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The water loss and chloride release from four different formulations of magnesium oxychloride cement have been studied. The formulations were all based on 25% m/m aqueous MgCl2 and the solutions employed to prepare cements consisted of either MgCl2:water or MgCl2:aqueous MCl (M = Li, Na or K, all MCl salts at 1 mol dm-3). Solutions were mixed with solid MgO at a ratio of 1:1 (m/v) and allowed to harden at room temperature for 1 hour. Five disc-shaped specimens (6 mm diameter × 2 mm thickness) of each were prepared, stored in a desiccating atmosphere and weighed at regular intervals. All were found to lose water rapidly, equilibrating within three hours. Water loss was Fickian to values of Mt/M∞ of around 0.7 in all cases. Diffusion coefficients were slightly smaller than for the additive-free cement (1.52 × 10-6 cm2/s) and fell in the range 1.27-1.39 × 10-6 cm2/s. Equilibrium water losses varied from 23.1% (with NaCl) to 20.0% (with KCl), but only with NaCl did the value differ significantly from that for the additive-free cement (20.7%). Chloride release was increased by the presence of the additives. It followed a consistent pattern, with a maximum generally at about 5 hours, followed by a reduction up to 2 weeks (336 hours). This shows that a proportion of the released chloride was taken back up as the cement matured.