Hydration mechanism and flow property of an aqueous suspension of plaster of Paris have been investigated from the viewpoint of viscosity. Viscosity was measured with a rotational coaxial viscometer under the conditions at a constant shear rate.
The results are summarized as follows.
(1) Sodium citrate, used as a retarder, was found to reduce the viscosity of the suspension.
(2) The increasing rate of viscosity (d log η_a/dt) of the suspension with the progress of hydration was independent of the water-to-plaster ratio but increased linearly with an increase in the rate of shear.
(3) Any equilibrium flow curve obtained from the constant shear rate test for the suspension at various states of hydration showed the pseudoplastic flow. Plots of the logarithm of Bingham yield value and of plastic viscosity obtained by applying Bingham equation to the flow curve, were linear against hydration time.
(4) The flow curves for the suspensions of various water-to-plaster ratios showed the pseudo-plastic flow and the logarithm of Bingham yield value and of plastic viscosity obtained from the flow curves decreased linearly with an increase in water-to-plaster ratio.
(5) Any suspension in the present paper showed thixotropy, that is, the viscosity decreased under the test of constant shear rate.

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The relationship between mineral contents, chemical composition and properties of products obtained by slow mixing of aqueous solutions of Ca²⁺ and PO₄^3- was studied by means of X-ray diffractometry, chemical analysis, IR spectroscopy, thermal analysis (TG-DTA) and observation of scanning electron microscope (SEM).
From those results, crystalline hydroxyapatite (HAp) was precipitated at a pH below 8-9. The lowest pH values for HAp formation in the reaction system (CaCl₂+Na₂HPO₄+NH₄OH) were 5. 5, 5.1 and 6.7 at HAp concentrations of 2.0 × 10^-2, 2.0 × 10^-3 and 2.0 × 10^-4 mol/l, respectively. Those pH values depended on the CaO/P₂O₅ mol ratio, concentration and impurities (Na⁺, NH⁴⁺, Cl^-, etc) in the reaction system (CaCl₂+Na₂HPO₄ or (NH₄) ₂HPO₄). Na⁺ and NH₄⁺ depressed formation of HAp. Na⁺ seemed to retard crystallization of HAp at high pH while Cl^- had no such effect.
The precipitates obtained from aqueous solutions of various pH's (5-11) at 25°C contained about 10-25% of H₂O. From heating loss and X-ray diffraction pattern, the above precipitates were considered to consist of HAp· (H₂O) ₆, HAp· (H₂O) ₈, HAp· (H₂O) ₁₂ or HAp· (H₂O) ₁₆ (amorphous). The reaction products (HAp^* : HAp· (H₂O) _n, n=6, 8, 12, 16) were identified by quantitative chemical analysis, X-ray diffractometry and SEM. The CaO/P₂O₅ mole ratio of the products were from 2.9 to 3.4. HAp^* precipitated as crystals with a lamellar structure and the crystallite size was about 100-200A ([001]) at a pH of above 5.5.

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Reactions related to “H₂O” in hydroxyapatite [Ca_10-z (HPO₄) _z (PO₄) _6-z (OH) _2-z·nH₂O] and fluorapatite [Ga_10-y (HPO₄) _y (PO4) _6-yF_2-y·mH₂O] during heating and a slight thermal discoloration of the apatites have been investigated by TG, X-ray diffraction, IR and ESR methods.
A new type reaction, PO₄^3-+H₂O→HPO₄^2-+OH^-→1/2P₂O₇^4-+OH^-+1/2H₂O↑, was presented to have to be required above ca. 250°C. The addition of this type to conventional reaction types resulted in apatite compositions of Ca_10-z (P₂O₇) _z (PO₄) _6-2z (OH) ₂ and Ca_10-y (P₂O₇) _y (PO₄) _6-2yF_2-y (OH) _y. The former converted to (1-z) Ga₁₀ (PO₄) 6 (OH) ₂+ 3zCa₃ (PO₄) ₂ at 700-800°C, while the latter was stable up to 1340°C. An ESR center giving a singlet, g=2.002-2.003 and ΔHpp=7-8G was found at room temperature in hydroxyapatite samples heated above 180°C. The ESR signal improved with heating temperature, and disappeared at 600-650°C. It was first pointed out that there might be a correlation among the disappearance, a trace weight loss and a local variation of P₂O₇^4- groups. The signal probably arised from trapped electrons in a locally distored structure induced by HPO₄^2-→1/2P₂O₇^4-+1/2H₂O↑.

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In a series of studies on natural hard tissues as inorganic and organic composite materials, relationship between the bending strength and the microstructure and the components of ivory were investigated by means of SEM, thermal analysis, IR spectroscopy and X-ray diffractometry.
1) The microstructure of ivory consists of inorganic and organic fibrous materials and fine tubes or vacant holes. In the center part the fibrous materials were oriented to the growth direction of ivory, whereas near the rim part the fibrous materials were entangled like yarn-balls. The fine tubes (2 pm in diameter) were radially distributed from the center to the edge. Many vacant holes with a diameter of 2-3 × 10 μm in the center and 10 pm in the rim part were observed.
2) The bending strength varied with the parts of ivory. The values of the bending strength of the center and rim parts were 2.4 -2.8 × 10³ kg/cm² and 1.8-2.1 × 10³ kg/cm², respectively. The bending strength was affected by water content. The modulus of elasticity was about 1.2 × 10⁵ kg/cm², regardless of the part.
3) Ivory was composed of about 7 wt% of water, about 30 wt% of organic materials, and inorganic materials which consists mainly of hydroxyapatite (H-Ap) and an apatite containing CO₃^2-. Specific gravity was 1.64 (center part) -1.82 (rim part).
4) Crystallite dimensions of (002) and (211) of H-Ap were 408A and 43 A, respectively, at the center. Those of the rim part were 272A and 54A, respectively. Heat treatment led to the formation of β-Ca₃ (PO₄) ₂ from H-Ap.

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