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White plaster is a combination of white marble sand or
dust (calcium carbonate), white portland cement and
water. As the plaster cures, referred to as hyrdration,
calcium hydroxide (lime) is formed as a by-product. This
calcium is in the “cream” or the surface of the finish, the
plaster dust. This is the softest and most vulnerable
element of the finish as it is highly reactive to the water
chemistry, soluble in nature and takes months to fully
cure. Today there are a variety of specialty finishes
available beyond the traditional white plaster or marcite mentioned previously. These finishes
use unique additives and materials mixed into a plaster application to offer a variety of color
and material options as well as increased strength and chemical resistance. Polymers and
Pozzolans, Ceramic, Quartz and exposed aggregates are a few of these innovative admixtures
that enhance, protect or replace the weakest link in traditional plaster, the calcium. In doing so
this greatly reduces the threat of staining, etching, mottling, checking, discoloration and other
concerns typically associated with white plaster.
- as well as silicones, protect the finish during the hydration (curing) process.
They increase hardness, improve bonding and reduce water penetration.
- are used to convert the calcium hydroxide in the plaster into a less soluble cement
compound for a more durable finish.
Adding one or more of these components may be done to enhance a traditional plaster finish
or they may be combined with quartz, ceramic, or pebbles to create one of the many specialty
exposed aggregate finishes described on the following pages.
- As mentioned earlier, traditional white plaster uses a marble sand or dust as the
aggregate. If you remember back to chemistry class and Moh’s table of hardness- (running on
a scale of 1-10; talc being a 1 and diamonds a 10) marble is a 3 and quartz a 7. So, it’s easy
to understand how adding a quartz or stone aggregate would result in a much harder surface
than a marble aggregate plaster. Quartz aggregate, unlike marble, is completely insoluble,
even in the presence of highly corrosive acid, so plasters using quartz are less affected by
fluctuations in the water chemistry.
This mix is a blend of choice quartz (sand, silica) along
with quartz that is colored and fired in a kiln. After the material has set to a particular hardness,
it is washed and brushed with water to expose the aggregate. For further exposure, acid
washing is done.
- New products have taken quartz aggregate and bonded with an exterior ceramic
coating. This coating – which contains inorganic pigments – provides a wide range of colors.
Color crystals are added to more traditional plaster mixes by replacing a portion of the marble
aggregate. Reducing the marble and consequently the calcium, and adding this highly durable
compound not only provides many color options it provides a longer lasting more durable finish
with higher resistance to chemical balance and mechanical abrasion.
All plaster finishes are subject to mottling, and shade variations
or discoloration. Colored plasters are even more likely to exhibit these traits.
Uneven shades
of color, streaking and pigment stains are not uncommon or considered a defect in colored
plaster pools, the darker the color the more this holds true. Also, variations of shade will exist
between color sample chips and mixed plaster and the color may fade completely or grow
gradually lighter over time. Once again, the best defense is closely following the start-up
procedures, chemical balance and care instructions for your particular pool finish.