Molecular simulations of cement based materials: A comparison between first principles and classical force field calculations

Abstract

The heterogeneity and complexity of the cement structure and processes makes the interpretation of experimental data challenging. Atomistic simulations allow investigations at the atomic level of interactions, thus having the potential to provide complementary information to experiments. In this regard, the investigation of the transferability of the available force fields as well their ability to predict the properties of interest is an important prerequisite. In this work, we compare CLAYFF force field against first principles Density Functional Theory (DFT) calculations focusing on its ability to predict structural, vibrational and thermodynamic properties of cement phases differing in the degree of hydration. The systems studied include tobermorite 9Å, 11Å, 14Å, gypsum, tricalcium aluminate and ettringite. Our results indicate that CLAYFF describes well the lattice parameters within acceptable errors. However for the vibrational properties, there is a significant alteration in the silicate, sulfate, water and OH frequencies in comparison to DFT and experimental results. DFT Bader charge analysis indicate that the charge on the interlayer calcium ions in tobermorite does not change with increase in hydration, implying that the nature inter-atomic bonding within the layers remain unchanged. For the thermodynamic quantities investigated (i.e. Helmholtz free energy, entropy and specific heat), CLAYFF results are in agreement with DFT calculations. Our findings indicate that water enhances the stability of the hydrated phases based on the lower values of the Helmholtz free energy. We demonstrate that CLAYFF can capture consistently the thermodynamic properties of cement phases.