Effect of Different Aluminum Substitution Rates on the Structure of Tobermorite

Tobermorite and Al-substituted tobermorite with varying Al/(Si + Al) molar ratios were synthesized via the static hydrothermal method. The effect of different Al substitution ratios on the structure of tobermorite was investigated by XRD, SEM-EDS, TG-DSC, FT-IR, ²⁹Si, and ²⁷Al MAS-NMR. The interplanar crystal spacing of (002) increased with the increment of the Al substitution ratio. The maximum ratio of the Al substitution for Si in the structure of tobermorite was about 15 at %, which was limited by the compositions of tobermorite and the electrostatic repulsive forces caused by Al substitution. The DSC curve of the tobermorite with an Al/(Al + Si) molar ratio of 0.15 showed a larger exothermic peak area at 850 °C, indicating that the Al-O-Si bond was formed in the structure of tobermorite and the Al participated in the recrystallization. The ²⁹Si and ²⁷Al MAS-NMR results confirmed that tetrahedral Al was introduced into the structure of the tobermorite. New silicon sites, Q²(1Al) and Q³(1Al), formed by the Al substitution for Si in bridging and non-bridging sites, were detected. Compared to the tobermorite without Al substitution, the adsorption capacity of Al-substituted tobermorite with Al/(Al + Si) = 0.15 to Na⁺ was improved by 44%.