Fe-containing Hydrates and their Fate during Cement Hydration : Thermodynamic Data and Experimental Study

Thermodynamic modeling is a versatile tool for predicting the chemical composition cement during the hydration of cement. The quality of the thermodynamic modeling depends directly on the quality and completeness of thermodynamic database used. One of the main limitations of modeling the hydration of cement is the lack of thermodynamic data for Fe containing hydrates. In addition, the formation of solid solutions between Fe- and Al-containing hydrates could stabilize mixed solids. However, it is unclear to what extent such solid solution formation occurs. Also experimentally it is very difficult to identify Fe-containing hydrates in hydrating cements by standard analytical techniques as the signals from Fe-containing phases significantly overlap with those from the corresponding Al-containing phases. Thus, in this study, potential Fe-containing hydrates like Fe-hemicarbonate (Fe-Hc), Fe-monocarbonate (Fe-Mc), Fe-monosulphate (Fe-Ms), Fe-Friedel's salt (Fe-Fr), Fe-strätlingite (Fe-St), Fe-katoite (C3FH6 ) and Fe-siliceous hydrogarnet (Fe-Si-Hg) were synthesised at 20, 50 and 80 °C. The solid phases were characterized by X-ray powder diffraction (XRD), Thermogravimetric analysis (TGA), scanning electron microscopy (SEM), vibrational spectroscopy (Raman and Infrared spectroscopy) and Extended X-ray absorption fine structure spectroscopy (EXAFS). The compositions of the liquid phases were analyzed using inductively-coupled plasma optical emission spectrometry and mass spectrometry (ICP-OES and MS). At ambient temperature Fe-Mc, Fe-Ms, Fe-Fr and Fe-Si-Hg were stable, while Fe-Hc, Fe-katoite and Fe-St were metastable. Fe-Mc, Fe-Ms, Fe-Fr and Fe-Si-Hg were stable also at 50 °C, but the Fe-AFm phases were unstable at 80 °C while Fe-Si-Hg were stable up to above 100 °C. 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 thermodynamic modeling of the hydration process, which predicts formation of stable Al/Fe-siliceous hydrogarnet in cement system. The determination of the solubility products of these hydrates will help to extend the thermodynamic data base of cement minerals and establish whether and to which extent Fe-containing hydrates are stable in fresh and in leached cementitious systems. The results from this study on the Fe speciation in cementitious systems are important for a better understanding of cement-water interactions with a view to the durability of cementitious materials.