Ceramics-Silikáty 54, (4) 341 - 344 (2010)

The water loss from four different formulations of magnesium oxychloride cement and two of magnesium oxysulphate cement has been studied. Five specimens of each were stored in a desiccating atmosphere and all lost water rapidly, equilibrating within three hours. For all the magnesium chloride cements, and for one of the magnesium oxysulphate cements (25 % m/m MgSO₄) water loss was Fickian to values of M_t/M_∞ around 0.8 for the oxychloride cements and 0.6 for the oxysulphate cement. However, plots did not go through the origin, as there was an induction time of between 3.1 and 6.3 minutes before true diffusion began. Diffusion coefficients were in the range 1.23-1.84 x 10⁻⁶ cm²/s. Equilibrium water losses varied from 27.4% (10 % MgCl₂) to 6.9% (75 % MgCl₂), and from 28.1 % (10 % MgSO₄) to 11.9% (25 % MgSO₄). Water transport properties were shown to be strongly influenced by the chloride or sulphate content of the cement formulation.