Belay Zeleke Dilnesa - Eawag-Empa Library

CHAPTER 4 FE-CONTAINING HYDRATES IN HYDRATED CEMENT
Thermodynamic calculations
Thermodynamic modeling was performed to calculate the composition of hydrated
synthetic Fe-cement in the presence of varying amounts of gypsum. The model predicted
the formation of C-S-H, portlandite and Fe-siliceous hydrogarnet in the absence of
gypsum (Fig. 86). It should be noted that in the presence of silica, no monosulfate but
only ettringite formation was predicted if more sulfate is present in the system. Fe-
siliceous hydrogarnet is calculated to be more stable than Fe-monosulfate under these
conditions. As the amount of gypsum increased the formation of Fe-ettringite and finally
the presence of unreacted gypsum was calculated. This is consistent with the XRD and
TGA results (Fig. 85) though Fe-siliceous hydrogarnet was not observed. Fe-siliceous
hydrogarnet was calculated to be unstable in the presence of gypsum and Fe-ettringite.
cm 3 /100g
80
70
60
50
40
30
20
10
Fe-Si-hydrogarnet
portlandite
Fe-ettringite
C-S-H
0
0 10 20 30
Gypsum %
40 50 60
Gypsum
Fig. 86 Calculated phase diagram of thermodynamic stable hydrate assemblages of synthetic Fe-
cement with different amounts of gypsum.
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