A simple synthetic method for preparing aragonite whiskers has been developed. A suspension of CaC1,-MgC1,-Mg(OH), with pH -9 has been prepared by adding Ca(OH), to MgCI, aqueous solution. CaCO, whiskers (aragonite phase) have been prepared easily by blowing C0,-containing gas into the suspension. The whiskers have high aspect ratios ranging from 20 to 80 with diameters of 0.5-1 pm.

A novel method for preparing calcium hydroxyapatite (Ca10(PO4)6(OH)2: HAp) fibers has been developed. HAp fibers can be prepared successfully by heating a compact consisting of calcium metaphosphate (ß‐Ca(PO3)2) fibers with Ca(OH)2 particles in air at 1000°C and subsequently treating the resultant compact with dilute aqueous HCl solution. The ß‐Ca(PO3)2 fibers and the Ca(OH)2 in the compact were converted into fibrous HAp and CaO phases by the heating, and the CaO phase was removed by acid‐leaching. HAp fibers obtained in the present work were 40‐150 µm in length and 2‐10 µm in diameter. The fibers had almost the same dimensions as those of the ß‐Ca(PO3)2 fibers.

Reductive decomposition of spent CaSO4 was studied using a packed-bed reactor to regenerate an alternative CaO sorbent. The reactor was operated at various process conditions including an increasing CO concentration, CO/CO2 concentration ratio (0.067−1), and temperature (1123−1273 K). In all cases, N2 was used as a balancing gas. The regeneration of CaO from CaSO4 was found to be most effective in the CO−CO2−N2 atmosphere and strongly depended on the CO/CO2 concentration ratio. At 1273 K, an apparent conversion value of 0.91 for the decomposition of CaSO4 to CaO was obtained in a 2 vol % CO and 30 vol % CO2 atmosphere. On the other hand, in a CO−N2 atmosphere, CaS was predominantly produced. The SO2 absorption capacity of CaO regenerated from CaSO4 was higher than that of limestone-calcined CaO. A larger pore diameter of the regenerated CaO was considered to be responsible for the higher SO2 absorptivity.

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Bone formation around three types of fibrous calcium-containing crystals has been examined histologically using rats. The implanted materials are (i) calcium metaphosphate (beta-Ca(PO(3))(2)) fibers having aspect ratios of 15-80 with 2-20 microm in diameter, (ii) beta-Ca(PO(3))(2)) fibers surface-modified using dilute NaOH and (iii) calcium carbonate (CaCO(3); aragonite phase) whiskers having aspect ratios of 15-40 with 0.5-3 microm in diameter. Beta-Ca(PO(3))(2) fibers show a mechanically high strength with a low modulus of elasticity, and the surface-modified fibers have a thin layer consisting of a calcium orthophosphate phase. CaCO(3) whiskers were used for comparison reasons. The materials were implanted for 4, 8, and 12 weeks into bone defects created in the bone marrow of rat tibiae. Cancellous bone formation was observed around beta-Ca(PO3)2 fibers, the surface-modified fibers and CaCO(3) whiskers after implantation for 12, 4 and 4 weeks, respectively. CaCO(3) whiskers were scarcely observed after 12 weeks for resorbing. The calcium phosphate fibrous materials show combined advantages of mechanically high strength for toughening a matrix phase and biological activities; thus, these materials may prove to be useful for novel applications in the biomedical field.