Assessment of the elastic properties of amorphous calcium silicates hydrates (I) and (II) structures by molecular dynamics simulation

J Fu and F Bernard and S Kamali-Bernard, MOLECULAR SIMULATION, 44, 285-299 (2018).

DOI: 10.1080/08927022.2017.1373191

Based on Hamid model of 11 angstrom tobermorite, amorphous calcium silicates hydrates (or C-S-H) structures (Ca4Si6O14(OH)(4)center dot 2H(2)O as the C-S-H(I) and (CaO)(1.67)(SiO2)(H2O)(1.75) as the C-S-H(II)) with the Ca/Si ratio of 0.67 and 1.7 are concerned. Then, as the representative globule' C-S-H, two amorphous C-S-H structures with the size of 5.352x4.434x4.556nm(3) during the stretch process are simulated at a certain strain rate of 10(-3)ps(-1) by LAMMPS program for molecular dynamics simulation, using ClayFF force field. The tensile stress-strain curves are obtained and analysed. Besides, elastic modulus of the globule' C-S-H is calculated to assess the elastic modulus of C-S-H phases (the low-density C-S-H - LD C-S-H - and the high-density C-S-H - HD C-S-H), where the porosity is a critical factor for explaining the relationship between globule' C-S-H at nanoscale and C-S-H phases at microscale. Results show that: (1) The C-S-H(I) structure has transformed from crystalline to amorphous during the annealing process, Young's moduli in x, y and z directions are almost the same. Besides, the extent of aggregation and aggregation path for water molecules in the structure is different in three directions. (2) Young's modulus of both amorphous C-S-H(I) and C-S-H(II) structures with a size of about 5nm under strain rate of 10(-3)ps(-1) at 300K in three directions is averaged to be equal, of which C-S-H(II) structure is about 60.95GPa thus can be seen as the elastic modulus of the globule' C-S-H. (3) Based on the globule' C-S-H, the LD C-S-H and HD C-S-H can be assessed by using the Self-Consistent Scheme (separately 18.11 and 31.45GPa) and using the Mori-Tanaka scheme (29.78 and 37.71GPa), which are close to the nanoindentation experiments by Constantinides et al. (21.7 and 29.4GPa).

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