Abstract
X-ray diffraction (XRD) patterns were calculated and compared to literature data with the aim of investigating the crystal structure of nanocrystalline calcium silicate hydrates (C-S-H), the main binding phase in hydrated Portland cement pastes. Published XRD patterns from C-S-H of Ca/Si ratios ranging from ~ 0.6 to ~ 1.7 are fully compatible with nanocrystalline and turbostratic tobermorite. Even at a ratio close or slightly higher than that of jennite (Ca/Si = 1.5) this latter mineral, which is required in some models to describe the structure of C-S-H, is not detected in the experimental XRD patterns. The 001 basal reflection from C-S-H, positioned at ~ 13.5 Å when the C-S-H structural Ca/Si ratio is low (< 0.9), shifts towards smaller d values and sharpens with increasing Ca/Si ratio, to reach ~ 11.2 Å when the Ca/Si ratio is higher than 1.5. Calculations indicate that the sharpening of the 001 reflection may be related to a crystallite size along c* (i.e. a mean number of stacked layers) increasing with the C-S-H Ca/Si ratio. Such an increase would contribute to the observed shift of the 001 reflection, but fails to quantitatively explain it. It is proposed that the observed shift could result from interstratification of at least two tobermorite-like layers, one having a high and the other a low Ca/Si ratio with a basal spacing of 11.3 and 14 Å, respectively.
Highlights
Nanocrystalline calcium silicate hydrates (C-S-H), synthetic phases, are the major hydration products and the main binding phases in Portland cement (Richardson, 2008; Richardson et al, 1994)
By analogy with other layered structures, line broadening is attributable to crystallite size in the nanometer range, whereas asymmetry is diagnostic of turbostratic disorder which is defined by the systematic occurrence, between successive layers, of random translations parallel to the layers and/or rotations about the normal, an assumption that has been recently validated by modeling of X-ray diffraction (XRD) patterns from four C-S-H samples having a calcium-to-silicon ratio (Ca/Si) ratio equal to $ 0.8 (Grangeon et al, 2013)
The present study investigates C-S-H structure by means of XRD calculations, using a mathematical formalism dedicated to defective structure, with the aim of testing if there is any evidence for the presence of a jennite-like structure in C-S-H of high Ca/Si ratio and what is the origin of the shift of the reflection at $ 7.4 2 Cu K when the C-S-H Ca/Si ratio varies
Summary
Nanocrystalline calcium silicate hydrates (C-S-H), synthetic phases, are the major hydration products and the main binding phases in Portland cement (Richardson, 2008; Richardson et al, 1994). By analogy with other layered structures (carbon black, layered silicates and manganates; Drits & Tchoubar, 1990; Grangeon et al, 2010; Manceau et al, 1997; Warren, 1941), line broadening is attributable to crystallite size in the nanometer range, whereas asymmetry is diagnostic of turbostratic disorder which is defined by the systematic occurrence, between successive layers, of random translations parallel to the layers and/or rotations about the normal, an assumption that has been recently validated by modeling of XRD patterns from four C-S-H samples having a Ca/Si ratio equal to $ 0.8 (Grangeon et al, 2013) This previous study did not discuss the evolution of the C-S-H XRD pattern as a function of the Ca/Si ratio, and especially the variation in position of the maximum at $ 7.4 2 Cu K , nor did it provide a calculated XRD pattern from nanocrystalline turbostratic jennite for comparison with nanocrystalline turbostratic tobermorite, and it may be wondered if XRD is accurate enough to distinguish between two highly defective lamellar structures of close chemistry and crystal structure. The present study investigates C-S-H structure by means of XRD calculations, using a mathematical formalism dedicated to defective structure, with the aim of testing if there is any evidence for the presence of a jennite-like structure in C-S-H of high Ca/Si ratio and what is the origin of the shift of the reflection at $ 7.4 2 Cu K when the C-S-H Ca/Si ratio varies
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