[Setting characteristics of calcia-bonded investments by hydration]

Calcium oxide (CaO) slurry mixed with water hardens and expands by hydration. The authors used this setting characteristic as a binder of zirconia-refractory investments. If a mixed slurry was placed in air, unreacted residual calcia powders absorbed water and reverted to calcium hydroxide (Ca (OH)2). This resulted in a large setting expansion that continued for several days. A carbon dioxide (CO2) gas atmosphere controlled this expansion of calcia. When mixed slurry was placed in a carbon dioxide gas atmosphere, calcium carbonate (CaCO3) was formed and prevented the further hydration of unreacted calcium oxide. Thus setting expansion values were controlled. The combined expansion of setting and thermal expansion, in CaO-CO2-bonded zirconia investments, was about 1.5-2.5%. On the whole, calcium oxide is expected to be a good binder for high temperature dental casting refractory.