Study of the structural properties of the C-S-H(I) by molecular dynamics simulation

Molecular dynamics have been used for a few years to study glass structures. Hydrogen properties were however difficult to simulate. For the study of cement hydrated structures, this technique has not been used. Calcium silicate hydrates (C-S-H) are the main hydrates of cement pastes. X-ray diffraction has not resolved their structure, but has demonstrated structural similarities with tobermorite. Si-29 magic-angle spinning nuclear magnetic resonance spectroscopy has revealed that as the Ca/Si molar ratio in the C-S-H is increased breaks occur in the chains of silicon tetrahedra, which are of infinite length in tobermorite. Molecular dynamics simulation gives atomic-level information. It complements Si-NMR, which does not give the full structure. It has been possible to simulate the partially covalent properties of the hydrogen and silicon bonds using pair and three-body potentials. Relaxation of the tobermorite structure (Ca/Si = 0.66 and 0.83) with 2500 atoms was studied in order to identify the sources of structural instability and to understand the breaking mechanisms in the C-S-K chains. The effects of cationic substitutions on the structure of tobermorite have been studied. Aluminum preferentially substitutes for the bridging silicon of the structure. The charge deficit introduced by the substitution is compensated by protons. Regardless of the substitution site, the aluminum is always flanked by two tetrahedrally coordinated silicon atoms. Calcium ions move to the substituted sites and complete the charge compensation of the protons. (C) 1997 Elsevier Science Ltd.