Molecular dynamics simulation of Al3+ and Na+ substitutions in the tobermorite structure

Molecular dynamics simulations have been used to study the effects of cationic substitutions on the structure of tobermorite. Before substitution, tetrahedrally coordinated silicon atoms form infinite chains. Aluminiums then preferentially substitute for the bridging silicon atoms of the structure, without causing rupture of the chains. The charge deficit introduced by the substitutions is compensated by protons. Non-bridging silicons can also be substituted. In this case, the compensation protons cannot belong to the second sphere of coordination of the aluminium atoms, and substitutions lead to chain rupture, allowing the protons to compensate the charge deficit created by the substitutions. Regardless of the substitution site, the aluminium atom is always flanked by two tetrahedrally coordinated silicon atoms. Calcium and particularly sodium move to the substituted sites and complete the charge compensation of the protons.