Quantum chemical determination of the Al-substituted site in tobermorite

Ab initio and semiempirical molecular orbital (MO) calculations have been performed to determine the position of the substitution of Al for Si in tobermorite. Replacement energies were calculated for the cluster models of tobermorite framework. Al was found to be substituted mainly in chain middle groups (Q2 sites). Correlation has been recognized between the site occupancy order in the unit cell and structural parameters such as bond lengths and bond angles. It was also found that the variation among energies required for double replacements at the different sites can be explained in terms of the Loewenstein rule.