We have investigated the solubility behavior of calcium sulfate dihydrate (gypsum, CaSO(4)2H(2)O) in aqueous NaCl solutions upon the addition of ammonium- or imidazolium-based ionic liquids (ILs), viz., ethylammonium lactate (EAL), 1-ethyl-3-methyl imidazolium hydrogen sulfate ([C(2)mim]HSO4), and 1-butyl-3-methyl imidazolium hydrogen sulfate ([C(4)mim]HSO4) at 30 degrees C. The addition of ammonium lactate ILs increased solubility significantly, similar to 500%, whereas the addition of imidazolium hydrogen sulfate ILs decreased the solubility by similar to 60%. The addition of ILs shifted the solubility maximum of CaSO(4)2H(2)O toward either higher or lower concentration of NaCl in solution depending on the nature of IL additive. Solution properties such as density (rho) and speed of sound (u) have been measured for the quaternary systems (CaSO(4)2H(2)O + NaCl + ILs + H2O) at 30 degrees C. Solution isentropic compressibility (kappa(S)) has been derived from measurements of u and rho. Physical and derived properties have been fitted with suitable polynomial equations. Crystal growth formation and surface morphology of CaSO(4)2H(2)O crystals recrystallized from different aqueous IL solutions have also been examined. IL additives have shown good potential for calcium sulfate scale removal and as calcium sulfate precipitation agent.