Thermodynamics and Molecular Mechanism of Al Incorporation in Calcium Silicate Hydrates

Quantitative description of thermodynamic and molecular mechanism of Al incorporation into calcium-silicate hydrates (C-S-H), the main binder in hydrated cement paste, is essential for development of novel cementitious materials with a lower CO, footprint. Thermodynamics integration based on ab initio molecular dynamic simulations was applied to estimate the Gibbs free energy of the Al exchange between different silica tetrahedral sites forming the dreierketten-chains at the C-S-H surface and aqueous Al(OH)(4)(-) anions. The calculations confirm that the Al substitute for Si into bridging. tetrahedral sites with an estimated equilibrium constant K-Al/si similar to 1. Al for Si substitution is further found to favor the cross-Al linking between adjacent chains of the same C-S-H layer. This result is in a good agreement with recent conclusions made from 27Al MAS NMR spectroscopy results. Mesoscale Monte Carlo simulations were performed with the calculated K-Al/si to interpret experimental observations of Al incorporation into C-S-H. The simulation results suggest that the chemical affinity of Al to C-S H is controlled by electrostatic interactions and the Al(OH)(4)(-)/Si(OH)(3)O- aqueous molar ratio.