DETERIORATION OF PHOSPHATE-BONDED INVESTMENT ON EXPOSURE TO 100% RELATIVE HUMIDITY ATMOSPHERE

Changes in the powdery composition and setting characteristics of the phosphate-bonded investment (Ceramigold) on exposure to 100% relative humidity atmosphere were investigated by means of Thermogravimetry-Differential Thermal Analysis (TG-DTA) and X-ray diffraction. The following results were obtained. 1. In the powder, NH4H2PO4 and MgO contained as the setting agents were converted into MgNH4PO4･6H2O through Mg(NH4)2H2(PO4)2･4H2O. 2. As compared with the result of the powder stored in the thermo-hygrostatic room (control=not exposed), the setting time of the mixture of the exposed powder with the colloidal silica solution decreased temporarily but increased after 4 days of exposure when the quantity of NH4H2PO4 in the powder became small and couldn’t be measured after 10 days of exposure. 3. The compressive strength and diametral tensile strength of the mixture at 2 hours increased slightly but decreased after 4 days or more of exposure and couldn’t be measured after 10 days of exposure. 4. The thermal expansion of the mixture at 800℃ produced little difference between the control and after 8 days of exposure. As for the setting agents the process of deterioration is similar to the setting reaction. The quantity of NH4H2PO4 in the powder seems to be related to the setting time and the strength of the set investment. The quantity of MgNH4PO4･6H2O in the powder may contribute to the extreme delay of the setting reaction