Post by DavidWhat added to plaster of paris will make it stronger? Even a tad? Metal
powders? Shredded rubber? Epoxy mixed in? Aluminium or steel mesh? Special
coatings? Perhaps the finish is the key? Perhaps heating or some sort of
heat treatment? Perhaps using something other than plaster of paris is best
but just a curiosity.
The most common additive is 'concrete binder" an acrylic latex solution,
cheap, and available from concrete yards. It also helps the fibers stick
within the plaster - Forton, Milestone and Winterstone are all variations of
this approach. Newsgroup members should also know that 'water base epoxies'
are starting to become common in Canada and USA - they offer superior
strength, but are mostly added to the "face mix" because of cost, and
usually in concrete more than plaster. Old school is alum solution, aluminum
sulphate and water, rubbed or soaked into the surface - there are many, many
old things come to think of it - soaking in milk was another - Let me know
if you want more. They sort of went out when the high strength plasters were
introduced, but maybe should come back with the high price of plaster these
days (natural gas price and product liability insurance related) .
When we are talking strength, we mean 'tensile' strength - the ability to
take a shock, 'flexural' strength - the ability to recover from a bend, or
'compressive' strength, the ability to take a sharp hammer. Plaster, out of
the $2 a lb bag, is less than 2,000 psi compressive, and the $7.50 a lb bag,
hydrostone, the 'strongest' plaster (which also contains portland cement)
will give you 8,000 psi. If you are in North America, also check out the
Georgia Pacific line of gypsum based plasters, which are more varied and
often better priced - especially "Densite". These prices are local Canada
retail by the way.
Plaster is still a great all round material and especially nice that it
cures fast and doesn't shrink. There is a whole mysterious world of exotic
cements that are much, much higher performance than plaster or portland but
information and materials are hard to come by, and expensive (or simply
unavailable) in less than ton quantities.
I almost forgot, heat curing or more porperly 'post curing' can make a
dramatic difference in the strength and reliability of casts - we are
talking maybe 100F temperature, for 8 hours, with good moving air and a
dehudifier. This is especially important with long casts, which will warp
while still 'green' - even though they may feel dry to touch.