Force Field for Tricalcium Silicate and Insight into Nanoscale Properties: Cleavage, Initial Hydration, and Adsorption of Organic Molecules
RK Mishra and RJ Flatt and H Heinz, JOURNAL OF PHYSICAL CHEMISTRY C, 117, 10417-10432 (2013).
Improvements in the sustainability and durability of building materials depend on understanding interfacial properties of various mineral phases at the nanometer scale. Tricalcium silicate (C3S) is the major constituent of cement clinker and we present and validate a force field for atomistic simulations that provides excellent agreement with available experimental data, including X-ray structures, cleavage energies, elastic moduli, and IR spectra. Using this model and available measurements, we quantify key surface and interface properties of the dry and superficially hydrated mineral. An extensive set of possible cleavage planes shows cleavage energies in a range of 1300 to 1600 mJ/m(2) that are consistent with the observation of faceted crystallites with an aspect ratio near one. Using pure and hydroxylated surface models that represent the first step in the hydration reaction, we examined the adsorption mechanism of several organic amines and alcohols at different temperatures. Strong attraction between -20 and -50 kcal/mol is found as a result of complexation of superficial calcium ions, electrostatic interactions, and hydrogen bonds on the ionic surface. Agglomeration of cleaved C3S surfaces in the absence of organic molecules was found to recover less than half the original cleavage energy (similar to 450 mJ/m(2)) associated with reduced Coulomb interactions between reconstructed surfaces. Additional adsorption of organic compounds below monolayer coverage reduced the attraction between even surfaces to less than 5% of the original cleavage energy (similar to 50 mJ/m(2)) related to their action as spacers between cleaved surfaces and mitigation of local electric fields. Computed agglomeration energies for a series of adsorbed organic compounds correlate with the reduction in surface forces in the form of measured grinding efficiencies. The force field is extensible to other cement phases and compatible with many platforms for molecular simulations (PCFF, COMPASS, CHARMM, AMBER, OPLS-AA, CVFF).
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