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College Park, Maryland      June 6 - 10 , 2004

T2-B3 (2:00 PM): Changes in Ettringite Crystalline Structure Associated with Thermal Dehydration

M. R. Hartman, R. Berliner (University of Michigan, Ann Arbor, MI 48109)

Neutron powder diffraction studies have been performed on deuterated ettringite, Ca6[Al(OD)6]2(SO4)3(~26D2O), synthesized in the laboratory by precipitation from solutions of Ca(OD)2 and Al2(SO4)3 in D2O. Structural studies were performed at 10K using Rietveld refinement on fully hydrated material as well as samples that had been thermally treated to 6 %, 15 %, 21 %, and 27 % weight loss. Changes in the lattice parameters were observed with the c-axis exhibiting a linear contraction from 21.34948(18) in the fully hydrated state to 21.2662(17) for the specimen with 27 % weight loss. Conversely, the a-axis was observed to exhibit a linear expansion from a value of 11.16474(7) in the fully hydrated state to 11.1849(10) in the specimen with 27 % weight loss. The occupations of the hydroxyl and water sites were refined on the dehydrated samples to determine the order in which these sites are depopulated during the dehydration process. In contrast to prior investigations employing thermogravimetric analysis and 27Al NMR, which proposed an orderly depopulation of specific crystalline sites, the present study showed that the water and hydroxyl sites are simultaneously depopulated in the dehydration process. Additionally, an in situ thermal dehydration of ettringite was performed at 50 C by passing dry nitrogen through a column of ettringite. The ettringite sample dehydrated in situ was dehydrated to a weight loss of 26.1 %. Comparison of the in situ dehydration data showed good agreement with the structural data of the statically dehydrated material.

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