Abstract
The structure of nanocrystalline calcium silicate hydrates (C-S-H) having Ca/Si ratios ranging between 0.57 ± 0.05 and 1.47 ± 0.04 was studied using an electron probe micro-analyser, powder X-ray diffraction, 29Si 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 Q3 and Q2 environments account, respectively, for up to 40.2 ± 1.5% and 55.6 ± 3.0% of the total Si, with part of the Q3 Si being attributable to remnants of the synthesis reactant. As the Ca/Si ratio increases up to 0.87 ± 0.02, the Si Q3 environment decreases down to 0 and is preferentially replaced by the Q2 environment, which reaches 87.9 ± 2.0%. At higher ratios, Q2 decreases down to 32.0 ± 7.6% for Ca/Si = 1.38 ± 0.03 and is replaced by the Q1 environment, which peaks at 68.1 ± 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 Å (for samples having a Ca/Si ratio <∼0.6) up to 12.25 Å at Ca/Si = 0.87 ± 0.02, probably as a result of a weaker layer-to-layer connectivity, and then decreases down to 11 Å when the Ca/Si ratio reaches 1.38 ± 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.
Highlights
Nanocrystalline calcium silicate hydrate (C–S–H) is a synthetic phase forming the main hydration product of many types of cements (Richardson, 1999, 2008), including ordinary Portland cement
Edge varies between 4047.9 eV and 4050.6 eV Within uncertainties, the main edge is at the same position in
The present study provides evidence for C–S–H being nanocrystalline and turbostratic tobermorite over the whole range of calcium to silicon (Ca/Si) ratio investigated
Summary
Nanocrystalline calcium silicate hydrate (C–S–H) is a synthetic phase forming the main hydration product of many types of cements (Richardson, 1999, 2008), including ordinary Portland cement. The evolution of the C–S–H structure as a function of its Ca/Si ratio is described by the existence of two phases having crystal structures close either to tobermorite or to jennite (Richardson, 2008; Taylor, 1986), depending on the Ca/Si ratio The former is assumed to be analogous to C–S–H for Ca/Si ratios lower than $1.3 and the latter is assumed to be analogous to C–S–H for higher Ca/Si ratios. These two minerals are layered structures built of Ca polyhedra (in sevenfold coordination in tobermorite, sixfold in jennite) with ribbons of wollastonite-like Si chains running at the surface.
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