Molecular insight into the fluidity of cement pastes: Nano-boundary lubrication of cementitious materials

MH Wang and KX Zhang and X Ji and P Wang and HY Ma and J Zhang and DS Hou, CONSTRUCTION AND BUILDING MATERIALS, 316, 125800 (2022).

DOI: 10.1016/j.conbuildmat.2021.125800

The workability mechanism of fresh concrete at the molecular level remains essentially unexplored. To under-stand the molecular origin for cement fluidity, molecular dynamics and Density Function Theory (DFT) were utilized to construct a shear model of Calcium-Silicate-Hydrate (C-S-H) layers. The structure, dynamics, and reactivity of ultra- confined pore solution between C-S-H gels are systematically investigated. Under shear loading, periodic oscillation of friction force is observed as the typically Couette flow and the interfacial friction force is reduced from 35.2 Kcal/mol.?, to 3.3 Kcal/mol.?, with water content increasing. All of the systems contain breakage of noncovalent bonds of water-Ca and water-water in the lubrication process. But, the obvious breakage of covalent bonds is found in the low-water content system, in which a part of Ca atoms is separated from the C-S-H matrix for lubricating that does not occur in the high-water content system. Furthermore, DFT studies monitor the reaction pathway for the dissociation of water molecules and calcium ions from silicon oxide tetrahedrons. In the energetic respect, it distinguishes covalent- ionic bond transformation and H-bond breakage. The atomic-level mechanisms provide new insights on workability design for fresh concrete.

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