Effects of humidity on calcite block fabrication using calcium hydroxide compact

Calcite has attracted attention as an artificial bone replacement material and as a precursor for the fabrication of carbonate apatite, which is also an artificial bone replacement material. In this study, the effect of humidity on calcite block fabrication was investigated using calcium hydroxide (Ca(OH)2) compact. Ca(OH)2 compact and Ca(OH)2 paste compact were exposed to CO2 at room temperature under 0%, 50%, and 100% humidity for two weeks. No carbonation was observed when Ca(OH)2 compact was exposed to CO2 under 0% humidity. In contrast, Ca(OH)2 compact transformed into pure calcite under 100% humidity. Forty percent of the Ca(OH)2 compact transformed into calcite under 50% humidity, while 30% of the Ca(OH)2 paste compact transformed into calcite. Interestingly, the diametral tensile strength of the Ca(OH)2 paste compact was four times higher than that of the Ca(OH)2 compact when both were exposed to CO2 under 100% humidity, despite the paste compact׳s lower conversion into apatite. After exposure to CO2, SEM observations revealed that in the case of the paste compact, the Ca(OH)2 powder was bridged with a precipitate, whereas in the case of Ca(OH)2 compact, no precipitate was found. Results obtained in this study demonstrated that carbonation of the Ca(OH)2 compact at room temperature was the result of a dissolution–precipitation reaction. Ca(OH)2 powder was dissolved into water to supply the Ca2+, and CO32− was supplied for the calcite precipitation from the interaction of CO2 and water. Excess humidity from the paste compact was the key to the precipitation of the calcite bridge. The presence of the calcite bridge resulted in a higher mechanical strength for the calcite block.