Tailoring cellulose/C-A-S-H interfacial adhesion through surface functionalization: A molecular dynamics study for sustainable geopolymer composites

Cellulose fibers offer a sustainable strategy to mitigate brittleness and microcrack susceptibility of geopolymers, due to their high tensile strength and biodegradability. However, their hygroscopic nature compromises interfacial adhesion with the geopolymer matrix, particularly with the primary binding phase calcium-aluminosilicate-hydrate (C-A-S-H) gel, thereby limiting reinforcing efficiency. The molecular mechanisms through which surface functionalization improves fiber-matrix interactions remain insufficiently understood. In this study, we systematically investigate how surface functionalization modulates interfacial behavior of cellulose/C-A-S-H by using molecular dynamics simulations. Our results demonstrate that hydrophilic groups capable of forming dual hydrogen bonds (e.g., CONH₂, COOH) significantly strengthen interfacial bonding. CONH2-modified cellulose shows the most substantial reinforcing effect, with a 47 % increase in interfacial bonding energy and a 25 % increase in maximum debonding force relative to unmodified cellulose. Groups that function primarily as either donors or acceptors (e.g., CHO) provide limited reinforcement with approximately17 % higher interfacial adhesion and 15 % higher debonding load. However, weakly polar and less favorably oriented groups (e.g., NH₂) reduce adhesion by around 6 % and lower debonding load by about 10 %. Incorporating spacer chain into hydrophilic groups further enhances interface strength by reducing steric hindrance and improving functional-group accessibility, as evidenced by the superior interfacial performance of CH2COOH over COOH, COCH3 over CHO, and CH2CH2OH over unmodified OH. Interfacial failure analysis reveals a dual-mechanism reinforcement: hydrophilic groups primarily increase the maximum load capacity, while the spacer chains contribute to greater energy absorption by extending the failure displacement. These findings provide a molecular-level foundation for designing high-performance, sustainable geopolymer composites through targeted cellulose surface functionalization.