Belay Zeleke Dilnesa - Eawag-Empa Library

The measured composition of the liquid phase was used to calculate the solubility
products at 20 and 50 °C and to derive the data for standard conditions (25 °C, 1 atm).
The solubility products of Fe-Fr was similar to the solubility product of Al-Fr, while the
solubility products of Fe-Mc and Fe-Ms were about 3 log unit lower than that of Al-Mc
and Al-Ms indicating that in Fe-Friedel’s salt is probably not stable in cements. The very
low solubility product of Fe-Si-Hg (5 to 7 log units lower than that of Al-Si-Hg) implies
that Fe-Si-Hg could be a stable phase in hydrated cements.
Also the mixed Al- and Fe-containing hydrates were synthesized to study the extent of
solid solution formation. Both XRD and thermodynamic modelling of the liquid
compositions indicated that Al- and Fe-monosulphate and Al- and Fe-Friedel’s formed
solid solutions with a miscibility gap, while Al- and Fe- monocarbonate existed as two
separate hydrates due to their different crystal structure (Al-Mc: monoclinic, Fe-Mc:
rhombohedral). The formation of solid solution between Al and Fe-siliceous hydrogarnet
seemed probable.
To understand to what extent the findings from the synthesised hydrates were relevant for
real cements, the speciation of iron was determined in hydrating cement using EXAFS
spectroscopy. Identification of Fe-containing hydrates and quantification of their
contributions was achieved by combining principal component analysis with iterative
target tests, and linear combination. The results show that several Fe species already
contributed to the overall Fe K-edge spectra of cement pastes during the first day of
hydration. While ferrite was the dominant Fe-containing phase in the unhydrated cement,
Fe-hydroxide was detected shortly after starting the hydration process. With time the
formation of stable Al/Fe-siliceous hydrogarnet was observed, while the amounts of Fe-
hydroxide and ferrite clinker slowly decreased. The latter finding agrees with results from
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