Structural characterization of C-S-H and C-A-S-H samples—Part II: Local environment investigated by spectroscopic analyses

Spectroscopic studies (¹H, ²³Na and ²⁷Al MAS NMR and Raman spectroscopy) have been used to characterize three series of C-S-H samples (0.8<Ca/Si<1.7): one C-S-H series, one aluminum inserted C-S-H series (named C-A-S-H series), and one sodium and aluminum inserted C-S-H series (named C-N-A-S-H series). Previous Rietveld analyses have been performed on the two first series and have clearly shown that (1) a unique ‘tobermorite M defect’ structural model allows to describe the C-S-H structure whatever the Ca/Si ratio and (2) the insertion of aluminum into the C-S-H structure led to the degradation of the crystallinity and to a systematic increase of the basal spacing of about 2 Å regardless the Ca/(Si+Al) ratio (at a constant Al/Si ratio of 0.1). Spectroscopic investigations indicate that the main part of the Al atoms is readily incorporated into the interlayer region of the C-S-H structure. Al atoms are mainly inserted as four-fold coordinated aluminates in the dreierketten silicate chain (either in bridging or paired tetrahedra) at low Ca/Si ratio. Four-fold aluminates are progressively replaced by six-fold coordinated aluminates located into the interlayer region of the C-S-H structure and bonded to silicate chains. Investigation of the hydrogen bonding in C-S-H indicates that the main part of the hydrogen bonds is intra-main layer, and thus explains the low stacking cohesion of the C-S-H structure leading to its nanometric crystal size and the OD character of the tobermorite like structures.