| Budget Amount *help |
¥4,200,000 (Direct Cost: ¥4,200,000)
Fiscal Year 1995: ¥1,200,000 (Direct Cost: ¥1,200,000)
Fiscal Year 1994: ¥3,000,000 (Direct Cost: ¥3,000,000)
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| Research Abstract |
Used gypsum is treated as a medical waste. We focused on the fact that gypsum follows reversible reaction, and aimed to recycle the used gypsum. According to our study, when a hardened commercial *-hemihydrate gypsum was heated to 180ﾟC for 210 minutes, and was pulverized to 106 mum of particle size to knead with water at a powder/water ratio of 0.50, the gypsum set again with some heat generation of exothermic reaction. The dry-compressive strength gained under this condition was 84.7 kgf/cm^2 as average, which was at one third or less of that of typical beta-hemihydrate gypsum.
The heating time of 210 minutes at 180ﾟC is too long for practical recycle application, therefore, we heated a used gypsum to a temperature of 700ﾟC which was available temperature level in ordinary dental furnace. The weight change of the used gypsum was measured at every minute.The weight loss after 10 minutes heating at 700ﾟC showed approximatlly the same weight of water applied to knead the gypsum powder. A
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n improved pulverizing method gave less than 45 mum of particle size, and the collecting ratio increased to a level of 60 %. When the heated and pulverized recycled gypsum powder was kneaded with water at a powder/water ratio of 0.43, the dry-compressive strength became 251.4 kgf/cm^2 as average, which was significantly high comparing with that obtained under the condition of heating at 180ﾟC for 210 minutes followed by pulverizing to 106 mum or less of particle size. The value of dry-compressive strength was a level of around 70 to 80 % of that of typical beta-hemihydrate gypsum. Presumable reasons of improved dry-compressive strength are that the improvement of pulverizing method provides nearly equal particle size with that of commercial *-hemihydrate gypsum and that less water mixing ratio provides sufficient treatability.
For further shortening the heating time, we tested to apply an electronic oven. When 300 g of *-hemihydrate gypsum was treated in an 1600 W electronic oven with 70 % of power output for 10 min., the resulted mass loss was nearly equal to the added water weight for kneading. When the gypsum was pulverized to less than 45 mum of particle size and was kneaded with water at powder/water ratio of 0.43, the gypsum re-set in exothermic reaction. When, however, the quantitity of gypsum placed in the electronic oven exceed 300 g, it did not re-set after kneading the pulverized gypsum powder with water.
Consequently, the use of electronic oven should not be applicable for recycling a large amount of gypsum at a time. Less
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