Structural characterization of C–S–H and C–A–S–H samples—Part I: Long-range order investigated by Rietveld analyses

Rietveld analyses on samples belonging to C–S–H and C–A–S–H series (0.8≤C/S≤1.7) were realized on X-ray powder patterns. The tobermorite M model was successfully used to refine all the powder patterns from C–S–H samples whatever the C/S value. This gives clear indication on the steady change in a unique structural description, corresponding to a ‘tobermorite M defect’ model, when passing from C–S–H(I) (C/S<1.0) to C–S–H(II) type (C/S>1.0). The possibility for both C–S–H types (from polymerized silicate chains to isolated silicate dimers) to accommodate the same structural model is explained by the continuous evolution of the occupancies of the cationic sites: the interlayer Ca atoms, the Si atoms from paired and bridging silicates. Accurate refinements of the structural and microstructural parameters evidenced the well crystallized feature of C–S–H phase combined with a small coherent domain size. Insertion of Al atoms in the C–S–H structure (C–A–S–H phase) involves a clear disruption into the layered atomic framework. The large increase of layer spacing observed when incorporating aluminum into C–S–H indicates that Al atoms should be located in the interlayer region of the structure in new crystallographic sites. Aluminum atoms are not substituted silicon crystallographic sites or interlayer calcium crystallographic sites.