Page 121 - Masonry and Concrete

Understanding Masonry
120 CHAPTER FOUR

produce some effects, they can also reduce compressive strength,
impair bond, contribute to efflorescence, increase shrinkage, or cor-
rode metal accessories and reinforcing steel.
Set accelerators, often mistakenly referred to as “antifreeze” com-
pounds, are sometimes used in winter construction to speed cement
hydration, shorten setting time, increase early strength development,
and reduce the time required for cold-weather protection. Calcium
chloride accelerators cause corrosion of embedded steel anchors and
reinforcement. Nonchloride accelerators are a little more expensive
but less damaging to the masonry. Chlorides should not be used in
mortar or grout which contains embedded metals such as anchors,
ties, or joint reinforcement. Automotive antifreeze should never be
used in masonry mortar or grout.
Set retarders extend the board life of fresh mortar and grout for as
long as four to five hours by helping to retain water for longer periods
of time. Set retarders are sometimes used during hot weather to coun-
teract the effects of rapid set and high evaporation rates. With soft, dry
brick or block, set retarders are also sometimes used to counteract
rapid suction and help achieve better bond. Mortar with set retarders
cannot be retempered.
Integral water repellents reduce the water absorption of hardened
mortar by as much as 60%. They must be used in mortar for concrete
masonry units that have also been treated with an integral water repel-
lent. Using water-repellent-treated masonry units with untreated mor-
tar, or vice versa, can reduce mortar-to-unit bond and the flexural
strength of the wall. Reduced bond also allows moisture to penetrate
the wall freely at the joint interfaces, so the intended moisture resis-
tance of the water repellent treatment is negated. To achieve good bond
when using treated concrete block, the block manufacturer should sup-
ply a chemically compatible admixture for use in the mortar.
The most commonly used admixtures are natural and synthetic
iron oxide pigments used to produce colored mortar. Iron oxides are
nontoxic, colorfast, chemically stable, and resistant to ultraviolet radi-
ation. Iron oxides come in yellows, reds, browns, and blacks. Carbon
black and lampblack (used to make blacks and browns) are less
weather resistant than the iron oxides used to make the same colors.
Synthetic iron oxides have more tinting power than natural oxides, so
less pigment is required to produce a given color. Synthetic oxides

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