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CHAPTER 3 SYNTHETIC FE-CONTAINING HYDRATES 3. SYNTHETIC FE-CONTAINING HYDRATES 3.1. Iron containing carbonate AFm phases 1 3.1.1. Introduction The main hydration products of Portland cements include C-S-H (calcium silicate hydrate), portlandite, ettringite and AFm (Al2O3-Fe2O3-mono) phases. AFm phases are formed from C3A (3CaOAl2O3) and C2(A;F) (Ca2(AlxFe1-x)O5) phases in the presence of carbonates, sulfates, chlorides and hydroxide during the hydration of Portland cement. The general formula is Ca2(Al,Fe)(OH)6XnH2O, where X denotes a single charged or half of a double charged anion which occupies the interlayer sites. Among possible anions are OH - , SO4 2- , CO3 2- and Cl - . AFm phases have a layered structure composed of two layers, a positively charged main layer [Ca2(Al,Fe) (OH)6] + and a negatively charged [XnH2O] - interlayer. The main layer consists of sheets of Ca(OH)6 octahedral ions, as in portlandite, in which every third Ca 2+ is substituted by Al 3+ and/or Fe 3+ . Cements are sensitive to carbonation which can lead to the formation of hemicarbonate 3CaO(AlxFe1-x)2O3(CaCO3)0.5(Ca(OH)2)0.5nH2O and/or monocarbonate 3CaO(AlxFe1- x)2O3(CaCO3)mH2O, x=0 to 1. Al-monocarbonate (Al-Mc) has a triclinic pseudohexagonal symmetry [66]. The solubility products of Al-monocarbonate and hemicarbonate have been determined experimentally in the range of 5 to 85 °C [7]. The stability of these phases has an impact on the bulk chemistry of cements as the formation of hemi- and/or monocarbonate indirectly stabilizes ettringite. This results in a higher volume of hydrated phases which can contribute to the improvement in mechanical properties of cements [65, 67]. It has been shown that Al-hemicarbonate (Al-Hc) and 1 This chapter has been published in Cement and Concrete Research, 41 (2011). 35
CHAPTER 3 SYNTHETIC FE-CONTAINING HYDRATES hydroxy-AFm are unstable with respect to Al-monocarbonate in the presence of calcite [2, 65]. OPC contains around 3-7 % of Fe2O3. During hydration, Fe-containing AFm and/or Fe- AFt phases may form. The extent to which Fe is present in AFm and AFt-phases will influence strongly the amount of AFt and AFm phases present and thus the volume of the hydrates and the properties of the hydrated cement. Understanding the characteristic of the hydrates in complex cement matrices is important since the material properties of cement-based materials are related to the chemical environment and the thermodynamic properties of the hydrated phases. Until recently only rough estimates of the solubility products of Fe-containing monocarbonate and hemicarbonate have been available [3], where the solubility has been estimated based on the solubility of the Al-containing phases. The first experimental data on the solubility of Fe-monocarbonate were estimated from Fe-ettringite experiments where contamination with CO2 led to the formation of Fe-Mc [15]. The formation of solid solutions can play an important role in stabilizing these solids. Solid solutions between anions in the interlayer structure of Al-containing AFm phases are common [52, 68, 69]. The existence of a solid solution between the Al- and Fe- ettringite has been reported [16]. It is unclear, however, to what extent Fe and Al in the main layer of AFm phases form solid solutions. In this section Fe-containing monocarbonate and hemicarbonate and the solid solution series with their aluminum analogues were synthesized to study their structure and solubility. Different techniques were used to characterize the synthesized solids. Synchrotron X-ray powder diffraction and Raman spectroscopy were used to determine the crystal structure of the Fe-monocarbonate. The solubility products of Fe-Mc and Fe- 36
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Fe-containing hydrates and their fa
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The measured composition of the liq
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ZUSAMMENFASSUNG Thermodynamische Mo
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konnten die Fe-haltigen Hydrate im
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CHAPTER 3 SYNTHETIC FE-CONTAINING H
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Intensity [arb. units] Fe-Fr CHAPTE
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CHAPTER 4 FE-CONTAINING HYDRATES IN
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(Al+Fe)/Ca (Al+Fe)/Ca 0.8 0.6 0.4 0
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k 3 (k) CHAPTER 4 FE-CONTAINING HYD
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differentiated relative weight Weig
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Early reaction CHAPTER 4 FE-CONTAIN
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weight loss in % differentiated rel
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5. GENERAL CONCLUSION AND OUTLOOK a
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Speciation of iron(III) in hydrated
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5. GENERAL CONCLUSION AND OUTLOOK
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Techniques XRD X-ray diffraction TG
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APPENDIX APPENDIX Appendix A: Addit
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Intensity (arb. Units) P Fe 2O 3 R
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APPENDIX Appendix B7 XRD patterns o
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REFERENCES REFRENCES [1] H.F.W. Tay
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REFRENCES [29] M. Vespa, R. Dähn,
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REFRENCES [54] M. Balonis, B. Lothe
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REFRENCES [81] H.F.W. Taylor, Cryst
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REFRENCES [107] K. Kyritsis, N. Mel
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REFRENCES [134] M. Wilke, F. Farges
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CURRCULUM VITAE Curriculum Vitae Na
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