Ab initio calculations for the substitutions of AI(OH)(4)(-) and SO42- with Si tetrahedra

We have studied the interaction phenomena for the substitutions of Al(OH)(4 )(-) and SO42- with both paired and bridging Si tetrahedra likely to be occ urred in C-S-H gel coexisting with ettringite system. Gaussian 94 program h as been used to obtain the total energy of the molecule in the optimized st ructure employing HF/3-21G* level of theory. It has been investigated that the net charge on the substitution site causes the modification of the orig inal structure during the substitution. The substitutions of Al(OH)(4)(-) w ith Si tetrahedra were found to be thermodynamically possible in both paire d and bridging Si tetrahedra at chemical equilibrium. The substitutions of SO42- with Si tetrahedra were found to be thermodynamically possible at the limited positions as well. In this work, these possibilities are also disc ussed with the estimated activated energies for the substitutions of Al(OH) (4)(-) and SO42- with paired Si tetrahedra.