Ab Initio Calculations for the Substitutions of Al(OH)- 4 and SO2- 4 with Si Tetrahedra

We have studied the interaction phenomena for the substitutions of Al(OH)- 4 SO2- 4 with both paired and bridging Si tetrahedra likely to be occurred in C-S-H gel coexisting with ettringite system. Gaussian 94 program has been used to obtain the total energy of the molecule in the optimized structure employing HF/3–21G* level of theory. It has been investigated that the net charge on the substitution site causes the modification of the original structure during the substitution. The substitutions of Al(OH)- 4 with Si tetrahedra were found to be thermodynamically possible in both paired and bridging Si tetrahedra at chemical equilibrium. The substitutions of SO2- 4 with Si tetrahedra were found to be thermodynamically possible at the limited positions as well. In this work, these possibilities are also discussed with the estimated activated energies for the substitutions of Al(OH)- 4 and SO2- 4 with paired Si tetrahedra.