Ceramics-Silikáty 55 (2) 183-187 (2011)

The influence of Group I metal chlorides on water loss and chloride release from magnesium oxychloride cements

E. Cannesson, S. Manier, J. W. Nicholson

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 M_t/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.

Keywords: Magnesium oxychloride; Alkali metals; Water diffusion; Chloride release