Effects of wet air on the formation and thermal stability of 12CaO⋅7Al
2O
3(C
12A
7) were studied in atmospheres with controlled water vapor content. The formation process of C
12A
7 in fired products was examined by X-ray powder diffraction and the water included in 11CaO⋅7Al
2O
3⋅Ca(OH)
2(C
11A
7⋅Ca(OH)
2) was identified by means of Fourier transform infrared spectrophotometer (FT-IR). Anhydrous C
12A
7 was prepared in dry atmosphere at temperatures above 1150°C. On the other hand, C
11A
7⋅Ca(OH)
2 including hydroxide ions was formed at above 1000°C in flowing wet air and the humidity accelerated the formation of the hydrate. It was difficult to make the distinction of X-ray diffraction patterns between C1
2A
7 and C
11A
7⋅Ca(OH)
2, but the stretching vibration band of O-H at about 3560cm
-1 measured by means of FT-IR enabled to identify the hydrate. In a flow of dried helium gas, the hydrate C
11A
7⋅Ca(OH)
2 decomposed into 5CaO⋅3Al
2O
3(C
5A
3) with dehydration when it was heated in the temperature range of 1100 to 1250°C. At temperatures above 1250°C, C
5A
3 changed into C
12A
7. On the contrary, C
5A
3 changed into C
11A
7⋅Ca(OH)
2 by heating at about 1100°C in a flow of wet air.
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