II masonry
cement and three parts sand. Type O mortar is
recommended for load-bearing, solid-unit
walls when the compressive stresses do not
exceed 100 pounds per square inch (psi) and
the masonry is not subject to freezing and
thawing in the presence of a lot of moisture.
The manner in which mortar is mixed has a lot to
do with the quality of the final product. In addition to
machine and hand mixing, you need to know the
requirements for introducing various additives,
including water, to the mix in order to achieve
Machine mixing refers to mixing large quantities
of mortar in a drum-type mixer.
Place all dry
ingredients in the mixer first and mix them for
1 minute before adding the water. When adding
water, you should always add it slowly. Minimum
mixing time is 3 minutes. The mortar should be
mixed until a completely uniform mixture is obtained.
Hand mixing involves mixing small amounts of
mortar by hand in a mortar box or wheelbarrow. Take
care to mix all ingredients thoroughly to obtain a
uniform mixture. As in machine mixing, mix all dry
materials together first before adding water. Keep a
steel drum of water close at hand to use as the water
supply. You should also keep all your masonry tools
free of hardened mortar mix and dirt by immersing
them in water when not in use.
You occasionally need to mix lime putty with
mortar. When machine mixing, use a pail to measure
the lime putty. Place the putty on top of the sand.
When hand mixing, add the sand to the lime putty.
Wet pails before filling them with mortar and clean
them immediately after emptying.
Mixing water for mortar must meet the same
quality requirements as mixing water for concrete.
Do not use water containing large amounts of
dissolved salts. Salts weaken the mortars.
You can restore the workability of any mortar that
stiffens on the mortar board due to evaporation by
remixing it thoroughly. Add water as necessary, but
discard any mortar stiffened by initial setting.
Because it is difficult to determine the cause of
stiffening, a practical guide is to use mortar within
2 1/2 hours after the original mixing. Discard any
mortar you do not use within this time.
Do not use an antifreeze admixture to lower the
freezing pint of mortars during winter construction.
The quantity necessary to lower the freezing point to
any appreciable degree is so large it will seriously
impair the strength and other desirable properties of
Do not add more than 2-percent calcium chloride
(an accelerator) by weight of cement to mortar to
accelerate its hardening rate and increase its early
strength. Do not add more than 1-percent calcium
chloride to masonry cements. Make a trial mix to find
the percentage of calcium chloride that gives the
desired hardening rate. Calcium chloride should not
be used for steel-reinforced masonry. You can also
obtain high early strength in mortars with
high-early-strength portland cement.
Concrete masonry walls should be laid out to make
maximum use of full- and half-length units. This
minimizes cutting and fitting of units on the job. Length
and height of walls, width and height of openings, and
wall areas between doors, windows, and corners should
be planned to use full-size and half-size units, which are
usually available (figure 8-6). This procedure assumes
that window and door frames are of modular dimensions
which fit modular full- and half-size units. Then, all
horizontal dimensions should be in multiples of nominal
full-length masonry units.
Both horizontal and vertical dimensions should be
designed to be in multiples of 8 inches. Table 8-2 lists
nominal length of concrete masonry walls by
stretchers. Table 8-3 lists nominal height of concrete
masonry walls by courses. When 8-by-4-by-16 units
are used, the horizontal dimensions should be planned
in multiples of 8 inches (half-length units) and the
vertical dimensions in multiples of 4 inches. If the
thickness of the wall is greater or less than the length
of a half unit, a special-length unit is required at each