(724h) A Molecular Dynamics Study of Interparticle Interactions in Fresh Cement Pastes

Juan Pablo Gallo-Molina^1,2, Adilson Alves de Freitas³, Karel Lesage², José Nuno Canongia Lopes³ and Ingmar Nopens¹

¹BIOMATH, Department of Data Analysis and Mathematical Modelling, Ghent University, Ghent, Belgium.

²Magnel-Vandepitte Laboratory, Department of Structural Engineering and Building Materials, Ghent University, Ghent, Belgium.

³Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal.

Due to the fact that it is one of the most produced materials in the world, its critical importance for infrastructure development and the environmental issues associated with the large masses involved, concrete has been attracting increasing research attention since the last decades[1]. For hardened concrete, civil engineers already possess a toolset that allow them to satisfactorily predict properties of interest (e.g. deformation, stress, etc.) as a function of material characteristics. However, when this material is encountered in a fluid state, the picture is completely different: only some empirical tests can relate some material variables (e.g. concrete composition) with relevant properties (i.e. flow speed, flow spread, etc.). Although this practice is longstanding, the increasing complexity of concrete mixtures - introduced by the growing use of mineral additions, chemical admixtures and non-traditional cement chemistries - has invoked the need for developing mechanistic models with predictive capabilities.

Typical concrete suspensions exhibit very broad polydispersity, with size ranges encompassing several orders of magnitude. Furthermore, composition and crystalline structures vary with both size and source. It follows that rheology is not controlled by a single mechanism, but rather by several competing ones. Indeed, hydrodynamic, van der Waals and electrostatic forces all play a role in determining flow behavior. Likewise, steric interactions become relevant if chemical admixtures are used and the aforementioned hydration reactions grow in importance after the first hours following water addition [2]. The population balance framework has been identified as a valuable tool that is capable of reflecting the emerging nature of rheology from the transient evolution of a given cement suspension microstructure[3]. However, to increase its effectivity, precise knowledge of the nature of colloidal interactions is needed, as they play a vital role in determining the values of aggregation efficiencies used by population balances models.

Considering this, we present in this work a molecular dynamics model focused on the main phases found in cement clinkers[4]. The objective of the modelling effort is to better understand the behavior of interparticle interactions in these systems since, as suggested before, they have a direct impact on agglomeration dynamics and subsequently rheology. We investigate the presence of interaction phenomena that cannot usually be assessed with classical formulations. Moreover, the effect of crystalline structure as well as that of interparticle distance is analyzed. The final aim is to gain relevant information for the construction of a more accurate representation of agglomeration/breakage dynamics in cement pastes.

References

[1] R. J. Flatt, N. Roussel, and C. R. Cheeseman, “Concrete: An eco material that needs to be improved,” J. Eur. Ceram. Soc., vol. 32, no. 11, pp. 2787–2798, 2012, doi: 10.1016/j.jeurceramsoc.2011.11.012.

[2] N. Roussel, A. Lemaître, R. J. Flatt, and P. Coussot, “Steady state flow of cement suspensions: A micromechanical state of the art,” Cem. Concr. Res., vol. 40, no. 1, pp. 77–84, 2010, doi: 10.1016/j.cemconres.2009.08.026.

[3] J. P. Gallo-Molina, K. Lesage, and I. Nopens, “Numerical Validation of a Population Balance Model Describing Cement Paste Rheology,” Materials (Basel)., vol. 13, no. 5, p. 1249, Mar. 2020, doi: 10.3390/ma13051249.

[4] A. A. Freitas, R. L. Santos, R. Colaço, R. Bayão Horta, and J. N. Canongia Lopes, “From lime to silica and alumina: Systematic modeling of cement clinkers using a general force-field,” Phys. Chem. Chem. Phys., vol. 17, no. 28, pp. 18477–18494, 2015, doi: 10.1039/c5cp02823j.