The application of nanomaterials in cement-based composites has attracted considerable research interest in recent years, because of their ability to enhance mechanical properties such as compressive, flexural, and fracture strength. Among these materials, nanocellulose, a bio-derived nanomaterial obtained from abundant renewable resources and characterized by excellent properties, has considerable potential for use in cement composites. Its surface functional groups can act as nucleation sites for calcium silicate hydrate (C-S-H) gel, leading to the formation of denser hydration products and refined pore structures. Furthermore, nanocellulose inhibits microcrack propagation through nano-filling and bridging effects, thereby synergistically improving the mechanical performance of cement-based materials. Despite these advances, the interfacial bonding behavior between nanocellulose and C-S-H gel at the nanoscale is not well understood because of limitations in conventional macroscopic experiments and microscopic characterization techniques. The reinforcing effect of nanocellulose has been proposed to arise primarily from a dual micromechanism of "short-circuit diffusion and bridging crack resistance". However, direct atomistic evidence supporting this hypothesis is lacking. In particular, the influence of nanocellulose surface functionalization (e.g., carboxylation) on interfacial interactions has not been systematically investigated. In this study, molecular dynamics (MD) simulations were used to investigate the interfacial structure, interaction energy, hydrogen bonding evolution, and failure behaviors of nanocellulose/C-S-H composites. A novel embedded composite model was constructed using a Monte Carlo-based strategy. The effects of hydroxylated and carboxylated nanocellulose on interfacial binding and mechanical performance were systematically compared.

The C-S-H gel model with a chemical composition of (CaO)_1.65(SiO₂)(H₂O)_1.75 was constructed following the realistic disordered structure proposed by Pellenq et al., which matches the experimental density (~2.6 g/cm³) and Ca/Si ratio (1.6-1.8). The modeling process started from the 11 Å tobermorite crystal structure, followed by random removal of bridging silicate tetrahedra, water saturation at 298 K, and subsequent energy minimization and NPT dynamic relaxation (298 K, 120 ps, 1 fs timestep) using the COMPASS force field. C-S-H models with different Ca/Si ratios (0.75, 1.31, 1.48, and 1.67) were also constructed by controlling the removal of silicate tetrahedra. Amorphous cellulose models (both hydroxylated and carboxylated) were built using cellobiose as the repeating unit. A three-dimensional periodic amorphous cellulose model (43.0 Å × 43.0 Å × 22.0 Å) was constructed by randomly packing five cellulose chains using the Grand Canonical Monte Carlo (GCMC) method, followed by NVT relaxation (298 K, 100 ps, 1 fs timestep). The final density of 1.32 g/cm³ agreed well with experimental values. The C-S-H/cellulose composite model was constructed by inserting the amorphous cellulose box into the interlayer of C-S-H, with a 5 Å vacuum layer to avoid initial atomic overlap. The system was first energy-minimized using the smart algorithm (energy tolerance 1.0×10^-4 kcal/mol, force tolerance 0.001 kcal/mol/Å), followed by NVT equilibration (298 K, 100 ps, 1 fs timestep). Uniaxial tensile simulations were performed along X, Y, and Z directions to obtain stress-strain curves, Young's modulus and tensile strength. Interfacial interaction energy was calculated as E_inter = E_{_total}-(E_{_C-S-H} + E_{_CNF}). Hydrogen bonding, radial distribution functions (RDF) and mean squared displacement (MSD) were also analyzed.

With increasing Ca/Si ratio from 0.75 to 1.67, the degree of polymerization of silicate chains decreased systematically, as evidenced by the decrease in Q₂ content from 100% to 20%. The C-S-H model with Ca/Si = 1.67 yielded a density of 2.44 g/cm³ and Q_n distribution (Q₀ = 11.6%, Q₁ = 65.12%, Q₂ = 20%), which closely matched the real molecular structure of C-S-H gel. Uniaxial tensile simulations revealed that Young's modulus decreased with increasing Ca/Si ratio in all three directions (X, Y, Z). For Ca/Si = 1.67, Young's modulus values ranged between 10-30 GPa, consistent with nanoindentation experimental results. The Y-direction exhibited the highest modulus due to the dominant contribution of silicate chains, while the X and Z directions were governed by the fractured hydrogen bond network. The interfacial interaction energies for both hydroxylated and carboxylated nanocellulose with C-S-H were negative, indicating spontaneous adsorption and thermodynamically stable interfaces. The absolute value of interfacial interaction energy for carboxylated nanocellulose/C-S-H (-1.70×10² kJ/mol) was significantly larger than that for hydroxylated nanocellulose/C-S-H (-0.73×10² kJ/mol), demonstrating stronger spontaneous adsorption and tighter interfacial bonding after carboxylation. The interaction energy mainly consisted of van der Waals and electrostatic contributions, with electrostatic energy accounting for over 90% of the total adsorption energy. Hydrogen bond analysis showed that in the C-S-H/carboxylated nanocellulose composite model, 56.1 hydrogen bonds were formed in the interfacial region, corresponding to a hydrogen bond density of 3.12 bonds/nm². In contrast, the hydroxylated nanocellulose/C-S-H interface gave only 25.7 hydrogen bonds with a density of 1.43 bonds/nm². The significantly higher hydrogen bond density after carboxylation strongly enhanced interfacial binding. RDF analysis revealed characteristic peaks at 0.97 Å and 2.05 Å for oxygen and hydrogen atoms in C-S-H, with atomic distances smaller than the hydrogen bond cutoff distance of 2.5 Å. The carboxylated system exhibited more pronounced peaks, indicating stronger bonding ability of carboxyl oxygen with interfacial hydrogen. MSD analysis showed that atoms at the interface of the carboxylated nanocellulose/C-S-H composite had lower MSD values than those in the hydroxylated system, indicating stronger atomic constraints and lower motional freedom. This enhanced interfacial stability enabled more effective stress transfer from the matrix to the reinforcing phase.

In this work, molecular dynamics simulations were used to investigate the interfacial interaction and reinforcement mechanism between nanocellulose and C-S-H. The results showed that the C-S-H model with a Ca/Si ratio of 1.67 constructed by the Monte Carlo method yielded a density of 2.44 g/cm³ and a Q_n distribution (Q₀ = 11.6%, Q₁ = 65.12%, Q₂ = 20%) that closely matches actual C-S-H gel. As the Ca/Si ratio increases from 0.75 to 1.67, the degree of polymerization of silicate chains decreases, and Young's modulus in all three directions decreases accordingly. Spontaneous adsorption occurs at the nanocellulose/C-S-H interface, where functional groups on the nanocellulose surface form hydrogen bonds to promote interfacial bonding. The number of hydrogen bonds formed at the interface of carboxylated nanocellulose/C-S-H (56.1) was significantly greater than that of hydroxylated nanocellulose/C-S-H (25.7). Both types of nanocellulose reinforce C-S-H, but the interfacial interaction energy of carboxylated nanocellulose/C-S-H (approximately -1.70×10² kJ/mol) is higher than that of hydroxylated nanocellulose, with electrostatic energy contributing over 90% of the total adsorption energy. These findings demonstrate that carboxylation modification significantly enhances interfacial bonding, providing an atomistic-scale theoretical basis for designing high-performance cement-based composites.