Nuclear Magnetic Resonance in Hemihydrate Gypsum

The proton spin-lattice relaxation times T1 and T1D along the static and dipolar magnetic fields have been measured at 18 MHz for the water molecule in powdered hemihydrate gypsum CaSO4·(1/2)H2O. The minima of T1 and T1D were observed at 103⁄T=4.3(233 K) and 7.0(143 K) respectively. These are ascribed to the 180° flip motion of water molecules, with an activation energy of 15.1 kJ/mol. At higher temperatures, the translational diffusion of water molecules becomes dominant; this is not observed in gypsum CaSO4·2H2O. The activation energy of this motion is estimated to be 32.2 kJ/mol. In hemihydrate gypsum, though the diffusion is supposed to be anisotropic, the jumping distance of the water molecule is tentatively examined by Torrey’s lattice diffusion theory.