Comparison of mechanical properties of C-S-H and portlandite between nano-indentation experiments and a modeling approach using various simulation techniques

Abstract This work focuses on elastic modulus of two main constituents of cement based materials: portlandite (CH) and Calcium Silicates Hydrates (C-S-H). At nano-scale, the single CH crystal using Density Functional Theory (DFT) is investigated and the homogenized elastic modulus is obtained to be assessed as the RVE unit, which is used in nano-indentation simulation. Then the monolithic C-S-H structure with the chemical formula: (CaO)1.67(SiO2)(H2O)1.75 is simulated during the stretch process at strain rate 10−3 ps−1 by Molecular Dynamics (MD) method using ClayFF field, and its averaged elastic modulus is used to assess Young's moduli of LD and HD C-S-H phases considering the porosity factor. Then at micro scale, FEM is used to simulate the nano-indentation test on ABAQUS software and Young moduli of CH and C-S-H phases are determined by the load-depth curve. Young modulus by the load-depth curve simulated is calculated to compare with the experimental one. The results show that: 1) the Young's modulus calculated by DFT and Reuss-Voigt-Hill (RVH) calculation is 45.46 GPa, which is in quite good agreement with experimental averaged value (39.88 GPa) and with the literature values (45.94 GPa by Laugesen, 52.4 GPa by Speziale et al., 44.69 GPa by Kerisit et al., 46.58 GPa by Holuj et al.). 2) Based on the elastic modulus of the monolithic C-S-H structure by MD simulations, the assessment results on LD C-S-H and HD C-S-H after homogenization are very close to nanoindentation experiments data. 3) By FEM method, the simulated P-h curve is adopted to compare the extent of deviation from the experimental values, which is within an acceptable relative error. The homogenized elastic properties of polycrystals can be obtained by elastic constants of single crystal (using DFT and RVH estimation), thus can be used to explain the relationship between structure and mechanical properties of CH from nano-scale to micro-scale. Results enable to provide useful parameters for composite cements systems modeling and a method to calculate elastic modulus of other similar structures.

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