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

Abstract

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.