Abstract
Phosphatization of waste gypsum (w-Gyp) was investigated in detail to search novel use for a recycling of waste gypsum boards as spherical beads composed of various calcium phosphates. The component of the w-Gyp powder was gypsum (CaSO4·2H2O) with some inorganic and organic impurities. The w-Gyp heated at 120 °C for 24 h was bassanite (CaSO4·0.5H2O) single phase as the same as the reagent gypsum. Mixture of heated w-Gyp and ion-exchanged water was successfully hardened as spherical gypsum beads in vegetable oil. The degree of phosphatization of w-Gyp and crystal phases of calcium phosphates formed were affected by treatment conditions, pH, temperature and reaction period. The w-Gyp was barely phosphatized in acidic pH with positively temperature depended increasing in reacted amounts. The phosphatization was completed in 6 to 24 h under neutral conditions and in 1 to 6 h under basic conditions. Dicalcium phosphate dihydrate (DCPD) could firstly be formed in all conditions and it changed into dicalcium phosphate anhydrous (DCPA) by dehydration of DCPD at over 60 °C. Under higher pH conditions, DCPA or DCPD was subsequently converted into hydroxyapatite, the most stable calcium phosphate in water. During the conversion, b-tricalcium phosphate (b-TCP) was also formed by the presence of Mg2+, a stabilizer of b-TCP. Spherical beads consisting of these calcium phosphates were prepared from spherical beads of the w-Gyp.
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