publications: articles

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Comparison of Computational Water Models for Simulation of Calcium-Silicate-Hydrate

Calcium silicate hydrate, or C-S-H, is the chief hydration product of Portland cement. The structure of the C-S-H phase within cement has been proposed and developed via molecular simulations. In such simulations, empirical interatomic potentials for water molecules within C-S-H are adopted to govern the position and relative motion of this key constituent. Initial simulations and force fields of C-S-H have assumed the simplest molecular model of H2O termed "single point charge" or SPC, but this choice has not been validated by comparison with other computational models of water that confer additional bond flexibility or charge distribution. To enable efficiently computational modeling of C-S-H and to explore the role that H2O plays in maintaining C-S-H structure and properties, the choice of an efficient and accurate water model is critical. Here, we consider five distinct, classical atomistic water models (SPC, TIP3P, TIP4P, TIP4P05, and TIP5P) to determine the effects of these computational simplifications on C-S-H properties. Quantitative comparison of all five water models shows that the appropriate water model depends on the C-S-H characteristics of interest. Among these models, both SPC and TIP5P models successfully predict key properties of the structure and elastic constants of C-S-H, as well as the dynamics within C-S-H.

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A model for the C-A-S-H gel formed in alkali-activated slag cements

For first time, an experimental and computational study has been conducted to define a structural model for the C-A-S-H gel forming in alkali-activated slag (AAS) pastes that would account for the mechanical properties of these materials. The study involved a comparison with the C-S-H gel forming in a Portland cement paste.

The structure of the C-A-S-H gels in AAS pastes depends on the nature of the alkali activator. When the activator is a NaOH, the structure of the C-S-H gel falls in between tobermorite 1.4 nm with a mean chain length of five, and tobermorite 1.1 nm with a mean length of 14. When waterglass is the activator the structure of the C-A-S-H gel is indicative of the co-existence of tobermorite 1.4 nm with a chain length of 11 and tobermorite 1.1 nm with a chain length of 14. This very densely packed structure gives rise to excellent mechanical properties.

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Time scale bridging in atomistic simulation of slow dynamics: viscous relaxation and defect activation

Atomistic simulation methods are known for timescale limitations in resolving slow dynamical processes. Two well-known scenarios of slow dynamics are viscous relaxation in supercooled liquids and creep deformation in stressed solids. In both phenomena the challenge to theory and simulation is to sample the transition state pathways efficiently and follow the dynamical processes on long timescales. We present a perspective based on the biased molecular simulation methods such as metadynamics, autonomous basin climbing (ABC), strain-boost and adaptive boost simulations. Such algorithms can enable an atomic-level explanation of the temperature variation of the shear viscosity of glassy liquids, and the relaxation behavior in solids undergoing creep deformation. By discussing the dynamics of slow relaxation in two quite different areas of condensed matter science, we hope to draw attention to other complex problems where anthropological or geological-scale time behavior can be simulated at atomic resolution and understood in terms of micro-scale processes of molecular rearrangements and collective interactions. As examples of a class of phenomena that can be broadly classified as materials ageing, we point to stress corrosion cracking and cement setting as opportunities for atomistic modeling and simulations.

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Impact of Chemical Impurities on the Crystalline Cement Clinker Phases Determined by Atomistic Simulations

The presence of chemical substitutions is believed to play a crucial role in the hydration reactions, structure, and elastic properties of cement clinker phases. Hence, substitutions are of great technological interest, as more efficient production of cement clinkers would result in a reduction of CO2 emissions, as well as possible economic benefits. Here we use a combination of classical and quantum mechanical simulation methods to study the detailed physicochemical changes of the clinker phases alite (Ca3SiO5) and belite (Ca2SiO4) when Mg2+, Al3+ and Fe3+ guest ions are incorporated into their structure. Using classical force field methods, we considered random substitutions among possible sites and different compositions in order to identify the preferential substitution sites on the crystalline structures. Then, the resulting structural changes that take place to accommodate the guest ions are investigated and discussed in detail. Using quantum mechanical density functional theory calculations the electronic structure of representative configurations has been computed to determine the potential impact of impurities on the reactivity.