A controlled process for precipitating calcium carbonate

A process for converting gypsum into precipitated calcium carbonate, comprising:

• providing a mixture comprising i) gypsum ii) a seed, a mineral acid, or both iii) at least one additive selected from the group consisting of ammonium sulfate, an organic acid, or an iron material;
• reacting the mixture with at least one carbonate source to produce precipitated calcium carbonate in the form of vaterite;
• wherein the reactants and the seed, the mineral acid, and/or the additive control the particle size of the vaterite.

19-28. (canceled)

A process for converting gypsum into precipitated calcium carbonate, comprising:

• providing a mixture comprising i) gypsum and ii) a seed, a mineral acid, or both;
• reacting the mixture with at least one carbonate source to produce precipitated calcium carbonate in the form of vaterite, such that the pH of the mixture is less than or equal to 8;
• wherein the reactants, reaction conditions and the seed and/or the mineral acid control the particle size of the vaterite.

30-38. (canceled)