THE THERMAL DEHYDRATION OF THE CA(H2PO4)(2)CENTER-DOT-H2O-SIO2 SYSTEM.1. MECHANISM

The thermal dehydration of the Ca(H2PO4)(2) . H2O-SiO2 system has been studied by a continuous thermogravimetric mode method, and by a batch method, heating the mixtures in the molar ratios SiO2/Ca(H2PO4)(2) . H2O = 0.5, 1, 2 and 3 at 150, 185, 245, 275, 320, 430 and 780 degrees C. The batches, after cooling, were examined by X-ray diffraction (XRD ) and infrared spectroscopy (IR) to identify the intermediate and fina l products. The final products were mainly Ca(PO3)(2) and SiP2O7, whil e pyro- and triphosphates and Si(HPO4)(2) were detected as intermediat e products. The quantity of SiO2 present affects the dehydration which takes place in five stages. The removal of water molecules in the fir st, second and fourth stages was increased, and in the third and fifth stages was decreased on increasing the amount of SiO2.