I've researched plenty, but cannot find this answer. If Diatomaceous Earth is 85% Sodium Silicate, could it be added to plaster of paris, with water, to form a hard surface, with the benefit of the diatomaceous earth to act as an anti mold agent, if making one's on coaster?
Diatomaceous earth is 80-90% silica (SiO2), not sodium silicate Na2SiO3 (or polymers).
Diatomaceous earth could provide strength gain for portland cement concrete (if it doesn't increase the water content), but is an inert filler for plaster. It might improve the properties you want in the final product, but I am unaware of its use for improving specific characteristics.
There are better fillers for plaster and concrete.