A series of synthetic C-S-H phases has been prepared, with the aim of studying structural changes with changes in C/S. Figure 1 shows the Raman spectra obtained from the fresh phases. Of particular interest are the O-Si-O silicate symmetrical stretching (SS) bands at ~1000 cm-1 and the Si-O-Si symmetrical bending bands at ~670 cm-1. Changes in the spectra revealed changes in C-S-H structure with changes in C/S. Silicate chains (Q2) dominate irrespective of C/S, whilst Q3 units were formed at low C/S and sorosilicates (silicate dimers) at high C/S. Sorosilicate formation was concurrent with that of portlandite.

The calcium-rich phases are very sensitive to carbon dioxide, and rapidly react to form calcium carbonate and a calcium-depleted C-S-H.

Figure 2 shows Raman spectra recorded from the fresh phase with C/S = 1.5, and from the same sample after exposure to air for 1 and 40 hours. The most obvious change is the appearance and growth of a band at 1080 cm-1, due to the formation of amorphous calcium carbonate. This is formed via carbonation of portlandite (i.e. loss of intensity at 356 cm-1) and also with structural changes of the C-S-H (i.e. loss of intensity at ~870 cm-1).