Additives stabilize calcium sulfate hemihydrate (bassanite) in solution

Recent work on the precipitation of the mineral gypsum (calcium sulfate dihydrate) has shown that contrary to long-standing opinion, this phase can precipitate from solution at room temperature via a bassanite (calcium sulfate hemihydrate, or plaster of Paris) intermediate phase. An amorphous calcium sulfate (ACS) phase can also precede precipitation of hemihydrate. Here, we profit from these observations to generate ACS and hemihydrate particles that have considerable stability in solution. By precipitating calcium sulfate in the presence of poly(acrylic acid), poly(styrene-4-sulfonate), sodium triphosphate and magnesium ions, we show that it is possible to use these additives to retard the transformation of these metastable mineral phases, and thereby readily isolate hemihydrate. These additives are also active in controlling the morphologies of the hemihydrate crystals, which can play a key role in defining properties such as porosity and mechanical strength. The results confirm the stepwise-precipitation of gypsum via amorphous and hemihydrate intermediates and suggest an alternative to the energy-intensive calcination processes which are currently widely used to prepare hemihydrate.