Synthetic C–S–H and C-A-S-H samples with different Ca/Si molar ratios (1.0 and 1.5) were prepared by precipitation from solution. The effects of incorporations of aluminum ion into C-S-H gels on their microstructure were studied by applying Fourier Transform Infrared Spectroscopy (FTIR), BET nitrogen adsorption and scanning electron microscopy (SEM). Results show that in FTIR spectra, peaks nearing 970 cm-1 and 450 cm-1 wave number change obviously with the substitution of Al3+into C/S1.0 C-S-H. In BET diagram, it also can be found that the substitution of Al3+into C/S1.0 C-S-H lower the specific surface area. Furthermore, SEM graphs also show that the morphology change in C/S=1.0 C-S-H after the substitution of aluminum ion. However, little difference can be found in FTIR spectra and BET diagrams after Al3+ doped into C/S1.5 C-S-H. So, it can be concluded that the effects of the substitution of aluminum ion on the structure and morphology of C-S-H are affected by Ca/Si molar ratio
