This study conducts basic research on formation of needle-like aragonite from spherical calcite. This study investigates formation of needle-like aragonite from spherical calcite. The single spherical vaterite phase was synthesized by adjusting the initial pH to 2.5 in the CaCl₂–Na₂CO₃–H₂O reaction system. Spherical vaterite was suspended in pure water at 60–92℃. At a suspension concentration of 2.5 mass% and a reaction time of 2 hours, vaterite dissolved and precipitated to form aragonite and calcite at temperatures above 77℃. The formation rate of aragonite was more than 85%. The long length of the formed needle-like aragonite was 35 μm and the aspect ratio was 30. The short length of aragonite increased with increasing temperature. The critical amount of vaterite to aragonite was determined by varying the suspension concentration and time. The critical amount increased with increasing temperature, reaching 3.6 g・100 cm^－3 at 90℃. The short length of the aragonite formed became longer as the suspension concentration increased.

　In the present study, adsorption ability of carbon dioxide of Shirasu-based materials mixed with wall material were investigated. The previously reported Shirasu-based material which was prepared from dried Shirasu (SSM) with calcination process followed by treatment in aqueous ammonia solution and exhibited adsorption ability of carbon dioxide was mixed with wall material, and adsorption ability of carbon dioxide of the mixed material was evaluated. As the previous report, improvement of adsorption ability of carbon dioxide on the mixed material were observed through water addition. From the result of the DRIFT spectra, water molecules adsorbed on the Brørnsted acid sites were identified, suggesting the effect of the adsorbed molecules on improvement of adsorption ability of carbon dioxide on the mixed material.

　Large quantities of spherical calcium carbonate pellets are generated in a water treatment plant in Okinawa to obtain soft water from hard water. This study investigated the use of these calcium carbonate pellets for cement aggregate as a substitute for silica stone. The mortars were prepared using standard sand for strength tests and calcium carbonate pellets A and B obtained from water treatment plants A and B. The bending strengths of the mortars were compared. The compressive strength of calcium carbonate pellets tended to decrease at diameters greater than 2 mm. Calcium carbonate pellet A contained 19% of 2 mm particle size. The bending strength of the mortar with calcium carbonate pellet B was the same as that with standard sand. In contrast, the bending strength of the mortar with calcium carbonate pellet A was lower than that with standard sand. However, the use of calcium carbonate pellet A, from which particles larger than 2 mm were removed, results in a mortar with bending strength equivalent to that of mortar using standard sand. Sufficient adhesion between the pellet and cement was observed by SEM and microscope. Based on these results, we considered that calcium carbonate pellets could be used as fine aggregate for mortar.