A Portland cement is a complex multi-component system and, to predict its elastic properties, an exhaustive database of the thermodynamic parameters of the main constituents is needed. Ettringite (ideally Ca6Al2(SO4)3(OH)12·27H2O, with a=b =11.21 and c =21.43 Å, Sp. Gr. P31c) is one of the most important crystalline phases in Portland cements: its crystallization, in the early hydration stages, governs the set rate of the highly reactive “C3A” phase (Ca3Al2O6), whereas in aged cements the formation of ettringite is commonly associated with degradation processes (Taylor et al. 2001). The crystal structure of ettringite is significantly complex with a H-bonding net which connects [Ca3[Al(OH)6]·12H2O]-columns (in which Al(OH)6-octahedra are alternated with triplets of Ca(OH)4(OH2)4-polyhedra) to sulphate groups (Gatta et al. 2019). Despite the previous studies at high pressure on this material (e.g., Cuesta et al. 2017, Clark et al. 2008), the linear bulk moduli (Ka and Kc) and a description of the deformation mechanisms at the atomic scale are still missing. In this light, we have investigated the compressional behavior of ettringite up to 4.2 GPa by means of in-situ single-crystal synchrotron X-ray diffraction, using a diamond-anvil cell (DAC) and the mix methanol:ethanol (4:1) as P-transmitting fluid. Ettringite shows a marked anisotropic compressional pattern (Ka 21(1) GPa, Kc 47(1) GPa). This anisotropic elastic scheme dramatically changes at P>3 GPa; KV0 drops from 26.6(5) to 10.4(8) GPa), which mainly affects the structure on the ab plane (Ka drops from 21(1) to 7.3(8) GPa whereas Kc decreases only moderately). Structure refinements reveal that the elastic softening reflects the collapse of the H-bonding network, due an average decrease of the Odonor···Oacceptor distances (up to 0.20 Å in some cases), which mainly affect the interaction between the sulphate groups and the Ca(OH)4(OH2)4-polyhedra. References: Clark S.M., Colas B., Kunz M., Speziale S., Monteiro P.J.M. 2008. Effect of pressure on the crystal structure of ettringite. Cem. Concr. Res., 38, 19-26. Cuesta A., Rejmak P., Ayuela A., De la Torre A.G., Santacruz I., . Carrasco F.L., Popescu C., Aranda M.A.G. 2017. Experimental and theoretical high pressure study of calcium hydroxyaluminate phases, Cem. Concr. Res., 97, 1–10. Gatta G.D., Hålenius U., Bosi F., Cañadillas-Delgado L., Fernandez-diaz M.T. 2019. Minerals in cement chemistry: a single-crystal neutron diffraction study of ettringite, Ca6Al2(SO4)3(OH)12·27H2O. Am. Mineral., 104, 73-78. Taylor H.F.W., Famy C., Scrivener K.L. 2001. Delayed ettringite formation, Cem. Concr. Res., 31, 683–693.
Anisotropic compressional behavior of ettringite / D. Comboni, P. Lotti, G.D. Gatta, M. Merlini, M. Hanfland. ((Intervento presentato al convegno Il tempo del pianeta Terra e il tempo dell'uomo: Le geoscienze fra passato e futuro tenutosi a Parma nel 2019.
Anisotropic compressional behavior of ettringite
D. Comboni
Primo
;P. Lotti;G.D. Gatta;M. Merlini;
2019
Abstract
A Portland cement is a complex multi-component system and, to predict its elastic properties, an exhaustive database of the thermodynamic parameters of the main constituents is needed. Ettringite (ideally Ca6Al2(SO4)3(OH)12·27H2O, with a=b =11.21 and c =21.43 Å, Sp. Gr. P31c) is one of the most important crystalline phases in Portland cements: its crystallization, in the early hydration stages, governs the set rate of the highly reactive “C3A” phase (Ca3Al2O6), whereas in aged cements the formation of ettringite is commonly associated with degradation processes (Taylor et al. 2001). The crystal structure of ettringite is significantly complex with a H-bonding net which connects [Ca3[Al(OH)6]·12H2O]-columns (in which Al(OH)6-octahedra are alternated with triplets of Ca(OH)4(OH2)4-polyhedra) to sulphate groups (Gatta et al. 2019). Despite the previous studies at high pressure on this material (e.g., Cuesta et al. 2017, Clark et al. 2008), the linear bulk moduli (Ka and Kc) and a description of the deformation mechanisms at the atomic scale are still missing. In this light, we have investigated the compressional behavior of ettringite up to 4.2 GPa by means of in-situ single-crystal synchrotron X-ray diffraction, using a diamond-anvil cell (DAC) and the mix methanol:ethanol (4:1) as P-transmitting fluid. Ettringite shows a marked anisotropic compressional pattern (Ka 21(1) GPa, Kc 47(1) GPa). This anisotropic elastic scheme dramatically changes at P>3 GPa; KV0 drops from 26.6(5) to 10.4(8) GPa), which mainly affects the structure on the ab plane (Ka drops from 21(1) to 7.3(8) GPa whereas Kc decreases only moderately). Structure refinements reveal that the elastic softening reflects the collapse of the H-bonding network, due an average decrease of the Odonor···Oacceptor distances (up to 0.20 Å in some cases), which mainly affect the interaction between the sulphate groups and the Ca(OH)4(OH2)4-polyhedra. References: Clark S.M., Colas B., Kunz M., Speziale S., Monteiro P.J.M. 2008. Effect of pressure on the crystal structure of ettringite. Cem. Concr. Res., 38, 19-26. Cuesta A., Rejmak P., Ayuela A., De la Torre A.G., Santacruz I., . Carrasco F.L., Popescu C., Aranda M.A.G. 2017. Experimental and theoretical high pressure study of calcium hydroxyaluminate phases, Cem. Concr. Res., 97, 1–10. Gatta G.D., Hålenius U., Bosi F., Cañadillas-Delgado L., Fernandez-diaz M.T. 2019. Minerals in cement chemistry: a single-crystal neutron diffraction study of ettringite, Ca6Al2(SO4)3(OH)12·27H2O. Am. Mineral., 104, 73-78. Taylor H.F.W., Famy C., Scrivener K.L. 2001. Delayed ettringite formation, Cem. Concr. Res., 31, 683–693.File | Dimensione | Formato | |
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