(plaster of Paris; Keene's cement; Parian cement; Martin's cement; Mack's cement). A group of cements that consist essentially of calcium sulfate and are produced by the partial dehydration of gypsum to the hemihydrate CaSO4•1/2H2O. They usually contain additions of various sorts. For example, Keene's cement contains alum or aluminum sulfate; Mack's cement contains sodium or potassium sulfate; Martin's cement contains potassium carbonate; and Parian cement contains borax.