Belay Zeleke Dilnesa - Eawag-Empa Library

CHAPTER 3 SYNTHETIC FE-CONTAINING HYDRATES
absence of silica, portlandite and amorphous Fe-hydroxide and possibly iron containing
carbonate phases will form. The formation of C4FHx was persistent in CaO-Fe2O3-H2O
system at room temperature. No solid solution formation occurred between C3AH6 and
C3FH6.
The Al and Fe siliceous hydrogarnet was more complex due to the formation of two
hydrogarnets with different silica contents. In addition, C-(A)-S-H co-precipitated during
the preparation of Si-hydrogarnet. Al-containing siliceous hydrogarnet did not form at
room temperature but only at 110 °C. A miscibility gap was observed between
C3AS0.41H5.18 and C3AS0.84H4.32 for samples synthesized at 110 °C. Hydroandradite formed
both at room temperature and at 110 °C. Again the formation of a solid solution was
observed with a miscibility gap between C3FS0.95H4.1 and C3FS1.5H2.96.
Possibly a simultaneous substitution of Al by Fe in the octahedral sites and Si by OH - in
the tetrahedral sites occurred with a miscibility gaps for Ca3(AlxFe1-x)2(SiO4)3-y(OH)4y.
The CaO-Al2O3-Fe2O3-SiO2-H2O is a complex system that exhibits multi ion substitution
in the structure and more detailed investigations are needed.
Implications for cementitious systems
In pure CaO-Al2O3-H2O systems C3AH6 is the stable phase. The addition of sulfate
destabilizes C3AH6 and leads to the formation of monosulfate or ettringite depending on
the amount of sulfate added [7]. The presence of carbonate leads to the formation of
monocarbonate [2, 67]. Similarly, the presence of silica could destabilize C3AH6 and
cause the formation of the thermodynamically more stable Si-hydrogarnet. However, this
process is kinetically hindered and observed only at higher temperature around 100 °C.
In the presence of Fe2O3 the phase assemblage of CaO-(Al,Fe)2O3-H2O is distorted to
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