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 substit ution, tetrahedrally coordinated silicon atoms form infinite chains. A luminiums then preferentially substitute for the bridging silicon atom s 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 compensa tion protons cannot belong to the second sphere of coordination of the aluminium atoms, and substitutions lead to chain rupture, allowing th e protons to compensate the charge deficit created by the substitution s. Regardless of the substitution site, the aluminium atom is always f lanked by two tetrahedrally coordinated silicon atoms. Calcium and par ticularly sodium move to the substituted sites and complete the charge compensation of the protons.