Effects of pH on the Transformation of Gypsum to Carbonate Apatite in the Presence of Ammonium Hydrogen Phosphate

摘要

Effects of pH on the transformation of gypsum to apatite in the presence of ammonium hydrogen phosphate was studied using NH{sub}4H{sub}2PO{sub}4, (NH{sub}4){sub}2HPO{sub}4 and (NH{sub}4){sub}3PO{sub}4. When set gypsum was immersed to ammonium hydrogen phosphate, pH of the solution decreased regardless of the solution. Although pH of the solution decreased, no significant compositional change was observed when gypsum was immersed in NH{sub}4H{sub}2PO{sub}4. On the other hand, apatite and small amount of dicalcium phosphate anhydrous (CaHPO{sub}4) was formed when gypsum was immersed in (NH{sub}4){sub}2HPO{sub}4 solution. Only apatite was formed when gypsum was immersed in (NH{sub}4){sub}3PO{sub}4 solution. Although the detailed mechanism for the pH dependent products has not been clarified, one of the causes may be the different thermodynamical difference between gypsum and apatite. We would like to recommend the use of (NH{sub}4){sub}3PO{sub}4 solution since this provides higher pH and thus larger thermodynamical difference between apatite and gypsum and resulting pure apatite block.