11Å tobermorite was synthesized from the mixture of cristobalite rock powder (≤10 μm in diameter, mainly opalline silica, SiO
2 82.21%, Al
2O
3 8.30%, Fe
2O
3 2.37%, CaO 0.40%, MgO 0.74%, Na
2O 0.50%, K
2O 0.82%, Ig. Loss 4.58%, total 99.92 wt %), lime and water with the Ca/ (Sid-Al) molar ratio of 0.8 and the water/solid weight ratio of 20 at a low temperature of 70°C or 90°C and atmospheric pressure.
For the early hydration step, the low crystalline C-S-H gel was formed, though after the reaction for 300 hours at 70°C or 72 hours at 90°C, 11Å tobermorite was formed with a basal spacing 11.7Å.
Well-crystallized 11Å tobermorite obtained at 90°C for more than 480 hours showed a lathlike crystal habit elongated parallel to b axis with (001) cleavage. When the well-crystallized 11Å tobermorite was heated in air at temperatures ranging from 300°C to 800°C for 20 hours, (002) reflection became weak and broad with increasing temperature, and the basal spacing was between 11.1Å and 9.6Å, until the conversion to wollastonite occured at about 800°C. Probably, the synthesized 11Å tobermorite is a mixed type (normal+anomalous ones).
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