We’re hearing questions about the
proper time to add
salt to newly plastered
pools that will have saltwater chlorine
generators (SWCG) installed:
Is it okay to add within the first
week of a new plaster pool being filled
with water? Or should the plaster
cure (hydrate) for 28 days before
adding salt? Does adding salt have any
negative effects on the new plaster?
To answer those questions, the
onBalance team conducted some
experiments in a laboratory to
determine what effects salt may have
on the curing and hydration process of
new plaster.
It was observed during the
experiments that when adding salt
to balanced water containing a new
fresh plaster coupon, the pH of the
water rose very quickly and much
higher than the other normal. Even
after adding acid repeatedly to lower
the pH, the pH rises again quickly
and generally above 8.5. This indicates
that calcium hydroxide (a plaster
component) is being dissolved and
removed (uniformly) from a plaster
surface when saltwater concentration
is present.
That detrimental effect causes
porosity of the plaster finish, which
weakens and ages the surface. Adding
salt at start-up also makes it more
difficult to get the pH down and
balance the pool water during the startup
period.
It was also observed that the
negative effect (increased calcium
hydroxide dissolution and pH rise)
of adding salt to new plaster pools
only lasts about two to three weeks.
That is because a very thin layer of the
new plaster coat (immediate surface)
typically becomes “carbonated”
(calcium hydroxide being converted
into calcium carbonate) over the first
two to three weeks after filling with
balanced water.
This (carbonation) conversion
creates a protective thin layer and more
durable plaster surface. It appears
that once a plaster surface has been
sufficiently and properly carbonated,
salt water (less than 5000 ppm) no
longer has the same negative effect.
Our experiment used well-made
plaster coupons, which received proper
curing, and positive LSI water.
The results obtained strongly
indicate that adding 3000 ppm of salt
has a detrimental effect on new plaster
when added at any time during the first
two to three weeks.
The bottom line: Waiting 30 days
before adding any salt is recommended
for plaster pools, including for quartz
and pebble pools. And, of course,
when adding salt to the pool, never
allow undissolved salt to lie and remain
on the plaster surface for any length of
time.
EXAMINING THE PROCESS IN
GREATER DETAIL
Let’s consider what happens in the
initial days and weeks of a new plaster
surface. When plaster is applied and a
new pool is filled, the cement portion
of the new, hardened pool plaster
contains about 20% calcium hydroxide,
the rest being primarily calcium
silicates, aluminates and carbonates.
These latter products are durable and
relatively insoluble in water.
However, calcium hydroxide,
which has a pH of about 12, is softer,
somewhat soluble, and can be
dissolved from a plaster surface even
by balanced tap water. Indeed, the
Saturation Index as a maintenance tool
to protect calcium carbonate is not
applicable to fresh new plaster (i.e.,
less than a few weeks old) because the
new plaster has a significant amount of
calcium hydroxide on the surface.
That is why the pH of the water in
a freshly filled pool typically rises as
the pool is being filled. The high pH
calcium hydroxide dissolves from the
plaster surface and enters the water,
causing the pH to rise. Fortunately,
after a few weeks, the new plaster
surface becomes carbonated (which
means calcium hydroxide has been
converted to calcium carbonate) and
the dissolution process and pH rise
are mitigated.
The calcium hydroxide that
dissolved into the pool water generally
converts into calcium carbonate.
That becomes what is often called
“plaster dust” in new plaster pools.
When undergoing a traditional start-up
process, carbonation of the surface
usually lasts about 1 1/2 to 2 weeks…
which is why new plaster dust is
generated for that long — and then
stops. At that time, the pH becomes
more stable because calcium hydroxide
is no longer being readily dissolved
into the pool water.
If in the field proper plastering
practices are not followed, producing a
lower quality finish and a more porous
surface, more time may be needed for
complete carbonation and before salt
should be added.