Phase and morphology of calcium carbonate precipitated by rapid mixing in the absence of additives.

Calcium carbonate is one of the most common minerals, and its polymorphic formation and transformation pathways from the amorphous to crystalline phases are well documented. However, the effects of locally created pH changes on the preferential formation of amorphous calcium carbonate (ACC) or its crystalline phase remain poorly understood. In this study, the influence of the initial solution pH on the precipitated polymorphs of calcium carbonate was investigated by the rapid mixing of each solution containing calcium or carbonate ions in the absence of additives. The results showed that the amount of recovered ACC particles was associated with the availability of fully deprotonated carbonate ions. A secondary crystalline phase was identified as the vaterite phase, but no polymorphic change to produce the more stable calcite was detected during 5 h of stirring. Interestingly, during the early stage of pouring, the vaterite morphology was dependent on the generated pH range, over which ACC particles were stabilized (pH > 10.3), followed by the hydration-condensation processes. When the pH was sufficiently low (pH < 10.3) for bicarbonate ions to participate in the carbonation reaction, croissant- or cauliflower-like aggregates with layered structures were obtained. In contrast, typical spherical vaterite particles were obtained at a high initial pH when the carbonate ions were dominant. Meanwhile, vaterite particles that were formed in the presence of an excess of carbonate ions were irregular and separate agglomerates. These results elucidate the formation of ACC and the morphologies of the vaterite products.

Effects of temperature on the carbonation of flue gas desulphurization gypsum using a CO2/N2 gas mixture.

The carbonation of flue gas desulphurization (FGD) gypsum using a CO2/N2 gas mixture was investigated to study the feasibility of using the flue gas directly in the gypsum carbonation. The effect of the reaction temperature on the carbonation reaction and the carbonation conversion efficiency of the samples were considered. In this study, the carbonation conversion efficiency was calculated using a new method for decreasing the error range from a sample containing unreacted gypsum. The carbonation reaction at 40°C was nearly twice as fast as the reaction at room temperature. In addition, the carbonation conversion efficiency at 40°C (96%) was nearly the same as that at room temperature. However, the efficiency decreased significantly with temperature, especially above 60°C. It can, therefore, be concluded that the direct use of flue gas in gypsum carbonation is most feasible at 40°C. The temperature of carbonation strongly affected the CaCO3 polymorphs and the morphological characteristics. Calcite with various shapes was the dominant (40-90%) phase at all temperatures. At temperatures below 40°C, spherical-shaped vaterite was pronounced, while needle-flower-shaped aragonite was dominant at temperatures above 80°C.

Mechanical characteristics and morphological effect of complex crossed structure in biomaterials: fracture mechanics and microstructure of chalky layer in oyster shell.

The complex crossed structures with a polymorph of calcite, termed a chalky layer, which make up much of the shell of an oyster, are composed of flames and leaflets. Two layers, folia and the chalky layer in the giant Pacific oyster (Crassostrea gigas) were examined using SEM (scanning electron microscope), micro-area-XRD (X-ray diffraction) and FT-IR (Fourier transform infrared spectrometer) to determine their morphologies and component characteristics. The chalky layer was also tested using microindentation to assess its mechanical properties, and a microcrack was generated to study the fracture mechanism of the chalky layer. From an analysis of the secondary protein structure, it was shown that the ordered structures of the two layers, α-helix and ß-structure, are similar but that the unordered structures are different. Moreover, the foliated rods at the interface of the chalky layer play a key role in the crystal growth of the chalky layers. Comparing the morphology and the preferred orientation of foliated laths, the advantages of the relatively high density and low hardness of the chalky layer have interesting implications regarding the development of sophisticated complex composites.