PSI - Issue 66

Mansi Gupta et al. / Procedia Structural Integrity 66 (2024) 122–134

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Mansi Gupta et al./ Structural Integrity Procedia 00 (2025) 000 – 000

has dimensions: a = 27.5 Å, b = 38.0 Å, c = 22.9 Å; α = 92.3°, β = 88.5°, γ = 88.3°; with molar ratio of Ca/Si to be 1.64. The overall density of simulated CSH gel is 1.64 g/cm3. The unit cell is subsequently replicated six times in the y direction and two times in the z direction to form a CSH beam-like structure. The final dimensions of duly obtained CSH nano-beam is 27.5 x 228.0 x 45.8 Å. The atomic structures for unit CSH cell and supercell is shown in Fig. 1.

(a) (b) Fig. 1. Atomic structure of (a) CSH unit cell [15], (b) CSH supercell. Colour indices- Calcium (Ca): Green, Oxygen (O): Red, Silicon (Si): Beige, Hydrogen (H): White. 3. Forcefield The present work adopts the reactive force field (ReaxFF) initially formulated by Adri van Duin et al. (2001), to encompass both inter- and intra-molecular interactions within the molecular structure. It is a potential based on bond order that is capable of simulating fracture and bond formations. The total energy in ReaxFF can be defined as: (1) Where, E bond is the bond energy, E over and E under are the energy penalty for over- and under-coordination of atoms. E vdwals , E Coulomb are the van der Walls and Coulombic energies, respectively. And E val , E pen , E tors , E conj are the valence angle energy, penalty energy, torsion energy, conjugation energy, respectively. The ReaxFF potential has been proven to be effective in simulating CSH bond fracture and failure mechanisms [Bauchy et al. (2015), Zhang et al. (2021), Kai et al. (2023)]. The overall atomic charges in the system were adjusted at each timestep by using the Charge equilibration (QEq) scheme. 4. Simulation details After getting a CSH beam-like structure, the generated supercell is first geometrically optimised by Conjugate Gradient (CG) algorithm. The energy and force tolerance were kept at 10-6 kcal/mol and 10-6 kcal/mol- Å, respectively. After energy minimization thermodynamic equilibrium is achieved by using isothermal-isobaric ensemble (NPT) at room temperature (300 K) and atmospheric pressure (101 kPa). The equilibrium settings are carried out for 300 ps (picoseconds) under Periodic Boundary Conditions (PBCs). After reaching the desired stage, a void is induced in the model as an edge crack. A through the thickness crack with 5 Å width and 9 Å height is made by deleting the atoms. In order to avoid interaction of atoms across the notch boundary, non-periodic boundary conditions are then enforced on the system. In x and y directions, non-periodic shrink-wrapped boundary condition are used; while the z axis have fixed boundary conditions. The simulated CSH cell with an existing notch is represented in Fig. 2. E system = E bond + E over + E under + E vdwals + E coulomb + E val + E pen + E tors + E conj