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Minerals in cement chemistry; a single-crystal neutron diffraction study of ettringite, Ca (sub 6) Al (sub l2) (SO (sub 4) ) (sub 3) (OH) (sub 12) .27H (sub 2) O

G. Diego Gatta, Ulf Halenius, Ferdinando Bosi, Laura Canadillas-Delgado and Maria Teresa Fernandez-Diaz
Minerals in cement chemistry; a single-crystal neutron diffraction study of ettringite, Ca (sub 6) Al (sub l2) (SO (sub 4) ) (sub 3) (OH) (sub 12) .27H (sub 2) O
American Mineralogist (January 2019) 104 (1): 73-78

Abstract

Ettringite, reported with ideal formula Ca (sub 6) Al (sub 2) (SO (sub 4) ) (sub 3) (OH) (sub 12) .26H (sub 2) O, is recognized as a secondary-alteration mineral and as an important crystalline constituent of Portland cements, playing different roles at different time scales. It contains more than 40 wt% of H (sub 2) O. The crystal structure and crystal chemistry of ettringite were investigated by electron microprobe analysis in wavelength-dispersive mode, infrared spectroscopy, and single-crystal neutron diffraction at 20 K. The anisotropic neutron structure refinement allowed the location of (22+2) independent H sites, the description of their anisotropic vibrational regime and the complex hydrogen-bonding schemes. Analysis of the difference-Fourier maps of the nuclear density showed a disordered distribution of the inter-column ('free') H (sub 2) O molecules of the ettringite structure, modeled (in the structure refinement) with two independent and mutually exclusive configurations. As the disorder is still preserved down to 20 K, we are inclined to consider that as a 'static disorder'. The structure of ettringite is largely held together by hydrogen bonding: the building units [i.e., SO (sub 4) tetrahedra, Al(OH) (sub 6) octahedra, and Ca(OH) (sub 4) (H (sub 2) O) (sub 4) polyhedra] are interconnected through an extensive network of hydrogen bonds. The ettringite of this study has ideal composition Ca (sub 6) Al (sub 2) (SO (sub 4) ) (sub 3) (OH) (sub 12) .27H (sub 2) O, with (Mn+Fe+Si+Ti+Na+Ba)<0.04 atoms per formula unit. The effect of the low-temperature stability of ettringite and thaumasite on the pronounced 'Sulfate Attack' of Portland cements, observed in cold regions, is discussed.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 104
Serial Issue: 1
Title: Minerals in cement chemistry; a single-crystal neutron diffraction study of ettringite, Ca (sub 6) Al (sub l2) (SO (sub 4) ) (sub 3) (OH) (sub 12) .27H (sub 2) O
Affiliation: Universita di Milano, Dipartimento di Scienze della Terra, Milan, Italy
Pages: 73-78
Published: 201901
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 49
Accession Number: 2019-016916
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus.
Secondary Affiliation: CNR, Istituto di Cristallografia, ITA, ItalySwedish Museum of Natural History, SWE, SwedenSapienza University of Rome, ITA, ItalyInstitut Laue-Langevin, FRA, FranceCentro Universitario de la Defensa de Zaragoza, ESP, Spain
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201911
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