is that mortar should be kept above freezing
until it is hard enough to knock on.
My rule comes from old bricklayers and
plasterers. I know this is hearsay, but I
believe people who work in the trades. This is
how I learned. Usually 5 to 7 hours is
plenty of time with no accelerators to set
properly before the temperature drops below
freezing. The freezing temperature for
mortar that is setting up is about 28
degrees. Bear in mind cement and concrete
generate some heat while they are setting up.
If cement mortar is allowed to freeze before
it sets, it turns to powdery mush. One
can write his name with his finger in frozen
mortar after it thaws. Bricks and blocks
can be lifted back out of the wall.
I use heaters and tents in cold
weather, but there are times when one pushes the
material to the limit. Generally, if we put
stucco on an occupied building with the heat on,
the wall temperature is above freezing just from
heat escaping from the building. Also, cement
mortar and does generate some heat while it is
In warm climates none of this is a concern, but
here in Northern Virginia and the
Washington, DC area, cold weather
protection is a necessity.
Using Gypsum plaster as
an accelerator in cement stucco:
I use Gypsum plaster as an accelerator
but very sparingly. Plaster is
Calcium Sulfate, the ingredient in Portland
cement that forms crystals and sets up
everything else. Molding plaster is used
in pre-cast concrete blocks, known as
stones. Molding plaster as an accelerator allows
several stones to be cast in a day, instead of
one a day. Gypsum is said to weaken the
concrete. but probably not much, if at all.
Plaster should never be used in a stucco
finish coat, ever. It will cause
My preference is to use brown mortar, or
basecoat plaster with no sand. It sets slower
than gauging plaster or Molding plaster. Gauging
plaster and Molding plaster are both white
plaster of Paris, the only difference is
that Molding plaster is ground finer.
Cement mortar using plaster as an accelerator
should never be re-tempered, and should be
thrown away when it sets. My rule is to use less
accelerator than you need, instead of too much.
It is better to stand around and wait for mortar
to set up than to mess up your whole wall.
Plaster should not be used as an
accelerator for cement mortar in government
work unless it is on the approved material
A word about heaters for
in the snow.
A blower type heater works
well, directing the heat allowing work
to proceed without a tent.
Some propane heaters come with a
fan to circulate the heat.
Propane costs less than kerosene,
but it does require a lot of time and
labor to take those heavy tanks to be
One should never use diesel fuel.
It stinks so bad and it will gag you
Portland type III,
(high early), sets fast, and if it
available, is a good alternative to
accelerators. We use it to build up thick
areas like window surrounds in cold
chloride is good as an accelerator if used
no more than 2%. It was used a lot
plastering swimming pools, where two coats
and same day completion is critical for a
good bond between coats and uniformity in
appearance. The new swimming pool plasters
premixed with acrylics have made calcium
chloride obsolete, except for maybe building
up round shapes and thick areas.
I hope this is of use to somebody and
hasn't arrived to late for this year. Next
winter will be here right on schedule.