College Park, Maryland      June 6 - 10 , 2004

T2-B2 (1:45 PM): Combined USANS/SANS Contrast Matching to Distinguish Calcium Hydroxide and Calcium-Silicate-Hydrate Gel Contributions to Hydrating Cement Microstructures

A. J. Allen (Ceramics Division, MSEL, National Institute of Standards and Technology), J.J. Thomas, J.J. Chen (Department of Civil Engineering, Northwestern University, Evanston, IL), J.G. Barker (NIST Center for Neutron Research)

Characterization and modeling of the microstructure of hydrating cement using both SANS and SAXS has long been pursued by groups from around the world. The durability and mechanical properties of concrete are intimately connected to the development of calcium-silicate-hydrate (CSH) gel within the hydrating cement component. SAS techniques are particularly useful because the CSH is amorphous, rendering diffraction ineffective, and is altered by sample preparation for electron microscopy. Also, the CSH gel structure (which exhibits fractal properties) extends over length scales appropriate for SAS. Quantifying CSH structure is complicated by the presence of water (H) in various states ranging from OH groups to trapped liquid in nanopores. A further complication for conventional SANS and SAXS, and even for USAXS, is the presence of calcium hydroxide (CH) crystals in intimate mixture with C-S-H, which makes it necessary to determine the fine CH structure at all length scales so that CSH can be unambiguously characterized. Recently, combined USANS and regular SANS measurements performed on contrast matched cement paste specimens have allowed the CH morphology to be revealed and surveyed over the full range of required sizes. All the H in CSH exchanges readily with deuterium oxide (D) in contrast variation studies, while CH is crystalline and does not take part in H/D exchange. Hydrated samples were studied by USANS and SANS in H/D mixes of 100 % H, 32 % D (CH contrast match), and 80 % D (effective CSH/D contrast match). Corresponding samples were also studied after the CH phase was removed by leaching. Comparing leached and unleached sample data for the various H/D mixes has allowed the CH and CSH components to be distinguished to an unprecedented degree, and has highlighted previously unobserved structures in the USANS range. A fully quantitative microstructure analysis of CSH over its whole scale range has thus been obtained for the first time.

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