Multiscale modification of seawater sea-sand cementitious composites via in-situ polymerization and its strengthening-toughening mechanism

In-situ PAA grafting onto PE fiber increases interfacial bond strength by 69%. • Multiscale hybrid system achieves 11.2% tensile strain and 8.3 MPa strength. • PAA-CW synergy refines crack width to 57.8 μm with 69 cracks, meeting marine durability. • In-situ PAA grafting modifies PE fiber surface via free-radical copolymerization. Combining in-situ polymerization with multiscale fibers offers an effective route to improve the tensile properties of seawater sea-sand cementitious composites. Herein, we developed a multiscale hybrid fiber system comprising polyacrylic acid (PAA) generated by in-situ polymerization of sodium acrylate (SA) as the nanoscale reinforcement, calcium carbonate whisker (CW) as the microscale reinforcement, and polyethylene (PE) fiber as the macroscale reinforcement. The effects of this system on the tensile performance, cracking behavior, and interfacial properties of the composites were systematically investigated, and the synergistic toughening mechanism was elucidated through single-fiber pullout tests and micromechanical modeling. The results show that with PE fiber content of 2 vol%, CW content of 0.3 vol%, and SA content of 3 wt%, the composite achieves a maximum tensile strain of 11.2%, a tensile strength of 8.3 MPa, and an average crack width of 57.8 μm, demonstrating its potential for application in marine environments. SA in-situ polymerization enables in-situ modification of the PE fiber surface via free-radical graft copolymerization, introducing a PAA graft layer rich in carboxyl groups. This increases the interfacial bond strength by 69% and the slip-hardening coefficient by 261%. A refined micromechanical model yielded strain-hardening strength index and energy index of this mix are 2.24 and 79.29, respectively, and the comprehensive performance index reaches 2.18 MPa 2 /μm. These results demonstrate an effective balance among strength, ductility and crack control, and provide a mechanistic basis for designing high-toughness structural materials for marine engineering.