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Structure of nanocrystalline calcium silicate hydrates: insights from X-ray diffraction, synchrotron X-ray absorption and nuclear magnetic resonance

Abstract : The structure of nanocrystalline calcium silicate hydrates (C–S–H) having Ca/Si ratios ranging between 0.57 AE 0.05 and 1.47 AE 0.04 was studied using an electron probe micro-analyser, powder X-ray diffraction, 29 Si magic angle spinning NMR, and Fourier-transform infrared and synchrotron X-ray absorption spectroscopies. All samples can be described as nanocrystalline and defective tobermorite. At low Ca/Si ratio, the Si chains are defect free and the Si Q 3 and Q 2 environments account, respectively, for up to 40.2 AE 1.5% and 55.6 AE 3.0% of the total Si, with part of the Q 3 Si being attributable to remnants of the synthesis reactant. As the Ca/Si ratio increases up to 0.87 AE 0.02, the Si Q 3 environment decreases down to 0 and is preferentially replaced by the Q 2 environment, which reaches 87.9 AE 2.0%. At higher ratios, Q 2 decreases down to 32.0 AE 7.6% for Ca/ Si = 1.38 AE 0.03 and is replaced by the Q 1 environment, which peaks at 68.1 AE 3.8%. The combination of X-ray diffraction and NMR allowed capturing the depolymerization of Si chains as well as a two-step variation in the layer-to-layer distance. This latter first increases from $11.3 A ˚ (for samples having a Ca/Si ratio <$0.6) up to 12.25 A ˚ at Ca/Si = 0.87 AE 0.02, probably as a result of a weaker layer-to-layer connectivity, and then decreases down to 11 A ˚ when the Ca/Si ratio reaches 1.38 AE 0.03. The decrease in layer-to-layer distance results from the incorporation of interlayer Ca that may form a Ca(OH) 2-like structure, nanocrystalline and intermixed with C–S–H layers, at high Ca/Si ratios.
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Sylvain Grangeon, Francis Claret, Cédric Roosz, Tsutomu Sato, Stéphane Gaboreau, et al.. Structure of nanocrystalline calcium silicate hydrates: insights from X-ray diffraction, synchrotron X-ray absorption and nuclear magnetic resonance. Journal of Applied Crystallography, International Union of Crystallography, 2016, 49 (3), pp.771 - 783. ⟨10.1107/S1600576716003885⟩. ⟨hal-01695800⟩

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