Dehydration processes of Mg (H₂PO₄) ₂·2H₂O and Ca (H₂PO₄) 2·H₂O in the presence of 85% H₃PO₄ were studied under a reduced pressure of 50mmHg at 180°C or 250°C. Some properties of the products; MgH₂P₂O₇, Mg (PO₃) ₂, CaH₂P₂O₇, and γ-Ca (PO₃) ₂, Obtained under the optimal conditions in these reactions were examined by means of measurements of density and surface area, and electron micrography. On each dehydration, two kinds of mixtures (phosphate : H₃PO₄= 10 : 20 and 10 : 1 in mixing ratio of g) were used.
The slurry mixtures of Mg (H₂PO₄) 2· 2H₂O with H₃PO₄ and Ca (H₂PO₄) ₂·H₂O with H₃PO₄, at a mixing ratio of 10 : 20, were heated at 180°C for 40h and at 250°C for 8h, respectively.
The dehydration processes proceeded along the following routes :
1) Mg (H₂PO₄) ₂· 2H₂O→ Mg (H₂PO₄) ₂→MgH₂P₂O₇→Mg (PO₃) ₂
2) Ca (H₂PO₄) ₂·H₂O→ [Ca (H₂PO₄) ₂] →CaH₂P₂O₇→γ-Ca (PO₃) ₂.
In route (2) Ca (H₂PO₄) ₂ was not detected because the dehydration of Ca (H₂PO₄) ₂·H₂O to CaH₂P₂O₇proceeded rapidly.
In the case of powder-like mixtures in a mixing ratio of 10 : 1, the process of the Mg (H₂PO₄) ₂·2H₂O-H₃PO₄ system was the same as route (1), but the final stage of the Ca (H₂PO₄) ₂·H₂O-H₃PO₄ system differed from that of route (2) :
3) CaH₂P₂O₇→ Ca₂HP₃O₁₀+amorphous→γ-Ca (PO₃) ₂.
It was found that these reactions in the presence of phosphoric acid occurred at lower temperatures than those in air or under reduced pressure.
MgH₂P₂O₇, Mg (PO₃) ₂, CaH₂P₂O₇, and γ-Ca (PO₃) ₂, obtained in this experiment had better crystallinity and more uniform grain size than those obtained by heating in air.

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The F^--uptake behavior of octacalcium phosphate (OCP) was studied at a solid/solution ratio of 1 g/50 ml, initial fluoride concentrations of 200 and 1000 ppmF^- and temperatures of 25-40°C, in comparison with that of other calcium salts.
At 200 ppmF^-, OCP lowered the [F^-] below 0.02 ppm at 25-40°C. Fluoride ions precipitated as fluorapatite (FAp). Similar removals of F^- were obtained also by the use of monetite at 25-40°C and brushite at 40°C. An inactivation of brushite for F^--uptake at 25-30°C suggested the F^--uptake process brushite→monetite→ FAp. At 1000 ppmF^- and 40°C, OCP could not lower the [F^-] sufficiently since the total amount of F^- was in excess of that needed for the reaction OCP→FAp, while monetite and brushite lowered the [F^-] until around [F^-] of the solubility of CaF₂ as the successive reaction FAp→CaF₂ proceeded. The rate-determining step for the reaction OCP→FAp was a diffusion process through the FAp dense product layer, and for the monetite→FAp it was a chemical reaction at the interface between monetite and FAp porous product layer. Activation energies were 12 kcal/mol for the former and 15 kcal/mol for the latter.

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Tricalcium aluminates, Ca₃Al₂O₆ were hydrothermally treated on addition of MgO under saturated vapor pressure at temperatures between 150 and 300°C and the influence of MgO on formation of hydrogrossular was studied by means of microscopy, XRD and SEM. Tricalcium aluminate was synthesized from chemicals at high temperature and starting mixtures for the hydrothermal treatments were prepared by blending mechanically the tricalcium aluminate and magnesia in desired proportions. Then, the mixtures thus obtained were transfered to silver tubes to be treated in hydrothermal bombs for desired durations. The results are summarized as follows :
1) Hydrogrossular, Ca₄Al₆O₁₀ (OH) ₆, portlandite, brucite, and new substances X and Y were detected as crystalline phases.
2) Magnesia was incorporated into hydrogrossular up to 3 mol %, corresponding to 1 mol % of Mg₃Al₂ (OH) ₁₂, and the hydrogrossular was almost pure.
3) Hydrogrossular trended to form metastably even at higher temperatures over its stable range (owing to the influence of MgO presence.
4) The crystalline X began to, form above 185°C and transformed to Y at 255°C
5) Chemical compositions for X and Y were 2CaO · 4MgO · 3Al₂O₃ · 7H₂O and 2CaO · 4MgO · 3Al₂O₃ · 5H₂O, respectively.

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