Evidence on the Dual Nature of Aluminum in the Calcium-Silicate-Hydrates Based on Atomistic Simulations

Authors

  • Mohammad Javad Abdolhosseini Qomi,

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    • Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge
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  • Franz-Josef Ulm,

    1. Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge
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  • Roland J.-M. Pellenq

    1. Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge
    2. Centre Interdisciplinaire des Nanosciences de Marseille, Centre National de la Recherche Scientifique and Marseille Université′, Marseille, France
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  • This work has been supported by the Concrete Sustainability Hub at MIT, with sponsorship provided by the Portland Cement Association (PCA) and the RMC Research & Education Foundation.

Author to whom correspondence should be addressed. e-mail: javad@mit.edu.

Abstract

Hydration of tri-calcium silicate (C3S) and di-calcium silicate (C2S) precipitates calcium-silicate-hydrate (CSH) which is the bonding phase responsible for the strength of cementitious materials. Substitution of part of C3S and C2S with aluminum-containing additives alters the chemical composition of hydration products by precipitating calcium-aluminate-silicate-hydrate (CASH). Incorporation of aluminum in the molecular building blocks of CSH entails structural and chemo-mechanical consequences. These alterations can be measured through solid state nuclear magnetic resonance (NMR) experiments. By conducting a wide spectrum of atomistic simulation methods on thousands of aluminum-containing molecular CASH structures, an overall molecular approach for determination of CASH nanostructure is presented. Through detailed analysis of different order parameters, it is found that aluminum can exhibit a tetra-/penta-/octahedral behavior which is fully consistent with the recent NMR observations. This corresponds to the formation of a class of complex three-dimensional alumino-silicate skeletons with partial healing effect in the CASH nanostructure potentially increasing durability and strength of hydration products. We explored the variation of mechanical observables by increasing aluminum content in CASH structures of varying calcium to silicon ratio. Finally, deformation of CSHs and CASHs of different chemical formula in a multi-scale fashion unravels the effect of chemical composition on the strength and kinematics of deformation in this particular type of composites.

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