As one of the serial studies on fibrous calcium compounds, the synthesis of fibrous C
3A⋅3CaCO
3⋅32H
2O was studied by adding CaCl
2 solution (6-21×10
-3mol dm
-3) to the solution prepared by introducing CO
2 gas or by adding Na
2CO
3 solution (CO
32-concentration: 1-5×10
-3mol dm
-3) into NaAlO
2 solution (7-22×10
-3mol dm
-3). Characteristics of the product obtained by liquid-phase reaction in CaCl
2-NaAlO
2-CO
2 and CaCl
2-NaAlO
2-Na
2CO
3 systems were determined by means of X-ray diffraction, thermal analysis (TG-DTA), scanning electron microscopy, infrared spectroscopy and chemical analysis. The formation of fibrous C
3A⋅3CaCO
3⋅32H
2O was affected remarkably by synthetic conditions such as pH, initial Ca/Al atomic ratio and concentration of CO
32-ion. When the concentration of CO
32-was 1-3×10
-3mol dm
-3, the initial Ca/Al atomic ratio to form fibrous crystals of C
3A⋅3CaCO
3⋅32H
2O was less than 0.3 in CaCl
2-NaAlO
2-CO
2 system and less than 2.0 in CaCl
2-NaAlO
2-Na
2CO
3 system. Thus, the fibrous crystals were stable at pH above 11.7. Therefore, when the initial Ca/Al atomic ratio exceeds 0.3 in the system of CaCl
2-NaAlO
2-CO
2, pH because less than 11.7, yielding no fibrous crystals. While, the reaction in CaCl
2-NaAlO
2-Na
2CO
3 system to form the fibrous crystals progressed successfully because the pH value was kept over 11.7 even in the initial Ca/Al atomic ratio 0.3-2.0. It was confirmed that fibrous C
3A⋅3CaCO
3 32H
2O crystallized as high crystalline fibers with aspect ratios above 300 elongated parallel to
c-axis in hexagonal through an unstable amorphous phase which was precipitated in the initial step. The fibrous shape was stable in heating up to 300°C. Furthermore, the packing state of the fibrous crystals as filler for paper was discussed.
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