Provided is a medical calcium carbonate composition that highly satisfies 1) tissue affinity, 2) in vivo resorbability, 3) reactivity, and 4) mechanical strength required for medical materials to be implanted in vivo, a medical calcium phosphate composition, a medical carbonate apatite composition, a medical calcium hydroxide porous structure, a medical calcium sulfate setting granules, and a bone defect regeneration kit related to the medical calcium carbonate composition, and methods for producing these. The medical composition calcium carbonate that highly satisfies the above described elements, and related medical compositions can be produced by controlling the polymorph or structure of calcium carbonate.

A method and a reactor to crystalize calcium carbonate and to form crystals in a liquid flow (F) with the concentration of 2.5% to 5% inside or on the surfaces of a fiber dispersion containing solid matter. The method includes feeding carbon dioxide (CO.sub.2) and lime milk (LM) (Ca(OH).sub.2) into the liquid flow and reacting the CO.sub.2 with the Ca(OH).sub.2 wherein the carbon dioxide (CO.sub.2) and/or lime milk (LM) is injected with a mixer crosswise into the liquid flow (F) and the injection is performed with a feed tube (2) to cause a dwell time of the crystallizing reaction of 0.5 to 10 seconds before a nozzle (20) of the reactor (1), after which the liquid flow (F) is mixed with the liquid or fiber suspension in container (4)