Ceramics-Silikáty 51 (2) 69-73 (2007)

Water transport in zinc oxy-chloride cements

J. W. Nicholson

The water uptake and water loss behaviour in three different formulations of zinc oxy-chloride cement have been studied in detail. Specimens of each material were subjected to a high humidity atmosphere (93% RH) over saturated aqueous sodium sulfate, and a low humidity desiccating atmosphere over concentrated sulfuric acid. In high humidity, the cement formulated from the nominal 75% ZnCl₂ solutions gained mass, eventually becoming too sticky to weigh further. The specimens at 25% and 50% ZnCl₂ by contrast lost mass by a diffusion process, though by 1 week the 50% cement had stated to gain mass and was also too sticky to weigh. In low humidity, all three cements lost mass, again by a diffusion process. Both water gain and water loss followed Fick's law for a considerable time. In the case of water loss under desiccating conditions, this corresponded to values of Mt/MĄ well above 0.5. However, plots did not go through the origin, showing that there was an induction period before true diffusion began. Diffusion coefficients varied from 1.56 x 10^-5 (75% ZnCl₂) to 2.75 x 10^-5 cm²/s (50% ZnCl₂), and appeared to be influenced not simply by composition. The drying of the 25% and 50% ZnCl₂ cements in high humidity conditions occurred at a much lower rate, with a value of D of 2.5 x 10^-8 cm²/s for the 25% ZnCl₂ cement. This cement was found to equilibrate slowly, but total water loss did not differ significantly from that of the cements stored under desiccating conditions. Equilibration times for water loss in desiccating conditions were of the order of 2-4 hours, depending on ZnCl₂ content; equilibrium water losses were respectively 28.8 [25% ZnCl₂], 16.2 [50%] and 12.4 [75%] which followed the order of ZnCl₂ content. It is concluded that the water transport processes are strongly influenced by the ZnCl₂ content of the cement.

Keywords: Zinc oxy-chloride cements, Water sorption, Water loss, Diffusion coefficient