The thing in calcium carbonate that does the buffering is the
carbonate, not the calcium. Don't know anything about plaster of
paris, but calcium sulfate is different from calcium carbonate.

Carbonate exists in water as an equilibrium between carbonate ion
(CO2, with a charge of -2) and bicarbonate ion (HCO2, with a charge of
-1). It has the ability to pick up or drop a hydrogen ion (H+). It's
the amount of free hydrogen ion that determines pH,

I'm not a chemist, but maybe the sulfate doesn't have the same
buffering capacity as carbonate.

Joan
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Calcium carbonate, in the
form of calcium sulfate, increases the KH when introduced into the
water at a pH of 7.8 . So how does increased levels of calcium
carbonate cause wild pH swings?

Regards,

Hal