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

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, 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 +/- 1.5% and 55.6 +/- 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 +/- 0.02, the Si Q(3) environment decreases down to 0 and is preferentially replaced by the Q(2) environment, which reaches 87.9 +/- 2.0%. At higher ratios, Q(2) decreases down to 32.0 +/- 7.6% for Ca/Si = 1.38 +/- 0.03 and is replaced by the Q(1) 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 similar to 11.3 angstrom (for samples having a Ca/Si ratio